Jan Lundvall

Changes of the pressure drop curve and of resistance along the vascular bed of skeletal muscle evoked by sympathetic stimulation

Abstract The changes of pressure and resistance along the vascular bed of skeletal muscle evoked by graded sympathetic activation (1–16 Hz) were studied by determinations of blood pressure at various sites in the vasculature and of regional blood flow. At all stimulation rates, the microvascular pressure and resistance showed a biphasic pattern of response whereas the increase of total vascular resistance ( R T ) rapidly reached a constant level. Initially during stimulation there was a preferential constriction of the arterial microvessels so that the pressure drop curve along the vascular bed became steeper and displaced distally towards the capillary section. The microvascular constriction reached a maximal level within 30 sec and was graded in relation to the stimulation rate; at intense stimulation (8–16 Hz) the resistance increase in the microvessels averaged no less than 3000% above control. This pronounced response, however, soon declined to a significantly lower, steady state level reached within 60 sec of stimulation. Concomitantly, a gradual constriction of the larger precapillary resistance vessels occurred so that the increase of R T remained constant. This redistribution of resistance within the vasculature during stimulation implied that the pressure drop curve was shifted back towards its prestimulatory configuration. Except for an initial transient phase, the vascular resistance distribution in skeletal muscle during sympathetic stimulation thus seems comparable to that prevailing in the resting, control state.

NERVOUS CONTROL OF THE MICROCIRCULATION IN SKELETAL MUSCLE

Publisher Summary The vascular bed of skeletal muscle is one of the most important targets for the central nervous sympathetic control of the peripheral circulation. This regulation in skeletal muscle is effected via an alpha-adrenergic, a beta-adrenergic, and a cholinergic link with partly different central representation and different functional organization in the periphery. The alpha-adrenergic constrictor component exerts its main control on large and small precapillary resistance vessels and on capacitance vessels; the beta-adrenergic dilator component exerts a preferential control on small pre- and post-capillary resistance vessels and on precapillary sphincter vessels; and the cholinergic dilator component exerts a relatively selective control of the large-bore precapillary resistance vessels. The chapter focuses on the microvascular beta-adrenergic dilator component. The vasomotor fiber control of peripheral vascular functions primarily subserves general circulatory homeostasis. The adjustment of capillary pressure forms the basis for reflex vasomotor control of plasma volume by determining the fluid distribution between the extravascular and the intravascular spaces.