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 (3 micrograms/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.