Original Research
Effect of Food Availability on the Physiological Responses to Water Deprivation in Ponies

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Abstract

Six ponies were deprived of drinking water and food and compared over 24 hours with nondeprived ponies, ponies deprived of water but with food available, and ponies deprived of food but with water available. When food was eaten during water deprivation, plasma osmolality rose 4% from 284 mOsm/kg to 295 mOsm/kg. During water and food deprivation, plasma osmolality failed to rise, even over 24 hours, and usually fell. Packed cell volume was higher when food but not water was available. Food and/or water deprivation had no significant effect on plasma protein concentration. When food was available, the ponies drank three times more water (13.1 ± 2.1 kg) than when water but not food was available (3.5 ± 1.4 kg). Blood volume changes were calculated from packed cell volume and plasma protein data, and it was found that blood volume did not change significantly with deprivation. Urine volume did not vary with deprivation, but free water clearance changed significantly, falling when food but not water was available. Under these conditions, blood volume is maintained, but the mechanisms are not clear. When deprived of both drinking water and food, ponies failed to develop the hyperosmolality expected under these conditions. Water deprivation while food is available is a more powerful challenge to water and electrolyte homeostasis than deprivation of both food and water.

Introduction

The mechanisms of initiation and termination of water drinking, that is, of thirst, have most often been studied in animals deprived of drinking water. It is sometimes assumed that the effects of water deprivation are always the same: an inevitable decrease of total body water and a consequent rise in osmolality of extracellular fluid as well as a decrease in blood volume [1]. Little notice has been made of the feeding conditions imposed during such water deprivation. An earlier study indicated that neither plasma osmolality nor blood volume of pigs falls when they are deprived of food, but only if water is also withheld [2]. Interest in this aspect of thirst arose from studies of the causes of prandial drinking. Typically, many animals, in particular humans, ponies, and rats, consume approximately three-fourths of their daily water in close temporal association with eating [3], [4], [5]. The welfare of horses either during transport or when food or water might be withheld is also a concern, and we wish to determine the least physiologically stressful means to restrict water. The objective of these experiments was to describe the pattern of changes in fluid balances with water deprivation in the presence or absence of food.

There are real-life situations in which these conditions occur. The animals can have food but no water. This occurs under drought conditions, when grass is available but water holes have dried up [6], and in the winter, when horses eat a lot to keep warm but their water source is frozen. The transported horse usually has hay available but is often heat stressed as well as water deprived. The confined neglected horse may have neither food nor water. The condition of water without food happens when a horse is fasted perioperatively. This can also happen in a desert environment when all the grass close to a water hole has been consumed.

Our hypothesis was that the presence of food affects the dehydration status of the animal but that the response of the horse may differ from that of other species, including other equine species such as the donkey or zebra. To test this hypothesis, we measured plasma protein, packed cell volume (PCV), blood and urine osmolality, and urinary as well as plasma sodium and potassium in ponies under four conditions: water but no food; food but no water; neither food nor water; food and water.

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Methods

This experiment was approved by Cornell University’s Institutional Animal Care and Use Committee.

Results

The ponies drank a mean of 13.1 ± 2.1 kg of water in the 24 hours when both food and water were available and 3.5 ± 1.4 kg when water but not food was available (t = 10.08, P < .001).

Discussion

The most dramatic effect of the different deprivation conditions was the marked rise of plasma osmolality when food was consumed during the water deprivation period (Fig. 1, Fig. 2). This indicates that the primary stimulus to drinking during water deprivation with food available was the rise in osmolality. The changes in these experiments approximate the rise of plasma osmolality needed to stimulate drinking, as has been reported for the pony [8].

Pigs and horses are species in which a volatile

Acknowledgments

The technical assistance of Joanne l’Anglais, Jacqueline Hunt, and Tiffany Cal during these experiments was greatly appreciated. The authors are grateful to H.F. Hintz for use of his facilities. Financial support was derived in part from National Institutes of Health grant R01 DK41383.

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