Effects of hyperosmolality and diuretics on heat-induced limb vasodilation in baboons.

Dehydration attenuates the increase in limb skin blood flow elicited by environmental heating (EH). This study sought to determine which of the two primary effects of dehydration, increased body fluid osmolality or decreased body fluid volume, was primarily responsible for this cutaneous vasoconstrictor bias in baboons. Unanesthetized chronically instrumented baboons were exposed to EH while in euhydrated state, after 65-69 h of water deprivation (dehydration), after infusion of a small volume of hypertonic (20%) saline to raise plasma osmolality and sodium concentration to dehydration levels, and after injections of the diuretic furosemide over a 64-h period to produce an isosmotic fall in extracellular fluid volume. EH consisted of an acute elevation of ambient temperature to 39.5-42.0 degrees C until internal temperature reached 39.5-39.8 degrees C. The normal increases in external iliac artery blood flow and iliac vascular conductance during EH were unchanged by hyperosmolality but were attenuated by 39 and 31%, respectively, after furosemide treatment and by 42 and 46%, respectively, during dehydration. Thus the fall in extracellular fluid volume is the component of dehydration that attenuates the increase in hindlimb blood flow during EH in the same way as dehydration itself.