Influence of vasoconstrictor systems on leg vasodilation during heating of dehydrated baboons.

This study, carried out in two parts, sought to determine the importance of vasopressin (VP), the renin-angiotensin system (RAS), and the sympathetic nervous system in the dehydration-produced attenuation of hindlimb (cutaneous) vasodilation during environmental heating (EH). Baboons, chronically instrumented for blood sampling and for measurement of mean iliac blood flow (MIBF), arterial pressure, and core temperature (Tc), were subjected to EH while in euhydrated and dehydrated (64-72 h of water deprivation) states. EH consisted of exposure to an elevated ambient temperature (40-42 degrees C) until Tc reached 39.5 degrees C. In part I, indexes of the above vasoconstrictor systems were measured. Base-line plasma renin activity (PRA) and VP and norepinephrine concentrations were all significantly elevated by dehydration. In addition, the increase in PRA during EH was accentuated by dehydration. In part II, the effects of blockades of the RAS, the pressor action of VP, and the innervation of the hindlimb on hindlimb vasodilation during EH were assessed. None of these blockades, singly or together, reversed the dehydration-produced attenuation of the increase in MIBF during EH. Thus we conclude that other mechanisms are responsible for the dehydration-produced attenuation of cutaneous vasodilation in baboons during EH.