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Department of Physiology, McGill University, Montréal, Québec, Canada.

1. Experiments were conducted using the whole-cell patch clamp technique to determine the electrophysiological properties and ionic currents of ovoid and rod-shaped single isolated calcium-tolerant rabbit atrioventricular (AV) nodal cells. 2. Action potential morphologies observed in these cells were similar to those obtained previously from intracellular recordings of intact atrioventricular nodal preparations: ovoid cells had N- or NH-like action potential configurations (see below), whereas rod-shaped cells had AN-like configurations. 3. Action potential restitution in AV nodal cells was characterized by a progressive increase in overshoot potential, maximal upstroke velocity (Vmax) and action potential duration, as well as a decrease in latency from stimulus to Vmax. In rod-shaped cells, premature stimuli could induce regenerative membrane responses before full action potential repolarization, whereas ovoid cells showed only post-repolarization refractoriness. In ovoid cells stimulated at the low stimulus intensities there was no shortening of the action potential duration and the most premature action potentials were often prolonged. 4. The quasi-steady-state current-voltage relationship of ovoid cells was significantly steeper, at both depolarized and hyperpolarized potentials, than that of either the rod-shaped AV nodal cells or atrial cells. The rod-shaped AV nodal cells and the atrial cells had similar current-voltage (I-V) relationships in the positive potential range, but the I-V curves crossed over at potentials of about-90 mV. 5. A hyperpolarization-activated inward current (I(f)) was apparent in the range between -60 and -90 mV in 95% of the ovoid cells (n = 75), whereas in 88% of rod-shaped cells (n = 16) I(f) was activated at more negative potentials. The magnitude of I(f) in ovoid cells, measured at -100 mV, was approximately 25 times that in rod-shaped cells. 6. A rapid inward current (INa) greater than 1 nA was found in all rod-shaped cells (n = 16) but in only 30% of ovoid cells (n = 75). A transient outward current (I(to)) was found in 93% of rod-shaped cells (n = 14) and in 42% of ovoid cells (n = 54). The combination of I(to) and INa was found in 93% of rod-shaped cells but in only 24% of ovoid cells. 7. These results suggest that there are at least two populations of isolated AV nodal cells with distinct action potentials and ionic current profiles that may contribute to the complex electrophysiological properties observed in the intact AV node.

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