Calcium conductance in relation to contractility in frog myocardium.

Abstract

Ca inward current and the corresponding phasic component of tension were measured in frog atrial muscle under voltage-clamp conditions in Na-free (Li) Ringer solution with tetrodotoxin (TTX) added. The quantity of Ca ions entering the cell upon depolarization, .DELTA.[Ca]i, was linearly related to peak phasic tension. The voltage dependence of the steady-state inactivation of the Ca-carrying system, f.infin., against voltage yielded similar relationships whether determined directly from variations of Ca inward current or peak phasic tension. The Ca system was almost fully available at potentials more negative than -45 mV and almost fully inactivated at potentials more positive than +10 mV. The time- and voltage-dependence of Ca current and of phasic tension are probably directly related. The time constants of Ca inactivation, .tau.f, were comparable in the range of membrane potential investigated (-20 to +25 mV), whether determined directly from the decay of Ca current or indirectly from peak phasic tension. The Ca current, ICa, apparently directly activates phasic contraction and that either parameter can be used as an indicator of the kinetics of the Ca-carrying system. Peak phasic tension was used to determine .tau.f further in the membrane potential range in which interference by other membrane currents renders direct analysis of Ca current difficult. The .tau.f against voltage relationship determined from phasic tension showed that the inactivation process of the Ca-carrying system is slowest at membrane potentials around -13 mV (.tau.f = 55 ms) and that the rate of inactivation increases with both increasing and decreasing depolarizations. Normal repolarization in frog myocardium possibly depends mainly on the decay of Ca inward current rather than on an increase of outward current.