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Myoblasts and fibroblasts in cultured derived from neonatal rat hearts were exposed to neuraminidase and phospholipases C (PLC) and A2 (PLA2). Calcium (Ca) and potassium (K) exchange of the cells was measured before and after enzymatic exposure. The exchange characteristics of control and treated cells were correlated with cellular ultrastructure including assessment of intramembrane particle (IMP) density and aggregation by freeze-fracture. Neuraminidase exposure (removal of sialic acid) known to produce marked increase in calcium permeability without change in potassium permeability in myoblasts produced no change in IMP configuration of these cells. PLC, however, produced marked increase in both Ca and K permeabilities, permitted entry of La, and was associated with IMP aggregation in myoblastic cells. PLA2 produced no change in ionic permeability and no alteration in intramembrane particle configuration in myoblasts. Exposure of fibroblasts to PLC caused no change in either Ca or K permeability and no change in IMP distribution. These results, coupled with those of previous studies of permeability changes induced by sialic acid removal, indicate that control of cellular Ca permeability resides in at least two separate sites at the cellular surface: (1) the glycocalyx and (2) the lipid bilayer. By contrast, K permeability control is based within the bilayer. Ultrastructural correlations suggest that IMP aggregation may be associated with changes in bilayer permeability.

circulation research

Correlation of alterations in cation exchange and sarcolemmal ultrastructure produced by neuraminidase and phospholipases in cardiac cell tissue culture.