Electrophysiology and ultrastructure of eustachian ridge from cat right atrium: a comparison with SA node

Intracellular microelectrode and electron microscopic techniques were used to investigate and correlate the electrophysiology of subsidiary pacemaker activity with the presence of cells having ultrastructural characteristics of pacemaker cells i.e. P cells, in Eustachian ridge tissue isolated from cat right atrium. In addition, the electrophysiological characteristics of subsidiary pacemaker activity and the ultrastructural characteristics of P cells in Eustachian ridge were compared to those of SA node obtained from the same hearts. Action potential recordings and morphological analysis were restricted to the endocardial site of earliest activation. Electrophysiological recordings revealed that spontaneously active Eustachian ridge tissues generate slow response action potentials with pacemaker characteristics similar, although not identical, to those of SA node. These included a relatively steep diastolic slope, low maximum diastolic potential (-70 mV), rate of rise (5.5 V/s), take-off potential (-52.5 mV), a relatively large overshoot potential (+7.7 mV) and a spontaneous cycle length (948 ms) about twice as long as SA node (434 ms). Morphological analysis revealed cells with ultrastructural characteristics of P cells, that were restricted to the endocardial site of earliest pacemaker activation. Morphological measurements indicate that Eustachian ridge P cells are not significantly different from P cells in SA node of the same hearts. However, Eustachian ridge P cells exhibit a unique apposition of subsarcolemmal cisternae between cells not seen in SA node. We conclude that pacemaker cells within the Eustachian ridge generate stable, spontaneous activity via slow response pacemaker action potentials. Cells responsible for this subsidiary pacemaker activity are most likely P cell types that are similar, although not identical, to P cells in SA node.