Electrophysiological study of the two main pacemaker mechanisms in the rabbit sinus node

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Abstract
By extensively mapping the rabbit sinoatrial node (SA node) area, plateau fibres (B cells) were found in a localised area between the caval border of the crista terminalis and the primary pacemaker area. The electrical characteristics of these plateau fibres were close to those of conducting tissue cells and therefore different from the primary pacemaker cells (A cells) . When the SA node was cut in small pieces (0.5 by 0.5 mm), the action potentials of the B cells exhibited a mean amplitude of 80 mV and a rate of rise of the ascending phase strongly depressed by 1.10+5 g·ml−1 of tetrodotoxin (TTX). The diastolic depolarisation was reduced by about 70% in the presence of 1.5 mmol·litre−1 of caesium ions (Cs). At this concentration, these ions are known to inhibit mainly the current if (a current activated at potentials more negative than −50 mV). In contrast, the action potentials of the A cells recorded in small pieces never exceeded 60 mV and were either barely sensitive or insensitive to the same concentrations of TTX. Their diastolic depolarisation (ranging from −60 to −35 mV) was not affected by concentrations of Cs which strongly depressed the diastolic depolarisation of the B cells, even when they were hyperpolarised in the same range of potentials. It seems therefore that, in the B cells, the pacemaker mechanism is in great part due to the onset of if while it is due in A cells to another mechanism, very likely the decay of iK associated with the onset of isi. We conclude that two distinct populations of pacemaker cells are clearly distinguishable in the rabbit SA node area when cut in small pieces. In the intact tissue, areas of gradual transition corresponding to the latent pacemaker cell areas are present between these two kinds of cells, accompanied by a variable and gradual percentage of the two main pacemaker mechanisms.