Key role of Ca in the production of noncoronarogenic myocardial necroses.

Skeletal and heart muscle fibers undergo severe functional and structural alterations, resulting in necrotization as soon as extracellular Ca ions penetrate excessively into the sarcoplasm, so that the capacities of the Ca binding or extrusion processes become insufficient. In mechanically injured skeletal muscle fibers, this necrotization process begins in the neighborhood of the membrane lesion where a large Ca inward transport takes place. Accordingly, elimination of Ca from the Ringer solution or an outward electric current which blocks the influx of extracellular Ca prevents the onset of necrotization, whereas additional Ca or an inward electric current which augments the influx of Ca potentiates the course of degradation. The crucial reaction in the production of necroses in skeletal and heart muscle fibers is a high energy phosphate deficiency which results (a) from excessive activation of Ca-dependent intracellular ATPases, and (b) from Ca-induced mitochondrial destruction. This applies especially to myocardial fiber damage caused by large doses of beta-adrenergic catecholamines such as isoproterenol. The number and size of the isoproterenol-induced cardiac lesions are obviously determined by the extent and, particularly, by the duration of the Ca-mediated high energy phosphate penury. Substances which sensitize the myocardium to catecholamine-induced necrotization (9-alpha-fluorocortisol, dihydrotachysterol, NaH2PO4) act by potentiating intracellular Ca overload and high energy phosphate breakdown. Conversely, verapamil D 600, and other Ca-antagonistic compounds protect the structural integrity of the heart muscle fibers by restricting transmembrane Ca influx and, consequently, ATP and creatine phosphate exhaustion.