ADRENERGIC EFFECTS ON THE BIOLOGY OF THE ADULT MAMMALIAN CARDIOCYTE

Background. To delineate the mechanism(s) of catecholamine-mediated cardiac toxicity, we exposed cultures of adult cardiac muscle cells, or cardiocytes, to a broad range of norepinephrine concentrations. Methods and Results. Norepinephrine stimulation resulted in a concentration-dependent decrease in cardiocyte viability, as demonstrated by a significant decrease in viable rod-shaped cells and a significant release of creatine kinase from cells in norepinephrine-treated cultures. Norepinephrine-mediated cell toxicity was attenuated significantly by beta-adrenoceptor blockade and mimicked by selective stimulation of the beta-adrenoceptor, whereas the effects mediated by the alpha-adrenoceptor were relatively less apparent. When norepinephrine stimulation was examined in terms of cardiocyte anabolic activity, there was a concentration-dependent decrease in the incorporation of [H-3]phenylalanine and [H-3]uridine into cytoplasmic protein and nuclear RNA, respectively. The decrease in cytoplasmic labeling was largely attenuated by beta-adrenoceptor blockade and mimicked by selective stimulation of the beta-adrenoceptor, but alpha-adrenoceptor stimulation resulted in relatively minor decreases in cytoplasmic labeling. The norepinephrine-induced toxic effect appeared to be the result of cyclic AMP-mediated calcium overload of the cell, as suggested by studies in which pharmacological strategies that increased intracellular cyclic AMP led to decreased cell viability, as well as studies that showed that influx of extracellular calcium through the verapamil-sensitive calcium channel was necessary for the induction of cell lethality. Additional time-course studies showed that norepinephrine caused a rapid, fourfold increase in intracellular cyclic AMP, followed by a 3.2-fold increase in intracellular calcium ([Ca2+]i). Conclusions. These results constitute the initial demonstration at the cellular level that adrenergic stimulation leads to cyclic AMP-mediated calcium overload of the cell, with a resultant decrease in synthetic activity and/or viability.