SMOOTH-MUSCLE CONTRACTION KINETICS AT DIFFERENT CALCIUM CONCENTRATIONS

Actin-myosin interaction kinetics of the intact rat portal vein were studied by analyzing force recovery after cessation of force-inhibiting length vibration. The time constant of postvibration force recovery averaged 0.86 +/- 0.04 s during shortterm activation (< 12 s), and increased up to 1.59 +/- 0.02 s (cross-bridge downregulation) during sustained activation of more than 10 min. After the depletion of intracellular calcium stores, the depolarized preparation developed maximum force 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 light chain kinase or by high activity of the 20-kDa myosin light chain phosphatase. Addition of the phosphatase inhibitor okadaic acid (10 mu M) during high calcium activation decreases the time constant to 8.04 +/- 0.86 s and appears to prevent the distinct retardation of the contraction kinetics. During submaximum activation (2.5 mM CaCl2), the time constant of postvibration force recovery stabilizes at 1.56 +/- 0.07 s, indicating downregulated cross-bridge kinetics, and is unaffected 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 okadaic did not affect contraction kinetics during barium activation. The pronounced retardation of contraction kinetics that was observed after the maximum calcium activation of the previously calcium depleted, depolarized preparation is probably due to high phosphatase activity. High phosphatase activity, on the other hand, does not seem to be involved in the process of cross-bridge downregulation or during barium activation.