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

Abstract

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.

Cardiovascular Adjustments to Heat Stress

“…In baboons, the magnitude of the diuretic-induced attenuation of the increase in leg blood flow during heating was virtually identical to that produced by 64-68 h of dehydration ( Fig. 11.10) (199). Since reductions in plasma volume produced by diuretics and by dehydration were very similar in that study (199), reduced body fluid volume may be the sole mediator of the attenuated increase in SkBF in the heated baboon.…”

“…11.10) (199). Since reductions in plasma volume produced by diuretics and by dehydration were very similar in that study (199), reduced body fluid volume may be the sole mediator of the attenuated increase in SkBF in the heated baboon. In support, the increase in leg blood flow during heating of dehydrated baboons is restored to normal after either oral ingestion (222) or intravenous infusion (223) of a sodium chloride solution that maintained the dehydration-produced hyperosmolality but reversed the hypovolemia.…”

“…When heat stress is superimposed upon a state of prolonged dehydration, the increase in SkBF in humans and baboons is attenuated (61,199,222,223,(249)(250)(251). After fluid restriction for 24 h and a bout of mild cxercise, the increase in human forearm SkBF during subsequent exercise in a warm environment was markedly reduced because of a 32% reduction in peak blood flow, a 39% reduction in the slope of the blood flow-internal temperature relationship, and a 02°C increase in threshold internal temperature for vasodilation (61) (Fig.…”

Naltrexone Modifies Thermoregulatory Symptoms and Lessens the Severity of Heat Stroke in Guinea Pigsa

“…The phenomenon of hyperthermia‐induced vasoconstriction (cancelling peripheral vasodilation despite the continuing IPH and high T h ) is likely to reflect the animal's inability to maintain cardiac output and blood pressure without shutting off blood flow in peripheral tissues. Occurring at the onset of heat exposure, water‐consuming heat‐defense responses (in our animals, polypnea) and thermoregulatory redistribution of blood toward the skin lead to dehydration and/or hypovolemia, both shown to increase the threshold T b for cutaneous vasodilation 8,9 . Our results demonstrate that hyperthermia‐induced vasoconstriction is opioid‐dependent because NTX, an opioid antagonist, prevents its occurrence.…”

Lesions of the anteroventral third ventricle region (AV3V) disrupt cardiovascular responses to an elevation in core temperature

“…The redistribution of blood flow from the visceral to the cutaneous circulations is strongly influenced by nonthermal inputs such as hydration status (14,29,31,36). Anatomic and in vitro evidence suggest that thermal and nonthermal information may be integrated within the AV3V (15,34,50).…”