Cellular mechanism of the functional refractory period in ventricular muscle

A premature action potential elicited in ventricular muscle during the functional refractory period of a preceding action potential requires an increased stimulus intensity for successful propagation. We measured the cellular basis for these relative decreases in tissue excitability during the recovery phase by performing parallel experiments on rabbit left papillary muscle and isolated rabbit ventricular cells in addition to conducting theoretical studies with numerical simulations of action potential initiation. For each experimental preparation, the pacing protocol consisted of a train of 10 stimuli (S1) at an S1-S1 interval of 500 msec with a premature stimulus (S2) of variable S1-S2 intervals following the tenth S1 action potential. The stimulus threshold for initiation of an S2 action potential (I2) was then measured as a function of the time of occurrence of the S2 stimulus relative to the time of 95% repolarization of the tenth S1 action potential (stimulus delay [SD] time). In the tissue preparation, the I2 increased sharply for SD times less than 0 msec to a value that was 100% above the S1 stimulus threshold for SD time = -5 +/- 2.4 msec (n = 8). Similar experiments on the isolated ventricular cell showed no increases in I2 as a function of SD time but rather significant decreases in both the action potential amplitude (APA) and the maximum rate of rise of the action potential upstroke (Vmax) of the S2 action potential. The APA and Vmax for the S2 action potential were decreased to 50% of the S1 action potential values for SD time = -5.2 +/- 2.1 msec and SD time = 0.3 +/- 1.6 msec, respectively (n = 8). Both parameters reached 100% recovery by SD time = 10 msec. These results and our numerical simulations are consistent with the hypothesis that the decreases in tissue excitability that occur with premature stimulation have a cellular mechanism as a result of a decrease in cellular responsiveness (APA, Vmax) rather than an intrinsic decrease in cellular excitability.