INTERRELATIONSHIP OF CALCIUM-MEDIATED ACTION POTENTIALS AND TENSION DEVELOPMENT IN CAT VENTRICULAR MYOCARDIUM

Studies of Ca-mediated action potentials and mechanical activity of cat papillary muscles and trabeculae partially depolarized and Na-inactivated by K-rich (13.7 mM) solution were performed. Variation in strength of muscle stimulation resulted in gradation of rate of rise, overshoot and duration of Ca-mediated action potentials, peak tension and time to peak tension. At constant stimulus strength the rate of rise, overshoot and duration of the Ca-mediated action potentials were markedly depressed when the frequency of stimulation was increased. Two seconds was maximum driving frequency. Within this range the frequency:tension relationship was similar to, although less than, the control. When steady state peak tension was low the Ca inflow was high, as indicated by dV/dtmax, overshoot and plateau positivity, and action potential prolonged. Following frequency change, dV/dtmax of the rising phase and overshoot decreased (increased) during the initial positive (negative) inotropic effect, and then tapered to steady-state by the end of the 1-2 min inotropic change. Hence, during staircase, as well as in steady state, the tension development was intimately interrelated to the Ca inflow, the underlying mechanism being discussed. Reducing [Ca]o to 0.25 mM completely inhibited Ca-mediated excitation as well as contraction within 15-20 min. Inhibitors of the slow Ca inward current (verapamil, D 600) similarly inhibited Ca-mediated action potentials, but left resting state excitation and contraction (1/min) essentially unaltered.