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Departments of 1 Humoral Regulation and 3 Circulation, Research Institute of Environmental Medicine, Nagoya University, Nagoya 464-8601, Japan; and 2 Department of Physiology, University of Leeds, Leeds LS2 9JT, United Kingdom
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 (as measured by the cell capacitance) of SA
node cells to elucidate the ionic mechanisms. The 10 mM 4-AP-sensitive
current recorded from rabbit SA node cells was composed of transient
and sustained components
(Itrans and
Isus, respectively). The activation and inactivation properties
[activation: membrane potential at which conductance is
half-maximally activated (Vh) = 19.3 mV,
slope factor (k) = 15.0 mV;
inactivation: Vh = 
31.5 mV, k = 7.2 mV] as well as the density of
Itrans (9.0 pA/pF on average at +50 mV) were independent of cell capacitance. In contrast, the density of
Isus (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
Isus in larger
cells (presumably from the periphery) can explain the shorter action potential in the periphery of the SA node compared with that in the
center. Thus variation of the 4-AP-sensitive current may be involved in
regional differences in repolarization within the SA node.
electrophysiology; patch clamp; transient outward current; regional difference
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