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1 Department of Physiology, University of Leeds, Leeds LS2 9JT, United Kingdom; 2 Research Institute of Environmental Medicine, Nagoya University, Nagoya 464-01, Japan; and 3 Department of Physiology, Iran University of Medical Sciences, Tehran, Iran
Regional differences in electrical activity in
rabbit sinoatrial node have been investigated by recording action
potentials throughout the intact node or from small balls of tissue
from different regions. In the intact node, action potential duration was greatest at or close to the leading pacemaker and declined markedly
in all directions from it, e.g., by 74 ± 4% (mean ± SE, n = 4) to the crista terminalis.
Similar data were obtained from the small balls. The gradient is down
the conduction pathway and will help prevent reentry. In the intact
node, a zone of inexcitable tissue with small depolarizations of <25
mV or stable resting potentials was discovered in the inferior part of
the node, and this will again help prevent reentry. The intrinsic
pacemaker activity of the small balls was slower in tissue from more
inferior (as well as more central) parts of the node [e.g., cycle
length increased from 339 ± 13 ms
(n = 6) to 483 ± 13 ms
(n = 6) in transitional tissue from
more superior and inferior sites], and this may help explain
pacemaker shift.
heart; cardiac; pacemaking
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