Action Potential (AP) Cycle Length in Connexin-43-Negative and Positive Single Isolated Sinoatrial Node Cells (SANC) does not Differ in the Basal State or during High-Level Beta-AR Stimulation

Abstract

SANC generate rhythmic APs that drive the heartbeat. Perspectives gleaned from the sinoatrial node(SAN) have been interpreted to indicate that smaller cells from the central SAN area control the SAN beating rate, while data from isolated SANC argue that spontaneous AP cycle length is independent of cell size, and that calcium cycling protein density is independent of cell area and size. It's well documented that the gap junction protein, connexin-43(Cx43), is highly expressed in peripheral vs. central SAN area. Since it is possible that cell size doesn’t completely discriminate between Cx43-negative and positive cells, we measured Cx43-immunolabeling, electrophysiological and Ca2+ cycling properties of single isolated SANC.Freshly isolated adult rabbit Cx43-negative SANC are, on average, smaller (592.3±9.2μm2, n=579) than Cx43-positive SANC (747.8±12.2μm2, n=571, p<0.001), but there is no difference in the spontaneous AP cycle length based on Cx43 expression (340.2±13.6ms n=30 in Cx43-negative cells, vs. 326.5±9.0ms n=50 in Cx43-positive cells, p=0.39). AP parameters also do not differ between Cx43-negative and positive cells, but the AP of later has a shorter repolarization time (APD90: 136.9±8.6ms vs. 110.2±4.8ms, p<0.01), which is not related to cell size. No significant differences are detected in the major characteristics of basal AP-triggered Ca2+-transient or spontaneous Local-Ca2+-Releases during diastolic depolarization between Cx43-negative and positive SANC. Acute β-AR stimulation (1μM isoproterenol) reduces the AP cycle length to the same level in both Cx43-negative and positive SANC (257.3±9.0ms n=9 vs. 271.6±7.7ms n=14, respectively, p=0.25), and no differences of APD90 are detected (p=0.17).Our results indicate that although different in size, there is no statistical difference between single isolated Cx43-negative and positive SANC of spontaneous AP cycle length or during maximal β-AR stimulation.