SMOOTH-MUSCLE CONTRACTION KINETICS AT DIFFERENT CALCIUM CONCENTRATIONS

Actin-myosin interaction kinetics of the intact rat portal vein were s tudied by analyzing force recovery after cessation of force-inhibiting length vibration. The time constant of postvibration force recovery a veraged 0.86 +/- 0.04 s during shortterm activation (< 12 s), and incr eased up to 1.59 +/- 0.02 s (cross-bridge downregulation) during susta ined activation of more than 10 min. After the depletion of intracellu lar calcium stores, the depolarized preparation developed maximum forc e at an extracellular calcium concentration in excess of 50 mM CaCl2. The time constant of postvibration force recovery rose to 12.31 +/- 1. 35 s after an activation period of 30 min. These retarded contraction kinetics may be caused either by low activation of the 20-kDa myosin l ight chain kinase or by high activity of the 20-kDa myosin light chain phosphatase. Addition of the phosphatase inhibitor okadaic acid (10 m u M) during high calcium activation decreases the time constant to 8.0 4 +/- 0.86 s and appears to prevent the distinct retardation of the co ntraction kinetics. During submaximum activation (2.5 mM CaCl2), the t ime constant of postvibration force recovery stabilizes at 1.56 +/- 0. 07 s, indicating downregulated cross-bridge kinetics, and is unaffecte d by phosphatase inhibition. For maximum barium activation, instead of calcium, 19.5 mM BaCl2 is required, which produced time constants of postvibration force recovery of 8.38 +/- 0.32 s. The addition of okada ic did not affect contraction kinetics during barium activation. The p ronounced retardation of contraction kinetics that was observed after the maximum calcium activation of the previously calcium depleted, dep olarized preparation is probably due to high phosphatase activity. Hig h phosphatase activity, on the other hand, does not seem to be involve d in the process of cross-bridge downregulation or during barium activ ation.