Interactive effect of osmoregulatory and volume regulatory stimuli on thermoregulation.

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To gain better insights into the interaction between fluid regulatory and thermoregulatory systems, we examined the effect of body fluid regulatory stimuli on thermoregulatory responses in human subjects.Upright posture reduced thermogenesis in response to body core cooling, compared with supine position. In contrast, upright posture enhanced peripheral vasoconstriction. Therefore, the thermoregulatory response to cold stress is influenced by volume regulatory system. The results also suggest that the mechanisms controlling peripheral vasoconstriction and thermogenesis during cold stress are appeared to be independent. We also examined the effect of plasma hyperosmolality on baroreceptor unloading-induced peripheral vasoconstriction. Plasma hyperosmolality augmented the reduction of forearm vascular conductance induced by lower body negative pressure during heat stress but not during normothermia. Therefore, there. apparently exists an interactive effect of plasma hyperosmolality and c … More entral hypovoloemia on the peripheral vascular response during heat stress, but not during normothermia. Our findings suggest that plasma hyperosmolality enhances baroreceptor-mediated reduction of thermal active peripheral vasodilation, but does not enhance the vasoconstrictor system.In a series of animal experiments, we examined the central mechanisms of the body fluid regulatory system. We quantified the permeability of capillaries in the subfornical organ of rat by a nuclear magnetic resonance method in combination with a venous injection of a relaxation reagent, Gd-DTPA^<2-> which could not pass through the blood-brain barrier (BBB). The proposed value may be 10-100 times larger than that of the blood-barrier in the cortex. We conclude that the capillaries in the SFO have one of the highest water permeability values among all of the capillaries in the brain, and this site may play an important role in osmoreception. In another animal experiment, we suggest that Mn^<2+> MRI is a useful technique for identifying in vivo the active brain region in the hypothalamus in response to various stimuli. Less