Heat acclimated hypohydrated rats: Age dependent vasomotor and plasma volume responses to heat stress

1. 1. Heat tolerance, tail blood flow (T_t), plasma volume (PV) and heart rates (HR) of heat stressed (at 40°C) young (YR) and 15 months old (OR) rats were measured before and after acclimation to heat (1 month at 34°C) with and without additional hypohydration stress. 2. 2. During heat stress, in non-acclimated rats, tail vasodilation in OR lagged behind YR, although T_t vs rectal temperature (T_re) were similar. Following hypohydration, vasomotor response of OR was retarded compared to that of hypohydrated YR and heat tolerance was significantly shorter. 3. 3. Heat acclimation improved heat tolerance of hypohydrated OR only (182 ± 31 min, compared with 166 ± 31 and 109 ± 15 for the hypohydrated non-acclimated YR and OR, respectively). Under that condition vasomotor differences between YR and OR disappeared. 4. 4. Heat induced plasma expansion of OR at the initiation of heating was smaller in OR both before and after acclimation. However, following hypohydration PV expansion was observed in OR only. 5. 5. It is suggested that the improved endurance of acclimated OR under condition of heat stress and hypohydration is associated with greater PV reserves and improved vasomotor response.