Sodium- and calcium-dependent steps in the mechanism of neonatal rat cardiac myocyte killing by ionophores. II. The calcium-carrying lonophore, A23187

Several lines of evidence indicate a role for elevated intracellular Ca²⁺ in mechanisms of cell killing induced by a wide variety of agents. Cardiac myocytes are susceptible to killing under various conditions, including ischemia and exposure to monensin. In order to delineate the Ca²⁺-dependent cell killing mechanism(s) to which cardiac myocytes are susceptible, we have investigated the mechanism by which they are killed by Ca²⁺ plus the divalent cation ionophore A23187. Evidence has been obtained for two Ca²⁺-mediated injury steps followed by a Na⁺-mediated step leading to cell death detected as membrane permeabilization to trypan blue dye. The first Ca²⁺-mediated step requires the presence of A23187 and low extracellular Ca²⁺ concentrations (1-100 μm) and is inhibited by Mn²⁺ and Ni²⁺ ions. The second Ca²⁺-dependent step requires extracellular Ca²⁺ concentrations in approximately the physiological range (>1 mm), is not dependent on ionophore, and is not inhibited by Mn²⁺. Arachidonic acid release occurs during both Ca²⁺-mediated steps, but mostly during the second step. The second injury step is characterized by visible cell swelling and release of lactate dehydrogenase enzyme activity. The Na⁺-dependent step requires extracellular Na⁺ equal to or greater than half the physiological concentration (i.e., ≥75 mm). Li⁺, which has a smaller ionic radius than Na⁺, can partially substitute for it in the Na⁺-dependent step, whereas K⁺, Cs⁺, Rb⁺, and NH₄⁺ (which have larger ionic radii) cannot.