Thermal dehydration: Central renin-angiotensin system and plasma fluid shifts and osmolarity

We measured relative changes in plasma volume (PV) and osmolarity during heat stress at 40 °C in untreated and in intracerebroventricular (icv) losartan (LOS)-adrninistered rats. The rats were in different hydration states and various phases of heal acclimation (at 34 °C). differing in response to LOS. Icv LOS accelerated the rate of water loss, which led to a marked increase in plasma osmolarity, and augmented plasma fluid efflux during heat stress in all experimental groups. The data obtained fit with the hypothesis that icv LOS mediates plasma composition and volume via a linkage to the adreno-sympathetic vasomotor thermoregulatory reflex and modulation of vasopressin release. The results suggest that during thermal dehydration central autonomic control may be involved in plasma fluid shifts. The temporally varying responsiveness ot this pathway with acclimation may suggest that this control mechanism is subjected to adaptive modulations. The relative importance of this pathway, however, remains to be elucidated.