Perturbation of the sarcolemmal membrane in isoproterenol-induced myocardial injury of the rat. Permeability and freeze-fracture studies in vivo and in vitro.

The mechanism whereby cardiotoxic doses of isoproterenol (ISO) induces early permeability alteration of the sarcolemmal membrane is unknown; both beta-receptor overstimulation and direct toxic effect of ISO oxidation products have been implicated. There has been no morphologic observation, furthermore, on the structural basis of permeability alteration during this process. The purpose of the present study was to compare the morphology of cardiocyte injury induced by ISO and oxidized ISO (ISO-O2) and to visualize perturbation of the sarcolemma correlating with the leaky membrane. The authors studied the left ventricular myocardium of rats 10 and 60 minutes after subcutaneous administration of 85 mg/kg ISO and isolated perfused rat hearts exposed for 10 minutes either to ISO or ISO-O2 in a dose of 100 mg/l (10(-4) M) to determine the permeability of the sarcolemmal membrane using the extracellular diffusion tracer horseradish peroxidase (HRP) by light and thin section electron microscopy, the morphology of the sarcolemmal membrane by means of freeze-fracture electron microscopy, and the density of intramembrane particles (IMP) in the sarcolemmal membrane by planimetry using freeze-fracture electron microscopy. In in vivo rat hearts both 10 and 60 minutes after ISO and in vitro (isolated perfused) rat hearts exposed to either ISO or ISO-O2 for 10 minutes, HRP labeled the sarcoplasm of focally located cardiocytes implicating leakiness of the sarcolemmal membrane. HRP positive cardiocytes (with the exception of the in vivo 10 minute group) showed characteristic features of contraction band necrosis (both on light and thin-section electron microscopy) in all groups. Freeze-fracture electron microscopy of sarcolemmal protoplasmic (P) membrane faces revealed two populations of cardiocytes in all groups. P-membrane faces in one population of cardiocytes appeared as in the control. In the other population of cardiocytes, P-membrane faces showed irregular tears. Planimetry demonstrated a significant decrease of IMP numerical densities in P-membrane faces with tears in the in vivo 10 minute group and both with or without tears in the in vivo 60 minutes group and the in vitro groups compared with the control values. Furthermore, with the exception of the 10 minute in vivo group, IMP densities significantly decreased in sarcolemmal membranes with tears compared with those without tears in all experimental groups. These observations are consistent with the view that catecholamine induced myocardial injury is, at least partly, related to the direct toxic effect of catecholamine oxidation products on the sarcolemmal membrane.(ABSTRACT TRUNCATED AT 400 WORDS)