Behavioral and physiological aspects of body fluid homeostasis in Fischer 344 rats

Abstract

Previous studies have shown that Fischer 344 rats, unlike many other strains, have neither a spontaneous preference for dilute NaCl solutions nor an excessive consumption of it after sodium depletion. The present studies examine some characteristics of water intake in Fischer 344 rats. Their spontaneous water intake was only about 50% of that of age-matched Sprague-Dawley rats, and the water-to-food ratio was about 30% lower. When water was added to the food, Fischer 344 rats decreased their fluid intake by a corresponding amount, whereas Sprague-Dawley rats continued to drink substantial amounts. In the absence of food, Fischer 344 rats reduced their water intake by a greater fraction than rats of the Sprague-Dawley strain. Physiological changes during these studies were as expected from the behavioral data, except that plasma protein concentration was consistently 10% higher in Fischer 344 rats. In contrast to this economy in their spontaneous drinking, water intakes of Fischer 344 rats were comparable to Sprague-Dawley rats in response to water deprivation, and administration of either hypertonic NaCl or angiotensin II, and in a sham-drinking paradigm. However, following treatment with either isoproterenol or polyethylene glycol, Fischer 344 rats drank considerably less than Sprague-Dawley rats. Possible reasons for, and implications of, these strain differences in drinking are discussed.