ELECTROPHYSIOLOGICAL PROPERTIES OF MORPHOLOGICALLY DISTINCT CELLS ISOLATED FROM THE RABBIT ATRIOVENTRICULAR NODE

1. Experiments were conducted using the whole-cell patch clamp techniq ue to determine the electrophysiological properties and ionic currents of ovoid and rod-shaped single isolated calcium-tolerant rabbit atrio ventricular (AV) nodal cells. 2. Action potential morphologies observe d in these cells were similar to those obtained previously from intrac ellular recordings of intact atrioventricular nodal preparations: ovoi d cells had N- or NH-like action potential configurations (see below), whereas rod-shaped cells had AN-like configurations. 3. Action potent ial restitution in AV modal cells was characterized by a progressive i ncrease in overshoot potential, maximal upstroke velocity (V-max) and action potential duration, as well as a decrease in latency from stimu lus to V-max. In rod-shaped cells, premature stimuli could induce rege nerative membrane responses before full action potential repolarizatio n, whereas ovoid cells showed only post-repolarization refractoriness. In ovoid cells stimulated at the low stimulus intensities there was n o shortening of the action potential duration and the most premature a ction potentials were often prolonged. 4. The quasi-steady-state curre nt-voltage relationship of ovoid cells was significantly steeper, at b oth depolarized and hyperpolarized potentials, than that of either the rod-shaped AV nodal cells or atrial cells. The rod-shaped AV nodal ce lls and the atrial cells had similar current-voltage (I-V) relationshi ps in the positive, potential range, but the I-V curves crossed over a t 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 = 1 6) 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-shifted cells. 6. A rapid inward current (I-Na) greater than 1 nA was found in all rod-shaped cells (n = 16) but in only 30% of ovoi d 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 c ombination of I-to and I-Na was found in 93% of rod-shaped cells but i n 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 compl ex electrophysiological properties observed in the intact AV node.