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1 Division of Cardiothoracic Surgery, Department of Surgery, and 2 Department of Pathology, Washington University School of Medicine, Barnes Hospital, St. Louis, Missouri 63110
This study reports the
comparative quantitative, morphological, and electrophysiological
properties of two pacemaker cell types, spider and spindle-shaped
cells, isolated from the rabbit sinoatrial node. Isolated nodal cells
were studied with perforated and ruptured patch whole cell recording
techniques. The basic spontaneous cycle length of the spider cells was
381 ± 12 ms, and the basic spontaneous cycle length of the
spindle cells was 456 ± 17 ms (n = 12, P < 0.05). The spider cells had a more positive maximum diastolic potential (
54 ± 1 mV) compared with the
spindle cells (68 ± 1mV, P < 0.05). The
overshoot and action potential amplitudes were also smaller in the
spider cells. The hyperpolarization-activated inward
(If) current density, measured from their tail
currents, was 15 ± 1.3 pA/pF for the spider cells and 9 ± 0.7 pA/pF for the spindle cells (P < 0.01).
If current activation voltage was more positive
in the spider cells than the spindle cells. Isoproterenol (1 µM)
decreased the spontaneous cycle length of the spider cells by 28 ± 3% and the spindle cells by 20 ± 1.5% (P < 0.05). Acetylcholine (0.5 µM) hyperpolarized the membrane potential
of the spider cells to 86 ± 0.7mV and the spindle cells to
76 ± 0.8 mV (P < 0.05). In summary, there are
at least two distinct pacemaker cell types in the sinus node with
different electrophysiological characteristics.
hyperpolarization-activated inward current; pacemaker current
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