PACEMAKER SYNCHRONIZATION OF ELECTRICALLY COUPLED RABBIT SINOATRIAL NODE CELLS

The effects of intercellular coupling conductance on the activity of t wo electrically coupled isolated rabbit sinoatrial nodal cells were in vestigated. A computer-controlled Version of the ''coupling clamp'' te chnique was used in which isolated sinoatrial nodal cells, not physica lly in contact with each other, were electrically coupled at various v alues of ohmic coupling conductance, mimicking the effects of mutual i nteraction by electrical coupling through gap junctional channels. We demonstrate the existence of four types of electrical behavior of coup led spontaneously active cells. As the coupling conductance is progres sively increased, the cells exhibit: (a) independent pacemaking at low coupling conductances, (b) complex dynamics of activity with mutual i nteractions, (c) entrainment of action potential frequency at a 1:1 ra tio with different action potential waveforms, and ((d) entrainment of action potentials at the same frequency of activation and virtually i dentical action potential waveforms. The critical value of coupling co nductance required for 1:1 frequency entrainment was <0.5 nS in each o f the five cell pairs studied. The common interbeat interval at a rela tively high coupling conductance (10 nS), which is sufficient to produ ce entrainment of frequency and also identical action potential wavefo rms, is determined most by the intrinsically faster pacemaker cell and it can be predicted from the diastolic depolarization times of both c ells. Evidence is provided that, at low coupling conductances, mutual pacemaker synchronization results mainly from the phase-resetting effe cts of the action potential of one cell on the depolarization phase of the other. At high coupling conductances, the tonic, diastolic intera ctions become more important.