Journal of Applied Physiology
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J Appl Physiol 71: 521-529, 1991;
8750-7587/91 $5.00
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Journal of Applied Physiology, Vol 71, Issue 2 521-529, Copyright © 1991 by American Physiological Society

ARTICLES

Effects of invariant sympathetic activity on cutaneous circulatory responses to heat stress

D. Richardson, Q. F. Hu and S. Shepherd
Department of Physiology and Biophysics, Sanders-Brown Center on Aging, University of Kentucky, College of Medicine, Lexington 40536-02301.

This study investigated the role of sympathetic withdrawal on blood flow responses in cutaneous arteriovenous anastomoses (AVAs) and capillaries to direct and indirect heat stress. This was achieved by clamping sympathetic activity (SC) to the tail of anesthetized rats so that constrictor tone remained invariant during exposure of either the animal's tail (direct heating) or body (indirect heating) to a 35 degrees C environment. Flow through the AVAs in the tail was evaluated by laser-Doppler flowmetry (LDF), while capillary flow was investigated by videodensitometry measurements of blood cell velocity (CBV) in single capillaries within the subepidermal vascular plexus. Both direct and indirect heating significantly increased LDF and CBV. In comparison to blood flow responses in sham-operated control rats, the SC procedure resulted in significantly lower LDF responses to both direct and indirect heat stress. By contrast, the response of CBV was not significantly affected by SC during either mode of heating. These results indicate that the withdrawal of sympathetic constrictor tone plays a role in the response of cutaneous AVAs, but not precapillary arterioles, to direct as well as indirect heat stress. Additional studies on unanesthetized animals showed that superimposing body heating on a base of local heating elicited a further increase in LDF, suggesting that local heating does not deplete neural mediated dilatory reserve.


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