Characteristics of static and dynamic regulatory mechanisms in myogenic microvascular control

The recently described static and dynamic components in myogenic microvascular control (Grände, Lundvall and Mellander 1977) were analysed in this study with regard to their stimulus-effector characteristics. Total and microvascular resistance responses in the sympathectomized vascular bed of skeletal muscle were analysed during graded changes of vascular transmural pressure (PT) applied at different rates (dPT/dt) in the range from +7.5 to -7.5 mmHg/s. The dynamic microvascular resistance responses, developing during the phase of changing PT, were pronounced and distinctly graded in relation to the magnitude of the dPT/dt stimulus, both with regard to amplitude of resistance response and rate of resistance change per unit time (dRmicro/dt). The static responses, revealed in the steady state phase of constant increased PT, were comparatively small and graded in relation to the amplitude of the PT increase. Rate-sensitivity in microvascular myogenic control was bi-directional, eliciting excitatory effects (constriction) in response to positive, and inhibitor effects (dilation) in response to negative, values of dPT/dt. The dynamic constrictor response to a given dPT/dt stimulus increased with increasing amplitude of PT and, thereby, increased duration of the dynamic stimulus. This effect might be explained by successive activation of myogenic "receptor units" with different thresholds. The described rate-sensitivity in the myogenic control system seems to increase its rapidity, stability, and sensitivity and thereby can contribute efficiently to well-adapted and refined microvascular adjustments.