Dependence of Cardiac Cell Ca2+Permeability on Sialic Acid-containing Sarcolemmal Gangliosides

The specific removal of negatively-charged sialic acid by neuraminidase produces a large increase in cardiac myocyte Ca uptake (17.3±1.1 mmol Ca/kg dry weight) and marked cell contracture. Importantly, the insertion of the negatively-charged amphiphile dodecyl sulfate in the sarcolemma eliminates the increased calcium uptake and preserves contractile function. In the present study, we further examine the role of sialic acid–Ca interaction and, specifically, the role of gangliosides (sialic acid-containing glycolipids) in cardiac cells» Ca permeability. Neonatal cell culture and adult ventricular myocytes were used. The major findings of this study are: (1) while dodecyl sulfate inhibits cellular calcium uptake and contracture development induced by sialic acid removal, cationic and neutral amphiphiles are without effect. (2) Ca channel blockers (nifedipine and protamine) and the Na/Ca exchange inhibitor Ni do not modify the effect of sialic acid removal. (3) A non-classical-channel related whole-cell current appears and increases after 21±2.2 min treatment with 0.02 U/ml neuraminidase (n=4). Incubation with neuraminidase in the presence of dodecyl sulfate greatly delays the appearance of these currents to 44.4±6.1 min (n=4). (4) The use of a specific probe for GM1 ganglioside, the cholera toxin B subunit (3μg/ml), induces a moderate but clear increase in cellular Ca (1.63±0.3 mmol Ca kg dry weight;n=8). However, this increase was not modified by treatment with dodecyl sulfate. (5) Finally, 50 mU/ml endoglycoceramidase, an enzyme which specifically cleaves the link between the sialic acid-containing oligosaccharide and ceramide of gangliosides, induced a significant increase in Ca uptake (4.4±0.9 mmol Ca kg dry weight,n=4). These results indicate the importance of negatively charged sialic acid-containing gangliosides in the maintenance of cardiac cell physiological Ca permeability. The increase in Ca uptake induced by the removal of sialic acid seems to be mediated by development of a Ca “leak” via other than classical cation channels.