On the Effects of Divalent Cations

A B $ T R A C T Resting and action potentials were recorded from superfused strips of frog ventricle. Reducing the bathing calcium concentration ([Ca2+]0) with or without ethylene glycol-bis(~-aminoethyl ether)N,N,N',N'-tetraacetate (EGTA) prolongs the action potential (AP). The change in the duration of the AP extends over many minutes, but is rapidly reversed by restoring calcium ions. Other changes (e.g., in resting potential and overshoot) are, however, only more slowly reversed. Reducing [Ca2+]0 with 0.2, 2, or 5 mM EGTA produces progressively greater prolongation of AP; maximum values were well in excess of 1 rain. This prolongation can be reversed by other divalent cations in EGTA (Mg 2+, Sr 2+) or Ca-free (Mn 2+) solutions, or by acetylcholine. Barium ions increase AP duration in keeping with their known effect on potassium conductance. D600, which blocks the slow inward current in cardiac muscle, is without effect on the action potentials recorded in EGTA solutions, or on the time course and extent of the recovery to normal duration upon restoring calcium ions. It is concluded that divalent cations exert an influence on membrane potassium conductance extracellularly in frog heart. The cell membrane does not become excessively "leaky" in EGTA solutions.