Patch clamp analysis of recovery of sodium channels from inactivation in mammalian myocardium in terms of a Markovian state model.

The kinetics of the recovery of Na channels from inactivation were investigated in ventricular cardiocytes of adult mice by a patch clamp technique. Single channel analysis showed an unaltered slope conductance of unitary currents during the recovery process to be 18.5 pS. The mean open time tau 0 and its monoexponential distribution remained also unchanged. The recovery time course of the apparent probability for a channel not to open coincided perfectly with the recovery time course determined from peak values of macroscopic currents which were recorded from cell-attached patches with up to 80 channels by averaging between 19 and 38 sweeps. Conclusively, the recovery was quantified by the time course of re-accumulation of the channels in a resting state. The initial recovery time course was sigmoidal. A serial Markov state model with three inactivated states in addition to the resting state was most appropriate for its description. The rate constants were assumed to be equal direction. A highly voltage-sensitive reaction towards the last inactivated state and a nearly voltage-independent reaction towards the resting state were obtained. Slow inactivation components were analyzed both during the recovery process (-100 mV ... -130 mV) after long preceding inactivation and at maintained depolarization (-50 mV ... -20 mV). For description, the model was extended by two further, serially coupled inactivated states. All four rate constants describing slow inactivation in the range of hundreds of milliseconds to seconds proved to be voltage-independent. Taken together, the results suggested the absorbing inactivated state I to be composed of five serially coupled inactivated states with only two transitions being steeply voltage-dependent.