Evidence for persistent activation of cardiac slow channels in low-calcium solutions

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.