Inhibition of vascular myogenic tone and reactivity by calcium antagonists.

In most organs basal vascular tone is maintained by the myogenic tone in the smooth muscles of the resistance vessels. Commonly used antihypertensive calcium antagonists reduce peripheral resistance and hence vascular tone, suggesting inhibition of myogenic tone. The findings supporting this hypothesis are reviewed here. Evidence for direct inhibition of myogenic tone by calcium antagonists includes findings in the hindleg of the anaesthetized cat, in which basal myogenic tone (prevailing resistance minus the resistance at maximal dilation) was dose-dependently reduced by felodipine. Furthermore, induced responses to vasomotor nerve stimulation were depressed by felodipine to the same relative extent as was the basal myogenic tone. The phasic spontaneous force development, which is of myogenic origin in the rat portal vein, was reduced after the administration of verapamil, diltiazem or felodipine. The adrenergic potentiation of this myogenic activity was also depressed to the same relative extent as felodipine depressed the spontaneous contractile force. The effect of calcium antagonists on myogenic reactivity seems to be even more pronounced than their effect on myogenic tone. Thus, transient vasoconstriction in response to a rapid increase in transmural pressure in the hindleg of the anaesthetized cat was reduced to a relatively greater extent by calcium antagonists than the basal myogenic tone. The attenuation of renal and cerebral blood flow autoregulation that has been observed after the administration of calcium antagonists also suggests a direct effect on myogenic reactivity. These results suggest that calcium antagonists interfere with activation in myogenically active vascular smooth muscle, resulting in reductions in myogenic tone.(ABSTRACT TRUNCATED AT 250 WORDS)