Effect of felodipine on the myogenic response to dynamic stretch in vascular smooth muscle.
In the present experiments we examined the effect of felodipine, a vasoselective dihydropyridine calcium antagonist, on contractile responses to dynamic and static stretch of the isolated portal vein of the rat. Dynamic stretch was applied to the vascular smooth muscle at graded rates (from 0.5-1.5% muscle length s-1). Earlier observations (Johansson & Mellander 1975) of a rate-dependent excitation of the vascular smooth muscle by dynamic stretch were confirmed. Addition of felodipine, 3 nM, reduced the spontaneous activity at static lengths to about 50% but resulted in much stronger inhibition of the dynamic stretch responses. Particularly the rate-dependent increase in active force was no longer evident since the response at high rates of passive lengthening was most clearly reduced by felodipine. By contrast, lowering of the extracellular Ca2+ concentration resulted in a comparable attenuation of the spontaneous contractile activity and of the dynamic stretch responses which still showed the typical rate dependence. Therefore, the pronounced inhibition by felodipine of the dynamic myogenic reactivity of the rat portal vein appeared to be a specific effect and not simply related to the overall reduction in contractile activity. We suggest that felodipine, in addition to its inhibition of action potentials and excitation-contraction coupling may exert a special negative influence on the mechano-electrical coupling, i.e. the process that couples dynamic stretch of the vascular smooth muscle to membrane excitation.