In order to examine the phenotypic consequences of genetic manipulation in the function of the sinoatrial (SA) node, it is a prerequisite to record the electrical activities of a single pacemaker cell of the SA node of mouse heart. In the present study, we isolated spontaneously beating pacemaker cells from the SA node by enzymatic digestion. The rate of spontaneous action potential firing was 294 ± 59 min−1 at 33–34 °C. The maximal diastolic potential (MDP) was −56.7 ± 7.4 mV and the overshoot was 22.7 ± 6.2 mV. With hyperpolarizing voltage clamp pulses, the hyperpolarization-activated, cyclic nucleotide-sensitive cation current (Ih or If) was recorded. Ih started to activate at −70 to ≈−80 mV, more negative than MDP. The half-maximal activation of Ih was obtained at −107.9 ± 10.4 mV. The inward-rectifier K+ current (IK1) was also recorded in spontaneously beating myocytes. However, the amplitude of outward IK1 was negligibly small (9.1 ± 3.4 pA at −60 mV). With depolarization, voltage-gated Na+ current, L-type Ca2+ current and T-type Ca2+ current were consistently observed. The sustained inward current (Ist) was also recorded in spontaneously beating pacemaker cells. E4031-sensitive, rapidly activating delayed rectifier K+ current (IKr) was activated by depolarization, although the amplitude was no more than 38.3 ± 22.2 pA at 0 mV. The chromanol 293B-sensitive, slowly activating delayed rectifier K+ current (IKs) was not present. The major repolarizing current was the slowly inactivating, 4-aminopyridine-insensitive outward current. We concluded that mouse pacemaker cells possess similar membrane currents, including Ist, to those of other species.
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