Effect of water or saline intake on heat-induced limb vasodilation in dehydrated baboons

Dehydration markedly attenuates the increase in hindlimb blood flow elicited by environmental heating (EH) in baboons. This study sought to determine the importance of gradually produced increases in body fluid osmolality and decreases in body fluid volume in producing this attenuation. The hindlimb blood flow increases during EH of seven unanesthetized chronically instrumented baboons were examined during euhydration, dehydration (64-68 h of water deprivation), and after ad libitum oral rehydration with either water or a hyperosmotic saline solution. EH consisted of acute exposure to ambient temperatures of 38-42 degrees C until internal temperature reached 39.5 degrees C. Dehydration depressed the maximal external iliac artery blood flow (MIBF) and iliac vascular conductance (IVC) attained during EH in the euhydrated state by 37 and 43%, respectively. Rehydration with either water or saline solution, however, restored maximal MIBF and IVC to euhydrated levels. Because plasma osmolality remained at dehydrated levels after rehydration with saline, hyperosmolality does not produce the dehydration-induced attenuation in hindlimb blood flow.