Maximum open probability of single Na+ channels during depolarization in guinea-pig cardiac cells.

Single Na+ channel currents were recorded from guinea-pig ventricular cells in cell-attached patches. The ensemble average current (I) of multi-channel recordings was used to calculate the variance (sigma 2) of current fluctuations around the mean in individual current recordings. The relationship between sigma 2/I and I was linear and allowed estimation of the number of functional channels in the patch of membrane. The unitary amplitude of channel current obtained from the relation sigma 2/I-I was in agreement with that obtained directly by measuring the original records. The number of channels determined at different depolarizing pulses was almost constant in a given patch. The value was nearly equal to that of the maximum current, measured at high depolarizing potentials when most channels are open, divided by the unitary current. The open probability of the channels at the peak time of mean current was calculated based on the estimated number of channels. It increased with increasing depolarization and saturated at about 0.6 at test potentials above -20 mV. The inactivation time-course of the mean current was fitted by a sum of two exponentials. The current amplitude extrapolated to time zero was much larger than the current which could be generated by all channels. This indicates that the inactivation of the Na+ channel develops with delay after the onset of depolarization. The finding is in agreement with a model in which the inactivation rate is accelerated with activation of the Na+ channel.