Effects of calcium on canine Purkinje fiber action potential duration in the presence of agents affecting potassium permeability.

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Abstract
Intracellular microelectrodes were used to study the effect of changes in extracellular Ca ion concentration [Ca2+]O on the transmembrane potentials of canine cardiac Purkinje fibers in control Tyrode''s solution and in the presence of agents thought to modify membrane permeability to K. In Tyrode''s solution decreasing [Ca2+]O from 2.7-0.9 mM increased action potential duration measured at -60 mV (APD-60) and at full repolarization (APD100) but did not significantly modify the normal linear relationship between cycle length and APD between cycle lengths of 500-4000 ms. 9-Aminoacridine (9-AA) was used to decrease K permeability. At concentrations between 0.01 and 1.5 .times. 10-5 M, 9-AA caused a concentration-dependent increase in APD-60 and APD100 and a significant increase in the slope of the line relating APD to cycle length. The usual effects of changes in [Ca2+]O on APD were potentiated by 9-AA. Lidocaine (L) was used to increase K permeability. At concentrations between 0.75 and 1.5 .times. 10-5 M, L significantly increased the slope of the line relating APD and cycle length. In the presence of L an increase in [Ca2+]O increased APD-60 and APD100, and a decrease in [Ca2+]O decreased both parameters. 9-AA potentiated and L reversed the effects of changing [Ca2+]O on APD. Changes in [Ca2+]O apparently modify repolarization by modifying slow inward current and K permeability and the extent to which the latter changes can determine the effect of [Ca2+]O on APD.