What are the effects of calcium antagonists (Mn, La, and D600) on action potentials and plateau currents in cardiac Purkinje fibers?
Calcium antagonists have multiple non-selective effects on plateau currents in cardiac Purkinje fibers, including decreasing slow outward current activation and increasing net outward time-independent plateau current.
We studied the influence of Mn, La, and D600 on action potentials and plateau currents in cardiac Purkinje fibers. The Ca antagonists each abolished the second inward current, but they failed to act selectively. Voltage clamp experiments revealed two additional effects: decrease of slow outward current (iotachi) activation, and increase of net outward time-independent plateau current. These effects occurred at inhibitor concentrations used in earlier studies, and were essential to the reconstruction of observed Ca antagonist effects on electrical activity. The inhibitory influence of Mn, La, and D600 on iotachi suggested that iotachi activation might depend upon prior Ca entry. This hypothesis was not supported, however, when Caomicron was varied: elevating Caomicron enhanced Ca entry, but iotachi was nevertheless depressed. Thus, the results suggested instead that Ca antagonists and Ca ions have rather similar effects on iotachi, possibly mediated by changes in membrane surface charge.
Kass et al. (1975) studied this question.