Effects of Hormones on Animals | Essay | Hormones | Endocrinology

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Effects on Hormones on Animals

Essay Contents:

1. Essay on the Hormonal Effects on Digestive Tract
2. Essay on the Hormonal Effect on the Integument
3. Essay on the Hormonal Effects on Skin Glands
4. Essay on the Hormonal Effects on Lactation
5. Essay on the Hormonal Effects on Pigmentation
6. Essay on the Hormonal Effects on Metabolism
7. Essay on the Hormonal Effects on Muscle
8. Essay on the Hormonal Effects on Gonads and Reproduction
9. Essay on the Hormonal Effects on Growth and Differentiation of the Young
10. Essay on the Hormonal Effects on Metamorphosis
11. Essay on the Hormonal Effects on Osmoregulation
12. Essay on the Hormonal Effects on Animal Behaviour

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Essay # 1. Hormonal Effects on Digestive Tract:

Hormones effect the secretion of digestive tract products, such as enzymes, hydrochloric acid, and bile salts. Gastrin is secreted from two sources, Gastrin₃₄ by the pyloric antrum and Gastrin₃₄ by the upper small intestine. Gastrin increases the secretion of hydrogen ions by oxyntic cells and thus promotes the secretion of gastric hydrochloric acid. It also increases the synthesis of pepsin by peptic cells.

Gastrin₃₄ acts on the pancreas and gall bladder. It increases fluid and enzyme content of the pancreatic juice. Gastrin also increases the contractions of the gall bladder. Secretin is another hormone secreted by the duodenum and upper jejunum. This hormone affects the fundus part of the stomach, intestine, pancreas and liver.

In stomach, it inhibits hydrogen ion secretion by oxyntic cells. In intestine, secretin stimulates the secretion of Brunners glands. It acts on the exocrine pancreas by increasing the flow and bicarbonate content of the pancreatic juice. It has a weak stimulatory effect on the pancreatic enzymes. Secretin brings about increase in the secretion of inorganic components of the bile juice such as sodium chloride and bicarbonate.

Pancreozymin – Cholecystokinin secreted by the duodenum and upper jejunum stimulates the enzyme component of the pancreatic juice and bicarbonate content. It helps in the release of bile juice by bringing contractions of the gall bladder and relaxing the sphincter of Oddi. In stomach, secretion of gastric hydrochloric acid is stimulated.

Gastric inhibitory peptide is secreted by the duodenum, jejunum and upper part of ileum inhibits acid secretion in stomach, and insulin by the pancreas. Enterogiucagon is found in the lower small intestine and colon. This hormone probably stimulates the secretion of insulin.

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Essay # 2. Hormonal Effect on the Integument:

Integument is anatomically and physiologically a very important tissue which exhibits considerable diversity reflecting the differences which exist in the physicochemical gradients between the vertebrates and their environment. Integument plays a role in animal’s osmoregulation, thermoregulation and respiration.

Integument also provides signs and signals that can promote social and sexual contact and can help the animal to blend in with its surroundings and thus protects the animal from predators, or helps it to catch food. Skin is the major interface between the animal and its environment.

Skin includes structures like scales, hair, feathers, pigment cells, secretory glands and certain sense organs. Many cold-blooded animals can rapidly alter the distribution of pigment in the skin so that they blend more closely with the shades and hues of their surroundings.

Seasonal changes occur in the integument, such as changes in pigmentation associated with breeding and alteration of color, length and density of fur and feathers in summer and winter. Hormones help in the maintenance of the nutritional and anatomical integrity of the skin as well as such processes as molting, pigmentation and the function of certain cutaneous glands.

Hormones that influence cutaneous function include several from the pituitary such as prolactin, melanocyte stimulating hormone, vasotocin, adrenocorticotrophic hormone, leutenizing hormone and thyroid stimulating hormone, and also thyroxin, catecholamines, corticosteroids, gonadal steroids and melatonin. Some of the actions of these hormones are confined to relatively few species while the effects of a hormone on the skin may be quite different in one species as compared to the other.

Hormones and Molting:

The epidermis in many animals undergoes a process of renewal as the outer layers drop off and are replaced by the underlying epithelium at regular intervals. This process may be continuous, or may occur at regular intervals ranging from a few days as in amphibians, to several months in certain lizards and snakes. Hair in mammals and feathers of birds also undergo such renewal at specific periods in a season.

The regular cyclical molting in fish, reptiles and amphibians reflects an autonomous rhythm in the skin upon which the hormones act in a permissive manner. The pituitary and thyroid glands are the main endocrines that influence molting in vertebrates. Removal of the pituitary gland prevents or prolongs the length of molting cycles in amphibians and reptiles, and blocks the seasonal molts observed in birds and mammals.

This effect is due to the lack of many hormones like thyroid stimulating hormone, prolactin and corticotrophin. In urodeles, lizards, birds and mammals, the thyroid hormones accelerate the molting process while removal of thyroid gland inhibits the process. In Bufo bufo corticotrophin and corticosteroids are necessary for successful molting.

Prolactin produces diverse actions in vertebrates, especially on the integument and its derivatives, the mammary glands. Injection of prolactin decreases the length of the sloughing cycle in the lizards Anolis carolinensis and Gekko gecko, and the urodele Notophthalamus viridescens. The seasonal changes in the pelage of mammals and plumage of birds are under the control of photoperiod. Changes in the day length are transmitted through eyes and hypothalamus to the pituitary gland.

The effects of photoperiod on the pelage and plumage are mediated through the action of the gonadal steroids, corticosteroids and thyroxin, which in turn are controlled by the hypothalamus and pituitary gland.

Cyclical changes in moting in poikilotherms are characterized by brief periods of cellular activity and rapid cell division interspersed by periods of low activity or resting periods. Thyroxin and prolactin shorten the resting periods. In toads the absence of pituitary prevents the shedding or casting off the epidermis. The shedding is promoted by corticotrophin and corticosteroids.

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Essay # . Hormonal Effects on Skin Glands:

In vertebrates skin possesses a variety of glands that perform different functions.

These glands are classified into:

(i) Mucous glands, and

(ii) Proteinaceous glands.

The proteinaceous type of glands include the venom glands in fishes and amphibians, preen glands in birds and the sebaceous, sweat glands and mammary glands of mammals. The maturation and function of sebaceous, sweat and odoriferous glands in mammals are influenced by sex hormones.

Injection of testosterone promotes the development of the submandibular chin glands in rabbits. Similar effects of the gonadal steroids are observed on sebaceous and sweat glands of other mammals. Mucous glands occur in the skin of fishes and amphibians. Mucous secretion is promoted by the injection of prolactin.

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Essay # 4. Hormonal Effects on Lactation:

Lactation or secretion of milk by mammary glands to feed the young is confined to the mammals. Milk is a nutrient solution containing fats, carbohydrates, proteins, and minerals. The mammary glands, like adipose tissue, can synthesize triglycerides and lactose. The mammary gland tissue synthesizes most of the proteins but a few are transferred from the plasma.

Morphological differentiation of the mammary gland and secretion of milk are both regulated by several hormones. The arrangement of the mammary gland tissue and the activities of hormones on them are shown in Figure 1.

Development of the alveoli is influenced by estrogens and progesterone, while the duct system is under the influence of estrogens. Oxytocin from the neurohypophysis is released by the stimulus of suckling on the nipple. It contracts the myoepithelial cells surrounding the alveoli resulting in expulsion of milk.

Initiation of the secretion of milk by the alveoli is dependent on prolactin released during parturition and suckling. For the proper functioning of the mammary glands, it is necessary that other hormones like thyroxin, corticosteroids and growth hormone must be present in adequate levels in blood. No single hormone is effective on one function but combinations involving ACTH or corticosteroids, prolactin, growth hormone, thyroid hormones, estrogen and progesterone are necessary.

At the cellular level lactation involves the following stages:

i. Mammary Gland Cell Proliferation:

Multiplication of the mammary tissue cells requires the presence of insulin and is enhanced by estradiol. Progesterone also helps to organize this process in an orderly manner

ii. Differentiation of the Mammary Cells:

This includes acquisition of the enzymes necessary for the formation of the milk constituents. This process requires the presence of insulin, cotisol and prolactin. Prolactin initiates transcriptional processes and the formation of messenger RNA, which mediates the formation of milk proteins and the enzyme lipoprotein lipase.

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Essay # 5. Hormonal Effects on Pigmentation:

The integument of most vertebrates contains pigment within the epidermis or dermis or in the appendages such as scales, hair and feathers. Pigmentation contributes to the animal’s camouflage, protects the internal organs from solar radiation, promotes the absorption or reflection of heat and light, helps in photoreception and contributes to the synthesis of vitamin D in the skin. Integumental color is also important in dimorphic sexual behaviour and reproduction. Pigments of different colors occur in the skin cells called chromatophores.

These cells usually contain a black or brown pigment called melanin and therefore are called melanocytes. If the intracellular distribution of the pigment is changed, they are known as melanophores. Xanthines and carotenes are yellow and red colored pigments present in xanthophores and erythrophores. Some chromatophores also contain pteridine platelets, which reflect light, giving an iridiscent appearance, and are thus called iridiophores.

The complex coloration observed in many vertebrates is the result of blending the colors reflected by the various chromatophores. Many vertebrates can alter their coloration in response to environmental and behavioural requirements.

These changes may be very slow (morphological color change) taking several days or weeks, or very fast taking only few minutes or hours (physiological color change). Both morphological and physiological color changes are influenced by the actions of hormones, especially the pituitary melanocyte stimulating hormone MSH.

The release of MSH plays a most important role in both physiological and morphological color change in vertebrates. The principal stimulus is the receipt of light usually by the lateral eyes but the pineal eye may also function as a photoreceptor.

In addition to directly influencing MSH release, light may also contribute to a cyclical photoperiodic release of this hormone in some mammals that seasonally change color of peelage. The secretion of MSH by the intermediary lobe of the pituitary is under the control of hypothalamus.

When the pars intermedia is transplanted to another part of the body away from the hypothalamus, or if its connections to this part of the brain are severed, MSH is secreted in an uncontrolled manner. MSH release – inhibiting hormone (MSH – R – IH) has been identified in the hypothalamus and this contributes to the control of the hormone’s release.

MSH – R – IH is a peptide probably formed in the hypothalamus. MSH releasing hormone (MRH) is also present in the hypothalamus, and this opposes the action of MSH-R-IH. Melatonin from the pineal gland mediates photoperiodic release of MSH by an action on the hypothalamus.

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Essay # 6. Hormonal Effects on Metabolism:

Hormones affect metabolic processes, both anbolic and catabolic in most cells. The synthesis and degradation of carbohydrates, lipids and proteins are controlled by hormones to meet specific energy or growth requirements of the individual. The transformation, storage and utilization of fats, proteins and carbohydrates are regulated by hormones.

Insulin:

Insulin plays a central role in intermediary metabolism, and this is associated, especially during fasting, with the actions of corticosteroids. Glucagon and adrenaline also contribute to the control system. Growth hormone and thyroid hormones modulate the processes involved in many chemical pathways.

Prolactin stimulates the formation of milk proteins in mammary glands and in the pigeon crop sac. Several hormones interact with each other in such metabolic processes, none can function normally in complete isolation from all the others.

Insulin decreases plasma glucose concentrations and in mammals reduces free fatty acid levels. In the absence of insulin, muscle wasting occurs due to the excessive mobilization of proteins.

The processes mediated by insulin are given below:

(a) Insulin increases the rate of uptake of glucose by skeletal muscle, cardiac tissue, adipose tissue, and mammary gland.

It facilitates formation of glycogen from glucose due to increase in the activity of glycogen synthetase in liver, muscle and adipose tissue.

(b) Insulin inhibits the mobilization of glycogen to form glucose.

(c) Gluconeogenesis from amino acids is inhibited. It also inhibits the mobilization of amino acids from proteins.

(d) Insulin increases the accumulation of fatty acids across the cell membranes of adipose tissue cells.

(e) It inhibits lipolysis of triglycerides to form fatty acids.

(f) This hormone promotes lipogenesis from glucose in adipose tissue

(g) It increases the uptake of amino acids by muscle and liver cells.

Thus, the effects of insulin can be divided into its actions on the accumulation of nutrients across the cell membranes and its facilitation or inhibition of metabolic synthesis in cells.

Catecholamines:

Adrenaline increases the concentration of both glucose and fatty acids in the blood. These effects are mediated in liver and muscles due to activation of phosphorylase, and in adipose tissue by the activation of lipase. Both of these effects are due to the formation of cyclic AMP.

Glucagon:

Glucagon exerts a hyperglycemic effect due to the mobilization of liver glycogen mediated by the activation of phosphorylase enzyme. Gluconeogenesis is promoted. Levels of fatty acids in plasma are elevated. Lipolysis is accelerated in adipose tissue.

Other hormones, which influence glucose and fatty acid metabolism are vasopressin, oxytocin and vasotocin.

Adrenocorticosteroids:

Adrenocorticosteroids exert profound effect on the intermediary metabolism, reproduction, growth and lactation. Corticosteroids increase blood glucose concentrations and promote gluconeogenesis and the deposition of glycogen in the liver.

He effects of corticosteroids on intermediary metabolism can be summarized as follows:

(1) Corticosteroids increase gluconeogenesis in the liver as a result of mobilization of proteins from skeletal muscle and the deamination of the released amino acids.

(2) They promote glycogenesis by increasing the activity of synthetase.

(3) Corticosteroids inhibit glycogenolysis.

(4) They reduce peripheral oxidation and utilization of glucose.

(5) Corticosteroids reduce the conversion of amino acids to proteins, and fatty acids to triglycerides.

Gonadal Steroids:

Estrogens and androgens produce widespread metabolic effects on the growth and differentiation of tissues, especially the reproductive organs. Andogens exert anabolic effects on skeletal muscle by promotion of the formation of proteins.

Estrogens increase plasma lipid levels in mammals and also have anabolic effects in the mammary glands and reproductive organs. They also promote the formation of lipoproteins in the liver, which are incorporated into the yolk of the egg. Progesterone increases the formation of avidin by the oviduct.

Thyroid Hormones, Growth Hormone and Prolactin:

These hormones regulate metabolism. In the absence of thyroid hormones consumption of oxygen is depressed and the hormones modulate the levels and activity of metabolic enzymes in cells. Growth hormone promotes growth and stimulates the formation of proteins in cells.

Its actions are mediated by somatomedin, a protein formed in the liver. Growth hormone promotes the uptake of amino acids by liver and muscle cells but inhibits the action of insulin on glucose uptake. Prolactin influences the intermediary metabolism of the mammary glands and promotes deposition of fat.

Hormones exhibit widespread actions on intermediary metabolism. Hormones directly influence one another’s release and so mimic or oppose the actions of other hormones. Intermediary metabolism is a well-integrated process, involving many tissues, chemical reactions and numerous metabolites. This is due to the actions of hormones at different sites, within the cells and in different kinds of cells (Figs 2 & 3).

Hormones and Calorigenesis:

Calorigenesis is the production of heat in the body.

The heat produced is:

(i) A byproduct resulting from the energy requirements of the body. This heat results from mechanical activity of muscle contractions or metabolic transformations of chemical substrates.

(ii) Production of heat is necessary for the maintenance of body temperature in homeotherms. Although all tissues contribute in heat development, skeletal muscles and liver are the most important.

Thyroid Hormones:

Tetraiodothyronine and triiodothyronine play an important role in maintaining the production of heat in mammals and birds. Oxygen consumption is reduced by 50% in the absence of the thyroid hormones while increased secretion of thyroid hormones increases the oxygen consumption to the same extent. Thyroxin injection doubles the basal metabolic rate. Thyroid hormones increase heat production by maintaining the metabolic machinery to function optimally.

Catecholamines:

Adrenaline and noradrenaline play an important role in regulating the production of heat in the body. Sudden exposure of homeotherms to cold increases production of heat. Mice can survive on transferring from a temperature of 25 °C to 0 °C by adjusting their metabolism.

However, if they are pretreated with drugs like propranolol which inhibit β-adrenergic effects of catecholamines, they die within three hours due to their inability to increase heat production. Under such conditions, thermal response is mediated by noradrenaline. Adrenaline also produces such an effect but as a second option.

Heat is produced due to increased turnover of ATP or by accelerated rate of mitochondrial respiration mediated by an increase in protein synthesis, which is influenced by thyroid hormones. Formation of cAMP from ATP mediated by hormones produce substantial quantity of heat. Hormones increase the supply of glucose and fatty acids that function as substrates for the production of energy in cells.

Catecholamines increase heat production from brown fat by a four-fold increase in the rate of oxygen consumption. This is due to the mobilization of fatty acids following the activation of lipase by cAMP. The effects of catecholamines on calorigenesis are facilitated by the action of thyroid hormones.

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Essay # 7. Hormonal Effects on Muscle:

Hormones affect contraction, relaxation, and metabolism of muscle. Adrenaline influences contraction and relaxation of muscles of heart, blood vessels and gastrointestinal smooth muscles. Oxytocin controls the contraction of the smooth muscles of the uterus and oviducts. Some steroid hormones profoundly affect anabolic and catabolic processes within the muscles.

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Essay # 8. Hormonal Effects on Gonads and Reproduction:

Hormones control reproductive processes, such as gonadal differentiation, maturation and gametogenesis. Reproduction in vertebrates involves physiological coordination. The sexual process involving union of the sperm and ova necessitates physiological, social and morphological arrangements to ensure ripening of the gametes at the same time. The growth and differentiation of the egg involves complex parental care.

All these events are successfully completed during certain seasons of the year when conditions are favorable. The coordination of all the processes involves hormones and the degree of complexity of their actions directly reflects the intricacies of the reproductive processes in a particular species.

The endocrine regulation of reproduction involves the secretions of the pituitary gland and the gonads. The influences of the hypothalamus and the median eminence on pituitary gonadotrophin release are vital in most groups of vertebrates.

The testicular cycle is controlled by the anterior lobe of pituitary. Removal of the pituitary results in regression of the testis involving the germ cells and Leydig cells. This is due to the deficiency of the FSH (follicle stimulating hormone) and ICSH (interstitial cell stimulating hormone). ICSH stimulates the production and release of testosterone.

Testosterone is necessary for the maturation of the sperm while FSH acts on the Sertoli cells to produce androgens that in turn mediate the maturation of the sperm. Testosterone is necessary for pre and postnatal development of the germ cells and it promotes the meiotic divisions of the spermatocytes. FSH is required for the maturation of the spermatids.

In the ovarian cycle, maturation of the ovum, ovulation and passage into the oviduct or uterus involves the coordinated activity of FSH, LH, prolactin, estrogens, progesterone and androgens in the ovary. Increase and fall in the levels of circulating concentrations of these hormones through their interactions in stimulating or inhibiting each other by a negative and positive feedback to the median eminence and hypothalamus.

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Essay # 9. Hormonal Effects on Growth and Differentiation of the Young:

Hormones are stimulatory or inhibitory to cell proliferation, and thus affect growth. Hormones regulate the earliest aspects of cell division, and differentiation of the fertilized egg. Reproduction is complete only when the young attain independence from the parents and are able to propagate the race. Growth, differentiation and maturation of the young, either in the egg or in the uterus of the mother are due to the coordination of chemical substances and hormones.

In human beings congenital thyroid deficiency leads to the disease called cretinism. Thyroid deficiency results in inadequate development of the nervous system, skeleton and reproductive organs. Thyroid hormones are more important in early postnatal life than fetal development. During fetal development, absence of pituitary stimulation due to lack of hypothalamic control leads to retardation of the development of the adrenal cortex.

The parathyroid glands also become functional in the fetal rat and removal of these glands results in hypocalcemia. The stage at which these hormones become functional and are necessary for fetal development varies very much. Experiments have been carried out to test the ability of sex hormones to modify sexual differentiation in different developing vertebrates.

These experiments involved parabiotically joining and crossing the circulation of embryos of the opposite sex, cross grafting of fetal gonads, and the administration of gonadal steroid hormones. Considerable changes including reversal of predetermined sex of the embryos were produced. Such effects depend on the age of the embryo, dose of the steroid hormones administered, the genital sex of the embryo, and the species.

The primordial germ cells have the potential to develop into either a testis or ovary. The primordial gonad is divided into outer cortex, which gives rise to the ovary and inner medulla that develops into the testes. The final development of this gonad either into testes or ovary is determined genetically. The Mullerian duct develops into uterus or oviduct, and the embryonic Wolffian duct in male becomes converted into the vas deferens (Fig. 4).

The appropriate development of the genetic male is dependent on the presence of androgens and that of the genetic female on estrogens. This pattern can be changed by removal of or antagonism of the natural hormones or by injecting hormones more typical of the opposite sex. The ultimate development, differentiation and persistence of the gonads depend on the secretion of androgens or estrogens by the definitive gonads.

The chemical Cyproterone is a specific antagonist of androgens. Injection of this chemical to male fetal rats leads to inhibition of the differentiation of the Wolffian ducts and prostate glands indicating that their development is dependent upon androgens. The differentiation of the gonads and the regression of the embryonic Mullerian ducts are not affected by the above treatment.

Joining of the male and female amphibian embryo results in inhibition in the growth of the ovary and conversion of the ovary into testis followed by parallel changes in the development of the Wolffian and Mullerian ducts. In male during normal development, the Mullerian duct becomes rudimentary but in association with the female embryo, their growth is stimulated.

As injecting sex steroids produces similar effects, it is evident that hormones initiate the changes produced by joining the embryos of the opposite sexes. In frogs, injection of androgens to female embryos leads to masculinizing effects. Estrogens produce the opposite effects by feminizing the male embryos.

Estrogens, progesterone and testosterone produce feminizing effects on genetic males so that sex reversal is possible only in the female direction in chondrichthyean fishes. In domestic fowl, injection of estrogens, in the early stages of incubation, into genetic males results in various degrees of feminization. They may become intersexes when the left testis becomes ovotestis or even a ovary. The Mullerian duct also persists.

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Essay # 10. Hormonal Effects on Metamorphosis:

Animals often exist in two or more distinct morphological and physiological forms during their life. The change from one phase to the other is known as metamorphosis. True metamorphosis involves preparation to live in a new habitat from the existing one, for example from seawater to fresh water or from aquatic life to terrestrial life. Thyroid hormones initiate metamorphosis in most amphibians.

The transformation of a purely aquatic animal into a terrestrial form that breathes air and moves on its four limbs involves many physiological and biochemical changes. Metamorphosis is influenced by genetic and environmental factors.

These factors activate the endocrine glands, especially the thyroid gland through the hypothalamus and pituitary gland. Feeding of thyroid gland extracts to tadpoles can induce metamorphosis earlier than the normal at the normal time.

Administration of antithyroid drugs such as thiouracil prolongs the process. Natural metamorphosis in tadpoles is accompanied by histological changes and an increase in the rate of uptake of iodine. The thyroid gland in frogs is under the control of thyroid stimulating hormone (TSH) produced by the adenohypophysis. Injection of TSH to tadpoles results in premature metamorphosis. Removal of the adenohypophysis results in failure of metamorphosis and giant tadpoles are produced.

The hypothalamus and median eminence control the activity of pituitary gland and thus metamorphosis. If the tadpole pituitary is transplanted to the tail, metamorphosis does not occur. This effect on growth reflects the lack of hypothalamic inhibition of the release of prolactin. Injections of prolactin to the tadpoles oppose the action of thyroxin and prevent metamorphosis. Prolactin antagonizes the peripheral actions of thyroxin and exerts an inhibitory effect on the thyroid gland.

Metamorphosis is divided into three stages:

a. Premetamorphosis:

A period of rapid growth,

b. Prometamorphosis:

A period of reduced growth but increased differentiation, and

c. Metamorphic Climax:

Involving drastic changes such as tail resorption, and terrestrial life.

During prometamorphosis thyroid hormone secretion is low and a low rate of TSH secretion. This condition is stabilized by the presence of large amounts of prolactin, reflecting the immaturity of the inhibitory action of the hypothalamus (prolactin release inhibiting hormone secretion). With the progressive maturation of the hypothalamus-pituitary axis in the beginning stages of prometamorphosis TRH gradually increases and releases TSH.

The thyroxin concentration thus increases during prometamorphosis. Under the influence of thyroxin through a positive feedback mechanism, the hypothalamus matures, and there is a massive stimulation of the thyroid through TRH and TSH establishing the third and last stage of metamorphosis the metamorphic-climax. In this stage the level of thyroxin decreases due to negative feedback of TRH (Fig. 5).

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Essay # 11. Hormonal Effects on Osmoregulation:

Hormones regulate the excretion and reabsorption of inorganic cations and anions. Sodium, potassium, calcium and phosphate ions are particularly affected. The physicochemical properties of the body fluids in animals differ from those of the external environment in which they live. Animals are continuously exposed to the ambient medium and this changes the composition of their body fluids.

Although the intracellular and extracellular fluids have similar osmotic concentrations, they differ in the type of solutes and equilibrium due to diffusion is established. The equilibrium is maintained by complex physiological phenomena involving cells, special tissues and organs concerned with osmoregulation.

The integration of the functions of these homeostatic tissues is dependent upon hormones. Hormones influence the functioning of kidneys, gills and gut, which are concerned with the regulation of the osmotic balance of the animals.

Hormones, which influence osmoregulation, are the neurohypophyseal hormones oxytocin and vasopressin (ADH), adrenocorticosteroids (mineralocorticoids), catecholamines and prolactin. Corticotrophin and angiotensin are indirectly involved by controlling the release of adrenocorticosteroids.

Active transport and secretion of ions like sodium, potassium, bicarbonate and chloride across or from the epithelial membranes of the osmoregulatory tissues is the basic phenomenon, which controls the physiological functioning of these tissues.

Hormones influence the activity of osmoregulatory tissues by the following processes:

(i) Hormones can alter the active transport of sodium and chloride and secretion of hydrogen ions, bicarbonate and potassium. Cortisol and aldosterone alter sodium and potassium movements across cell membrane. Vasotocin and ADH promote sodium transport. Catecholamines increase the transport and secretion of chloride ions and inhibit the effects of ADH.

(ii) Hormones influence the osmotic and diffusional movements of water and sodium ions across the epithelial membranes. Vasotocin and ADH increase the permeability of the renal tubules, skin of amphibians and urinary bladder to water. Prolactin reduces the permeability of the gills of some teleost fishes to sodium.

(iii) Catecholamines, vasotocin and vasopressin can change the diameter of blood vessels and influence the functioning of different osmoregulatory tissues. Flow of urine is influenced by changes in the rate of filtration of plasma in the glomerulus and the above-mentioned hormones can alter this process.

In mammals the effects of hormones on osmoregulation can be summarized as follows:

a. Vasopressin:

This hormone reduces urinary losses of water (antidiuresis) due to increased osmotic reabsorption of water from the kidney tubules.

b. Adrenocorticosteroids:

Aldosterone controls sodium and potassium levels. Excretion of sodium from the kidney, sweat glands and salivary glands is reduced while potassium loss is increased. Aldosterone promotes sodium reabsorption from the large intestine and the ducts of mammary glands. Corticosterone also exerts similar effects. Thus adrenocorticosteroids help in conservation of sodium and excretion of potassium ions.

c. Adrenaline:

This hormone plays a prominent role in osmoregulation. It promotes the secretion of sweat glands and antagonizes the release and actions of ADH on the kidney.

d. Angiotensin:

Angiotensin initiates the release of aldosterone and promotes the reabsorption of sodium from the kidney tubules and colon (Fig. 6).

Calcium and Phosphate Balance:

Calcium plays a vital part in many aspects of cellular function. Intracellular calcium concentration is itself subject to regulation. Inspite of the tendency of calcium to enter the cell down the electrochemical gradient, intracellular concentration is normally low and stable.

This is due to the binding of calcium to specific binding protein or to mitochondria and due to a steady efflux in exchange to other cations, such as sodium. Alterations in intracellular calcium levels always are accompanied by changes in cellular activities.

The important functions carried out by calcium in the body are:

1. Structural:

Bones, teeth and connective tissue, and intercellular cementing substance aiding cell adhesion contain calcium.

2. Excitable Cells:

Stability of the nerve membranes, release of chemical transmitters at nerve endings, excitation-contraction coupling in skeletal muscle, action potentials in certain types of smooth muscle, action potential generation, and activation of the contraction in cardiac muscle are some of the important functions of calcium.

3. Endocrine Glands:

Calcium and cAMP are involved in intracellular regulation of endocrine activity. Calcium is involved in release of exocytosis of stored hormones (stimulus-secretion coupling in adrenal medulla, release of
insulin, anterior pituitary hormones and neurosecretion in the neural lobe of pituitary gland.

4. Exocrine Glands:

Calcium is necessary for the secretion of water and enzymes by the salivary glands and exocrine pancreas and for acid and enzyme secretions by the stomach.

5. Enzymes:

The activity of many enzymes depends on the concentration of calcium. Blood clotting requires calcium, as the enzyme converting prothrombin to thrombin is calcium dependent.

Bones and teeth contain 99% of the calcium in the body, and the rest is found in within the cells and in extracellular fluid. Since calcium is vital in controlling the activities of the body, it is necessary that the extracellular free calcium concentration is regulated within narrow limits. The free calcium concentration of the interstitial fluid is closely similar to that of plasma, thus regulation of the free calcium concentration in plasma ensures stability in the extracellular fluid.

Most of the calcium is bound to protein or forms complexes with anions such as phosphate, citrate and bicarbonate. The complex with phosphate is the most important as it is involved in the calcification of bone. Increased phosphate levels facilitating bone deposition reduce free calcium level and low phosphate levels are hypercalcemic.

The pathways by which the secretion of parathormone (PTH) during hypocalcemia and that of Calcitonin (CT) during hypercalcemia serve to maintain the ionized calcium concentration within the normal range are shown in Figure 7.

(i) Hypocalcemia stimulates PTH secretion and suppresses CT secretion,

(ii) PTH acts on labile bone causing the release of calcium into the extracellular fluid,

(iiia) PTH acts on kidney tubules to decrease phosphate reabsorption lowering plasma phosphate and promoting bone reabsorption,

(iiib) to increase calcium reabsorption.

(iv) PTH facilitates intestinal absorption of calcium, and

(v) Accelerates the conversion of 25-hydroxy chole calciferol into 1, 25 dihydroxy chole calciferol in the kidney,

(vi) Hypercalcemia promotes the secretion of CT.

(vii) CT favors the incorporation of calcium into the bone and lowers blood calcium.

Phosphate Metabolism:

Phosphate occurs in three major forms in blood, inorganic phosphate, ester phosphate and lipid phosphate. Plasma phosphate refers to inorganic phosphate. Parathormone (PT) is the predominant hormone influencing plasma phosphate concentration. Changes in phosphate concentration are secondary consequences of the primary role of PTH in the control of plasma calcium level.

Parathormone mobilizes both calcium and phosphate by stimulating bone resorption. Any increase in phosphate level is counterbalanced by the phophaturic action of parathormone. Therefore, increase in both calcium and phosphate concentration and consequent formation of calcium phosphate is avoided.

Permissive Actions of Hormones:

Hormones exert a permissive action on the effects of other hormones. The effectiveness of certain hormones is enhanced by the action of another hormone. The permissive hormone may not produce any effect by itself but it may be an absolute requirement for the actions of other hormones.

Because the majority of peptide hormones stimulate cells by the activation of cAMP, it is expected that one peptide hormone will enhance the activity of another peptide hormone in an additive manner and for one hormone to potentiate the activity of another, the mechanisms of action of the two hormones has to be different.

The permissive effects of hormones are restricted mainly to the actions of steroid and thyroid hormones, where the ligands enhance the action of each other or they enhance the action of other hormones working through membrane receptors.

The following mechanisms account for the permissive actions of steroid or thyroid hormones (Fig. 8):

(i) Steroid and thyroid hormones, through their actions on specific mRNA synthesis cause an increase in the number of membrane receptors, which increase the production of cyclic nucleotides, thus leading to an increased cellular response to hormones acting on the plasmalemma.

(ii) These hormones increase or decrease the quantity of cyclic nucleotide dependent protein kinases or the amount of substrate available for phosphorylation by cAMP or cGMP dependent protein kinases.

(iii) Thyroid and steroid hormones increase the synthesis of a protein that could act as an inhibitor of another protein whose action is antagonistic to cyclic nucleotide action.

‘Synergism’ is the physiological response of a tissue to a combination of two hormones that greatly exceeds the individual actions of either hormone. For example, FSH has no detectable effect on enzyme activity of testicular interstitial tissue and LH only minimally stimulates such activity. However, in the presence of FSH, the actions of LH are greatly increased. This is due to an FSH induced increase in LH receptors known to occur in other steroid synthesizing tissues in response to these two gonadotropins.

Methylxanthines when used at low concentrations may slightly increase the basal activity of cells. Hormones when used in low concentration exert only a minimal effect. A hormone and methylxanthine, in combination produce a dramatic response. The action of the hormone is to increase cyclic nucleotide levels, whereas the action of methyxanthine is to inhibit degradation of cAMP by the enzyme phosphodiesterase.

The action of one hormone may require the action of one or more other hormones that individually are relatively inactive. This phenomenon is known as the permissive action of hormones. Synergism occurs when a tissue’s response to two or more hormones in combination is greater than the sum of the individual actions of the hormones. It is difficult to separate the permissive and synergistic actions of a hormone into distinct physiological entities.

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Essay # 12. Hormonal Effects on Animal Behaviour:

Hormones play important part in animal behaviour. During the reproductive cycles, hormones control sexual and aggressive behaviours. Gonadal and pituitary hormones control maternal behaviour. There are numerous examples of behaviour whose development or expression is under hormonal control. The most famliar examples are seen in the sexual behaviour of animals.

In temperate zone birds, the progressive increase in day length with approaching springtime induces growth of the gonads, testicular weight in some species increasing by 500 times. This is accompanied by a large rise in the levels of sex hormones, which in turn promotes the development of various aspects of sexual behaviour, such as selection of territory and singing in the male and nest building in the female.

Although hormones play a direct role in such behaviour, they also play indirect roles by promoting the differentiation of sex related morphological features, such as plumage, color and voice. The behaviour of one sex also releases behavioural responses in the other sex. An interesting example is the stimulation of nest building behaviour in female canaries by the singing of courting males.

The poorer is the singing, the less is the nest building by female. Although increased progesterone levels in the female promote the nest-building behaviour itself, it is nevertheless dependent on the appropriate sensory input. Similarly, the singing of the male, though promoted by testosterone is dependent for development of quantity and quality on exposure to the of songs the more experienced, older males.

Some behavioural differences between male and female mammals are due to differences in levels of sex hormones. Increased levels of androgens circulating in the blood lead to greater sexual receptivity in females and to greater sexual aggressiveness in males thus increased sexual arousability in both the sexes.

The more general behavioural consequences of castration in male domestic animals and humans are well known. An aggressive bull is converted into a placid ox. Aggressive behaviors can be restored in castrated mice by administration of either estradiol or testosterone. Simultaneous administration of progesterone interferes with the induction of aggressive behaviour.

Behavioural change by hormones shows that hormones can modify the properties of certain nerve cells. Recordings from single units in the lateral areas of the hypothalamus of rats at different times during the estrus cycle show that the percentage of neurons synaptically inhibited by such stimuli as pain, cold, or mechanical stimulation of the cervix is higher during estrus than at other times.

Administration of estrogen to rats produces similar augmentation of synaptic inhibition in that part of the hypothalamus. The neurons in another part of the hypothalamus, the septum, respond differently to estrogen. These cells show a decrease in reflex inhibition in response to cervical stimulation, pain and cold during estrus or when estrogen is administered.

Thus an increase in the plasma level of estrogen, whether due to ovarian secretion during estrus or to administration of the hormone, increases the inhibitory effects of senzory input in some neuros while decreasing it in others. Progesterone and prolactin also produce specific changes in neural excitability in various parts of the hypothalamus.