Fluid deprivation increases isotonic NaCl intake, but not hypertonic salt intake, under normal and heated conditions in obese Zucker rats

In the course of exposure to fluid deprivation and heated environment, mammals regulate their hydromineral balance and body temperature by a number of mechanisms including sweating, water and salt intakes. Here we challenged obese Zucker rats, known to have a predisposition to hypertension, with 0.9%NaCl alone or with 2%NaCl solution + water to drink under fluid deprivation and heated conditions. Food and fluid intakes, body weight, diuresis and natriuresis were measured daily throughout. Serum aldosterone levels and Na+ concentration were also analyzed. Data showed that obese and lean rats presented similar baseline measurements of food, 0.9%NaCl and fluid intakes, diuresis and fluid balance; whereas hypertonic 2%NaCl consumption was almost absent. Before and during fluid deprivation animals increased isotonic but not hypertonic NaCl intake; the obese showed significant increases in diuresis and Na+ excretion, whereas, total fluid intake was similar between groups. Heat increased isotonic NaCl intake and doubled natriuresis in obese which were wet on their fur and displayed a paradoxical increase of fluid gain. Fluid deprivation plus heat produced similar negative fluid balance in all groups. Body weight losses, food intake and diuresis reductions were amplified under the combined conditions. Animals exposed to 2%NaCl showed higher circulating levels of aldosterone and obese were lower than leans. In animals which drank 0.9%NaCl, obese showed higher serum levels of Na+ than leans. We conclude that in spite of their higher sensitivity to high salt and heat obese Zucker rats can control hydromineral balance in response to fluid deprivation and heat by adjusting isotonic NaCl preference with sodium balance and circulating levels of aldosterone. This suggests a key hormonal role in the mechanisms underlying thermoregulation, body fluid homeostasis and sodium intake.