Calcium conductance and tension in mammalian ventricular muscle

Voltage, membrane current and contraction were simultaneously measured in voltage clamp experiments (single sucrose gap) on cat ventricular trabeculae. The pulse programs allowed the determination of the potential dependence of the steady state activation and inactivation as well as the restoration of the calcium-carrying system (slow inward current). 1. The steady state activation variable (d infinity) rose in a sigmoid manner from -50 mV (d infinity nearly 0) to 0 mV (d infinity nearly 1). The experimental values can be described by the function 1/1 + exp [(Vh-V)/S] where half activation (Vh) = -22.5mV and S = 7.6 mV. 2. The steady state inactivation variable (f infinity) declined from 1 at -60mV to 0 at 10mV. The best fit curve is nearly a mirror image of the activation curve with Vh = -28 mV and s = -8.3 mV. 3. The voltage dependence of the (normalized) peak tension was well described by the steady state conductance variables except at potentials positive to +20mV. A "steady state" tension (superimposed on "tonic tension") was found in the potential range where a steady state conductance is predicted by the curves describing steady state activation and inactivation. 5. Following inactivation, the time courses of restoration of the calcium-carrying system and tension were identical. Time courses were exponential with tau = 118 msec at -80 mV, 144 msec at -60 mV, and 198 msec at -40 mV. 6. Two possible models of excitation-contraction coupling in line with the present results are discussed.