Sympathetic baroreflex control of vascular resistance in comfortably warm man. Analyses of neurogenic constrictor responses in the resting forearm and in its separate skeletal muscle and skin tissue compartments

Abstract

Resting forearm vascular resistance changes elicited in male volunteers by graded reflex sympathetic activation evoked by graded lower body negative pressure (LBNP) were studied at room temperatures of 24-25 and 20-21 degrees C. The latter condition caused strong suppression of skin flow and permitted preferential analysis of muscle responses and, by comparison with responses at 24-25 degrees C, secondary estimation of circulatory reactions in the skin. Short-lasting LBNP-bouts (1.5 min) allowed analyses of reflex vascular reactions to high and barely tolerated LBNP (85 mmHg) and thereby to high levels of circulatory stress and sympathetic nerve discharge, yet without risks for the subjects under study. Both muscle and skin reacted intensely and in a graded manner to graded sympathetic activation with very pronounced resistance change (74-77% flow decline; 350-400% resistance rise above control level) at high LBNP. Therefore, the sympathetic vasomotor fibres can exert a very potent control of vascular resistance both in skeletal muscle and in skin under thermoneutral conditions, and both tissues apparently can serve as major targets for powerful sympathetic homeostatic baroreflexes. Evidence indicated that this control is exerted from both low-pressure cardiopulmonary and high-pressure arterial baroreceptor areas. These conclusions deviate from previous literature, in which baroreflex sympathetic vasoconstriction in the human limb has been proposed to be more or less selectively mediated from cardiopulmonary receptors and, further, muscle to respond fully already at mild circulatory stress without further constriction if the stimulus is increased.

Effect of axillary brachial plexus blockade on baroreflex‐induced skin vasomotor responses: assessing the effectiveness of sympathetic blockade

“…On the other hand, in humans the concept that arterial baroreceptors have only a minor influence on limb vascular resistance was developed (17,18). In contrast to these data, Edfeldt and colleagues have demonstrated in their studies using lower body negative pressure that deactivation of the arterial baroreceptors significantly contributes to the regulation of forearm resistance, which involves active participation of the skin microvasculature (19–21). In a recent study, Bernardi and colleagues reported that laser Doppler flowmetry fluctuations were responsive to sinusoidal neck suction, indicating a response to sympathetic modulation.…”

The Effects of Venlafaxine on Autonomic Functions in Healthy Volunteers

“…However, physiological experiments in healthy humans give hint that vessels of the glabrous skin of the fingers are not under cholinergic control. 10 Reduced activity in central noradrenergic neurons and peripheral synaptosomal reuptake of norepinephrine are responsible for termination of VRs of the vessels of the fingers. Sustained activation of central and/ or peripheral sympathetic neurons may account for the observed prolongation of VR observed by us after subchronic administration of venlafaxine.…”

The thermoneutral zone implications for metabolic studies

“…As a consequence the TNZ in supine position is shifted upward relative to the TNZ in upright position. Furthermore, arterial baroreceptor reflexes that accompany a supine position cause vasodilatation both in muscle and skin tissue (+57% blood flow in femoral artery), which decreases insulation and contributes to the upward shift of the TNZ in supine vs. upright position (84).…”