Heterogeneity of 4-aminopyridine-sensitive current in rabbit sinoatrial node cells

The electrophysiological properties of sinoatrial (SA) node pacemaker cells vary in different regions of the node. In this study, we have investigated variation of the 4-aminopyridine (4-AP)-sensitive current as a function of the size las measured by the cell capacitance) of SA node cells to elucida te the ionic mechanisms. The 10 mM 4-AP-sensitive current recorded from rab bit SA node cells was composed of transient and sustained components (I-tra ns and I-sus, respectively). The activation and inactivation properties [ac tivation: membrane potential at which conductance is half-maximally activat ed (V-h) = 19.3 mV,slope factor (kappa) = 15.0 mV; inactivation: V-h = -31. 5 mV, kappa = 7.2 mV] as well as the density of I-trans (9.0 pA/pF on avera ge at + 50 mV) were independent of cell capacitance. In contrast, the densi ty of I-sus (0.97 pA/pF on average at +50 mV) was greater in larger cells, giving rise to a significant correlation with cell capacitance. The greater density of I-sus in larger cells (presumably from the periphery) can expla in the shorter action potential in the periphery of the SA node compared wi th that in the center. Thus variation of the 4-AP-sensitive current may be involved in regional differences in repolarization within the SA node.