Correlation of calcium exchange, structure, and function in myocardial tissue culture

Abstract

45Ca exchange was studied in cultured myocardial cells using the online scintillator-disk flow cell technique. The pattern of exchange was modified by altering the temperature and rate of beating, application of neuraminidase to remove sialic acid, and exposure to La3+. Correlative ultrastructural studies were done. Of the exchangeable Ca in these cells, 90-100% exchanged as rapidly as the flow cell could be exchanged and was displaced by 0.5 mM La. Ultrastructurally the cells demonstrated a well-developed glycocalyx and free sarcoplasmic reticulum (SR) system and a prominent junctional SR. La stained the cell membranes and glycocalyx exclusively. Intracellular structures remained unstained. Increased temperature and rate of beating increased exchangeable Ca by 65%, all of which was rapidly exchangeable and La-displaceable. Sialic acid removal by treatment with neuraminidase increased exchangeable Ca by over 4-fold, and more than 80% of this Ca was La-displaceable. Extensive La deposits were found within the mitochondria in these cells. These cultured cells differed from those previously reported in having 62% less total Ca, all of which was attributable to the loss of a non-La-displaceable, exchangeable component. The La-displaceable and inexchangeable components were virtually equal in both groups of cells and the response to sialic acid removal was similar. In the cells not exposed to neuraminidase the kinetics and La response indicated that the exchangeable Ca was localized at the cell surface and that a component of this Ca participated in contractile activation and returned to the surface without mixing intracellularly.