The dynamics of cross-bridge movement in vascular smooth muscle estimated from a single isometric contraction of the portal vein: The influence of temperature and calcium

The velocity of the contractile elementVCE of the rat portal vein during a single isometric twitch was calculated from the ratio of the rate of tension increase (dT/dt) to the stiffness (dT/dL) of the series elastic element. This stiffness was considered as a linear function of tension (dT/dL=k · T), and thus the respective term wasVCE=(dT/dt)/(kT). A polynome of the 10th order was fitted to the time course of tension change within the first seconds of stimulation. From this function, the instantaneous values of forceT, rate of tension increasedT/dt, as well as the ratio(dT/dt)/T were computed. The series elasticity was treated as the respective stiffness factor ‘k’.Vmax isVCE extrapolated to zero load, and this value gives an index of the turnover rate of the myosin cross-bridges. The experiments were carried out at different temperatures (37° C, 30° C, and 25° C) as well as at different intracellular calcium levels occurring as a staircase phenomenon in the first contractions after a period of non-stimulation of 20 min.