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.
Bibliographical noteFunding Information:
This research was supported in part by grants from the American Heart Association, Minnesota Affiliate. and from the National Institute ofGenera Medical Studies (GM 33479). D. J. DuBourdieu is the recipient of a T. H. Rowe11 Graduate Fellowship.