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Electrophysiological study of the 2 main pacemaker mechanisms in the rabbit sinus node


, : Electrophysiological study of the 2 main pacemaker mechanisms in the rabbit sinus node. Cardiovascular Research 19(5): 304-318

By extensively mapping the rabbit sinoatrial node (SA node) area and plateau fibers (B cells) were found in a localized area between the caval border of the crista terminalis and the primary pacemaker area. The electrical characteristics of these plateau fibers 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 .cntdot. ml-1 of tetrodotoxin (TTX). The diastolic deplorization was reduced by about 70% in the presence of 1.5 mmol .cntdot. l-1 of Cs ions (Cs). At this concentration, these ions 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 depolarization (ranging from -60 to -35 mV) was not affected by concentrations of Cs which strongly depressed the diastolic depolarization of the B cells, even when they were hyperpolarized in the same range of potentials. In the B cells, the pacemaker mechanisms is in great part due to the onset of if while it is due in A cells to another mechanism, very like the decay of iK associated with the onset of isi. 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 cells area are present between these 2 kinds of cells, accompanied by a variable and gradual percentage of the 2 main pacemaker mechanisms.

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