Rate-dependent myogenic response of vascular smooth muscle during imposed changes in length and force.

A recent study of electrical and mechanical responses to stretch in isolated vascular smooth muscle revealed a clear-cut and graded influence of the rate of change in length, dL/dt (Johnansson and Mellander, 1975). This myogenic "dynamic" response at high rates of stretch was much more pronounced than the "static" response to constant, increased (+40%) length. In this previous study the mechanical responses were recorded as active force. In view of the fact that myogenic reactions in vivo must be associated with changes in vessel caliber, it was considered of interest to investigate the responses to stretch also in smooth muscle undergoing active shortening. In the present study, as in the one referred to above, electrical and mechanical activity in the isolated rat portal vein was studied by the sucrose gap method. However, in the present experiments the mechanical responses were recorded not only as active force but also as active shortening, making possible a comparison of the myogenic responses under these two types of smooth muscle contraction. Dynamic passive stretch was found to be associated with marked increase in spike discharge and mechanical activity under both these experimental conditions and the quantitative relation between spike discharge and rate of passive stretch (or shortening) was similar. Thus, active shortening of the smooth muscle did not interfere with the ability of the vessel to respond myogenically to passive stretch. A further analysis of the results suggested that, for the preparations as a whole, the dynamic excitatory response was more closely related to the rate of change of passive force, dP/dt, than to dL/dt.