Toxicosis by
monensin, a Na(+)-selective
ionophore, induces skeletal and cardiac muscle
necrosis. Cultured neonatal rat cardiac myocytes were killed by
monensin (greater than 0.2 micrograms/ml) beginning at 30 min and completing by 60-90 min. Other cultured cell types presumably lacking excitable membranes were not killed by
monensin under these conditions. Cardiac myocytes were also killed by
nigericin and
nonactin (
monovalent cation-carrying
ionophores with low ion selectivity), but not by
valinomycin, which has a high selectivity for K+.
Monensin-induced killing was associated with formation of
blebs in cell membranes and subsequent swelling of the cells during the early phases of killing, whereas surface membranes of cells permeabilized to
trypan blue dye contained discrete small holes visible by scanning electron microscopy.
Monensin-induced killing occurred at extracellular Na+ concentrations greater than or equal to 10 mM, but not when Li+, K+, Cs+, Rb+, or
choline ions replaced Na+ at concentrations up to 0.15 M. Killing was prevented at extracellular pH values less than or equal to 6.4 and was enhanced by
ouabain, an inhibitor of Na+/K(+)-
ATPase-mediated Na+ transport. Several characteristics of
monensin-induced cardiac myocyte killing were similar to those observed during killing induced by the Ca(2+)-carrying
ionophore,
A23187 plus Ca2+, including a requirement for extracellular Ca2+ concentrations greater than 0.5 mM, inhibition by Mn2+ and Ni2+, and an associated stimulation of
arachidonic acid release. The cell killing characteristics are consistent with a
monensin-induced Na+ influx which admits toxic levels of extracellular Ca2+ to the cytoplasm of cells with excitable membranes, possibly via Na+/Ca2+
antiporter protein(s).