Cellular mechanisms of delayed recovery of excitability in ventricular tissue

It is well established that ventricular tissue, under some conditions, exhibits the phenomenon of postrepolarization refractoriness (PRR) in which the tissue excitability is depressed after an action potential. We have done parallel experiments on rabbit papillary muscles and on isolated rabbit ventricular cells to explain the cellular basis of this phenomenon, using elevated extracellular K+ concentration ([K+]o) (8 mM) to depolarize the tissue and the isolated cells. For isolated cells, we could separately measure cellular excitability (the inverse of the cellular current threshold) and the cellular responsiveness (the ability of the cell to generate inward current after excitation has occurred). We present two hypotheses that could explain the magnitude and time course of tissue PRR in terms of either changes in cellular excitability or changes in cellular responsiveness. We show that, although small changes in cellular excitability do occur, the predominant cellular mechanism for tissue PRR is the time course of recovery of the cellular responsiveness.