Characteristics of extracellular potentials recorded from the sinoatrial pacemaker of the rabbit.

SUMMARY We have identified the extracellular potential changes associated with electrical activity of the sinoatrial (SA) pacemaker. Unipolar Ag-AgCI electrodes and DC amplification were used to record unipolar electrograms from the endocardial surface of the isolated rabbit atrium. Transmembrane action potentials (TAP) from the SA pacemaker were recorded to identify the pacemaker region and to monitor intracellular potential changes simultaneously with the electrogram. The SA electrogram showed two characteristic potentials when the electrode was in immediate proximity to pacemaking cells: (1) during phase 4 there was a steady slope (of the order of magnitude of —30 to —90 /xV/sec at the cycle lengths studied) opposite in polarity to the slope of phase 4 of the TAP; (2) during the transition from phase 4 to phase 0 of the TAP the slope of the electrogram increased smoothly to approximately -400 to -800 ptV/sec. We have called these two deflections the diastolic slope and the upstroke slope. As cells in the perinodal region and crista lerminalis depolarized, these low frequency potential changes were interrupted by high frequency deflections. Tetrodotoxin (2-10 mg/liter) rendered the atrial muscle inexcitable and delayed and then abolished the high frequency activity in the SA electrogram, which then appeared as a continuous smooth tracing similar to the SA nodal TAP but reversed in polarity. The correlations between transmembrane and extracellular records were maintained when sinus automaticity was suppressed by cooling (36.5°C to 30°C). A potential resulting from pacemaker depolarization also could be recognized when the SA node was depolarized in a retrograde fashion by a paced atrial rhythm. These results demonstrate that rabbit SA pacemaker cells produce detectable and characteristic extracellular potential changes.

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