REMOVAL OF SIALIC-ACID ALTERS BOTH T-TYPE AND L-TYPE CALCIUM CURRENTS IN CARDIAC MYOCYTES

The whole cell configuration of the patch-clamp technique was used to test the hypothesis that the presence of sialic acid residues influences both T- and L-type Ca2+ currents (I(Ca,T) and I(Ca,L)) in cultured pacemaker cells isolated from the rabbit sinoatrial node. Removal of these anionic sugar moieties by neuraminidase (1.0 U/ml for 5-20 min) increased I(Ca,T) in five of nine cells (by a factor of 2.2-5.1) and I(Ca,L) in three of six cells (by a factor of 1.2-1.6). In cells that did not exhibit such an increase, the enzyme reduced I(Ca,T) but had no significant effect on I(Ca,L). In cells that exhibited an increase in I(Ca,T), exposure to neuraminidase also shifted the activation curve to more negative potentials and increased the slope of the inactivation curve. The enzyme did not influence the gating of I(Ca,L) or the rates of inactivation of either I(Ca,T) or I(Ca,L). The enhancement of I(Ca,T) and I(Ca,L) could not be mimicked by including neuraminidase in the patch pipette or by adding a contaminant of the enzyme preparation, phospholipase C, to the bath. When external Ca2+ was replaced by Ba2+, neither I(Ca,T) nor I(Ca,L) was increased significantly by neuraminidase. It is proposed that by removing sialic acid residues neuraminidase might directly alter the gating of T-type Ca2+ channels. On the other hand, the increased amplitudes of I(Ca,T) and I(Ca,L) might be due to a rise in intracellular Ca2+.