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 elementV_CE 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 wasV_CE=(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.V_max isV_CE 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.With increasing temperature,T remained constant but there was an increase inV_max (Q₁₀=1.9), peakV_CE (Q₁₀=1.8) and in (dT/dt)_max (Q₁₀=1.8). Furthermore, this so-called tachytropic effect of temperature showed a reduction of the time to the maximumdT/dt (Q₁₀=1.4), and to the peakV_CE (Q₁₀=1.2).During a staircase cycle the parameters describing the contractile state, i.e. theV_max, the time to maximumdT/dt, and the time to peakV_CE remained constant.T, maximumdT/dt and peakV_CE were, however, considerably increased. These results are typical of the socalled polytropic effect of calcium.The experimental results obtained from the isometric force-velocity relation were compared with those calculated by means of isotonic contractions in previous experiments. There were similar changes in the dynamics of contraction, irrespective of the method used for calculation ofV_max; this was the case in experiments with varied temperatures and calcium levels. Therefore the determination ofV_max from a single isometric contraction seems to be a suitable method of describing the elementary process of contraction in vascular smooth muscle. This method showed a higher time resolution as compared with other methods using isotonic contractions.