AJP - Heart and Circulatory Physiology, Vol 242, Issue 5 827-H833, Copyright © 1982 by American Physiological Society
Evidence for persistent activation of cardiac slow channels in low-calcium solutions
J. Linden and G. Brooker
Action potentials were recorded from frog ventricular strips superfused
with calcium-free solutions. Very long action potentials (3-60 s) were
induced by chelating residual calcium with 1-5 mM
ethyleneglycol-bis(beta-aminoethylether)-N,N'-tetraacetic acid (EGTA). The
kinetics of changes in action potential duration suggest that calcium
influences action potential duration at an intracellular site. In
low-calcium solutions isoproterenol was found to lengthen action
potentials. The lengthening effect of isoproterenol became progressively
greater as [Ca2+] was reduced by elevating [EGTA]. In the presence of 2 mM
EGTA, 0.1 microM isoproterenol increased action potential duration from 4.3
+/- 0.4 to 43 +/- 14 s. Verapamil produced a greater than 90% reduction in
the duration of very long action potentials (60 s) induced by EGTA,
isoproterenol, or both. After rapid or prolonged depolarizations in
low-calcium solutions the last 10-30 mV of repolarization took 10-15 s. We
speculate that this slow phase of repolarization may be a manifestation of
persistent slow channel activation. The data suggest that in low-calcium
solutions slow channels can remain activated for many seconds.
