Role of calcium in isoproterenol cytotoxicity to cultured myocardial cells

Primary cultures of rat myocardial cells were used to evaluate the cellular dynamics of calcium accumulation after exposure to isoproterenol (ISO). Non-toxic concentrations of ISO (2.4 × 10^-7 M) caused a gradual increase in myocyte calcium uptake. These effects peaked 3 min after exposrue and returned to control levels within 2 min. Toxic concentrations of ISO caused a biphasic increase in calcium uptake. The initial phase peaked 1 min after exposure and returned to control levels by 3 min. A second phase was characterized by a progressive increase in calcium uptake that plateaued 10 min after exposure. Ascorbic acid (AA, 5 × 10^-3 M) and sodium bisulfite (SB, 9.6 × 10^-4 M) did not modify the calcium uptake of the initial phase, whereas propranolol (1 × 10^-6 M) and verapamil (1 × 10^-5 M) prevented the initial rise in calcium uptake. In contrast, the antioxidants prevented the second phase of ISO-induced calcium uptake, whereas verapamil and propranolol did not. The toxic accumulation of calcium induced by ISO may be due to oxidative damage of the sarcolemma. Antioxidants may prevent the formation of oxidative metabolites from ISO and the subsequent calcium overload. Our results show that agents which modify slow calcium-channel transport do not prevent ISO-induced calcium overload in our cell culture system.