Thermal dehydration-induced thirst in spontaneously hypertensive rats

Spontaneously hypertensive (SH) rats and normotensive Wistar-Kyoto (WKY) rats were exposed to either 25 or 37.5 degrees C for 3.5 h, and their thermal and water balance responses were compared. After exposure, either a blood sample was obtained or the rats were allowed to rehydrate for 4 h. SH rats had both higher core temperatures and evaporative water losses during heat exposure. Measurements of hematocrit, hemoglobin concentration, plasma protein and sodium concentrations, and plasma osmolality indirectly showed that the SH rats were dehydrated relative to the WKY rats after exposure to either 25 or 37.5 degrees C. SH rats drank significantly more water but also had significantly higher urine volumes than the WKY rats and thus rehydrated only slightly better than the WKY rats. SH and WKY rats had similar levels of water intake and urine output after 24 h of water deprivation. The elevated thermal response of SH rats to heat exposure does not appear to lead to uncompensatable changes in body water status.