Glycosylation influences voltage-dependent gating of cardiac and skeletal muscle sodium channels

The role of glycosylation on voltage-dependent channel gating for the cloned human cardiac sodium channel (hHla) and the adult rat skeletal muscle isoform (μl) was investigated in HEK293 cells transiently transfected with either hHla or μl cDNA. The contribution of sugar residues to channel gating was examined in transfected cells pretreated with various glycosidase and enzyme inhibitors to deglycosylate channel proteins. Pretreating transfected cells with enzyme inhibitors castanospermine and swainsonine, or exo- glycosidase neuroaminidase caused 7 to 9 mV depolarizing shifts of V₁/₂ for steady-state activation of hHla, while deglycosylation with corresponding drugs elicited about the same amount of depolarizing shifts (8 to 9 mV) of V₁/₂ for steady-state activation of μl. Elevated concentrations of extracellular Mg²⁺ significantly masked the castanospermine-elicited depolarizing shifts of V₁/₂ for steady-state activation in both transfected hHla and μl. For steady-state activation, deglycosylation induced depolarizing shifts of V₁/₂ for hHla (10.6 to 12 mV), but hyperpolarizing shifts for μl (3.6 to 4.4 mV). Pretreatment with neuraminidase had no significant effects on single-channel conductance, the mean open time, and the open probability. These data suggest that glycosylation differentially regulates Na channel function in heart and skeletal muscle myocytes.