DOMESTIC ANIMAL ENDOCRINOLOGY
Vol. 9(1):1-11,1992
OSMOTIC AND HORMONAL REGULATION OF THIRST IN DOMESTIC ANIMALS M.J. McKinley, B.J. Oldfield, and L. Vivas Howard Florey Institute of Experimental Physiology and Medicine University of Melbourne, Parkville, Vic. 3052, Australia Received July 29, 1991
INTRODUCTION The volume and osmolality of both the intracellular and extraceilular fluids of mammals are normally maintained within a quite narrow range. For instance, if an animal becomes dehydrated, several compensatory physiological mechanisms come into operation. Vasopressin, the antidiuretie hormone, is secreted from the posterior pituitary gland and renal water losses diminish, because a more concentrated urine is formed (1, 2). In addition, the response to dehydration includes enhanced renal excretion of solutes, particularly sodium chloride (3). While these renal mechanisms can partially correct the hypertonicity and minimise further losses of body water, only intake of water can fully restore the volume and composition of body fluids. Thus, an urge or desire to drink water, termed thirst, is an important component of a series of co-ordinated responses which have the resultant homeostatic effect of maintaining body fluid volume and osmolality. Thirst is a subjective human feeling or sensation, and there can be no absolute certainty that animals experience thirst analogous to that of people. It can only be assumed that the strong urge to imbibe water, which is obvious in many species when they become depleted of water, represents the presence of a type of thirst in these animals. For the purpose of this article, the assumption will be made that when animals drink water in response to a deficit of either the intracellular or extracelular fluid or both, they experience a drive to drink water, termed thirst. Studies of water drinking in domestic animals such as dogs, goats, sheep, cattle and pigs have provided much of our knowledge of mechanisms of thirst. This article reviews the osmotic and hormonal factors which influence the thirst associated with deficits in the intracellular or extracellular fluid volume of these animals. OSMOREGULATORY
THIRST
At the start of this century, Mayer (4) concluded that thirst was related to the osmotic pressure of the body fluids. However, it was not until the 1930s, with G i l m a n ' s investigations o f the water drinking of dogs (5), that it was recognised that the effective osmotic pressure (tonicity) of the plasma, (rather than the absolute osmotic pressure of the solution relative to an ideal semi-permeable membrane) was a determinant of thirst. These, and subsequent studies in dogs, goats, rats, sheep, cattle, and domestic fowls have utilised different solutes to make solutions which were hyperosmolar to plasma, and tested the dipsogenic effects of systemic infusion of these solutions (6,7,8,9,10,11). The results show that when concentrated solutions of NaCI, sucrose, fructose, sorbitol or mannitol are infused systemically, animals drink considerable quantities of water (Figure 1). These solutes do not penetrate cell membranes quickly: when they are infused into animals, an osmotic gradient is established across cell membranes; water then leaves cells by osmosis, resulting in cellular dehydration. By contrast, when concentrated solutions of urea or glycerol are infused systemically to cause an equivalent increase in plasma osmolality, much less water drinking results. These two molecules move across cell membranes relatively quickly, so that an osmotic gradient is not maintained, little water leaves cells, and cellular dehydration is minimal. Thus, the osmotic stimuli that are the most effective dipsogens are also those that cause cellular dehydration, and the idea has developed that thirst results from the the cellular dehydration of specific sensor cells in the body, termed osmoreceptors (12). Copyright © 1992 Butterworth-Heinemann
1
0739-7240/92/$3.00
2
MCKINLEY
• SIGNIFICANTLY DIFFERENT %= PERCENTAGE OF TRIALS
ET AL.
FROM INFUSION OF 0.15 M NaCI P
Vol. 9(1):1-11,1992
OSMOTIC AND HORMONAL REGULATION OF THIRST IN DOMESTIC ANIMALS M.J. McKinley, B.J. Oldfield, and L. Vivas Howard Florey Institute of Experimental Physiology and Medicine University of Melbourne, Parkville, Vic. 3052, Australia Received July 29, 1991
INTRODUCTION The volume and osmolality of both the intracellular and extraceilular fluids of mammals are normally maintained within a quite narrow range. For instance, if an animal becomes dehydrated, several compensatory physiological mechanisms come into operation. Vasopressin, the antidiuretie hormone, is secreted from the posterior pituitary gland and renal water losses diminish, because a more concentrated urine is formed (1, 2). In addition, the response to dehydration includes enhanced renal excretion of solutes, particularly sodium chloride (3). While these renal mechanisms can partially correct the hypertonicity and minimise further losses of body water, only intake of water can fully restore the volume and composition of body fluids. Thus, an urge or desire to drink water, termed thirst, is an important component of a series of co-ordinated responses which have the resultant homeostatic effect of maintaining body fluid volume and osmolality. Thirst is a subjective human feeling or sensation, and there can be no absolute certainty that animals experience thirst analogous to that of people. It can only be assumed that the strong urge to imbibe water, which is obvious in many species when they become depleted of water, represents the presence of a type of thirst in these animals. For the purpose of this article, the assumption will be made that when animals drink water in response to a deficit of either the intracellular or extracelular fluid or both, they experience a drive to drink water, termed thirst. Studies of water drinking in domestic animals such as dogs, goats, sheep, cattle and pigs have provided much of our knowledge of mechanisms of thirst. This article reviews the osmotic and hormonal factors which influence the thirst associated with deficits in the intracellular or extracellular fluid volume of these animals. OSMOREGULATORY
THIRST
At the start of this century, Mayer (4) concluded that thirst was related to the osmotic pressure of the body fluids. However, it was not until the 1930s, with G i l m a n ' s investigations o f the water drinking of dogs (5), that it was recognised that the effective osmotic pressure (tonicity) of the plasma, (rather than the absolute osmotic pressure of the solution relative to an ideal semi-permeable membrane) was a determinant of thirst. These, and subsequent studies in dogs, goats, rats, sheep, cattle, and domestic fowls have utilised different solutes to make solutions which were hyperosmolar to plasma, and tested the dipsogenic effects of systemic infusion of these solutions (6,7,8,9,10,11). The results show that when concentrated solutions of NaCI, sucrose, fructose, sorbitol or mannitol are infused systemically, animals drink considerable quantities of water (Figure 1). These solutes do not penetrate cell membranes quickly: when they are infused into animals, an osmotic gradient is established across cell membranes; water then leaves cells by osmosis, resulting in cellular dehydration. By contrast, when concentrated solutions of urea or glycerol are infused systemically to cause an equivalent increase in plasma osmolality, much less water drinking results. These two molecules move across cell membranes relatively quickly, so that an osmotic gradient is not maintained, little water leaves cells, and cellular dehydration is minimal. Thus, the osmotic stimuli that are the most effective dipsogens are also those that cause cellular dehydration, and the idea has developed that thirst results from the the cellular dehydration of specific sensor cells in the body, termed osmoreceptors (12). Copyright © 1992 Butterworth-Heinemann
1
0739-7240/92/$3.00
2
MCKINLEY
• SIGNIFICANTLY DIFFERENT %= PERCENTAGE OF TRIALS
ET AL.
FROM INFUSION OF 0.15 M NaCI P
