Abstract

Sulfatide (3′-sulfo-β-galactosyl ceramide) is a glycosphingolipid present in mammalians in various fatty acid isoforms of which the saturated 16 carbon-atom length (C16:0) is more abundant in pancreatic islets than in neural tissue, where long-chain sulfatide isoforms dominate. We previously reported that sulfatide isolated from pig brain inhibits glucose-induced insulin secretion by activation of ATP-sensitive K⁺ channels (K_ATP channels). Here, we show that C16:0 sulfatide is the active isoform. It inhibits glucose-stimulated insulin secretion by reducing the sensitivity of the K_ATP channels to ATP. (The half-maximal inhibitory concentration is 10.3 and 36.7 μmol/l in the absence and presence of C16:0 sulfatide, respectively.) C16:0 sulfatide increased whole-cell K_ATP currents at intermediate glucose levels and reduced the ability of glucose to induce membrane depolarization, reduced electrical activity, and increased the cytoplasmic free Ca²⁺ concentration. Recordings of cell capacitance revealed that C16:0 sulfatide increased Ca²⁺-induced exocytosis by 215%. This correlated with a stimulation of insulin secretion by C16:0 sulfatide in intact rat islets exposed to diazoxide and high K⁺. C24:0 sulfatide or the sulfatide precursor, β-galactosyl ceramide, did not affect any of the measured parameters. C16:0 sulfatide did not modulate glucagon secretion from intact rat islets. In βTC3 cells, sulfatide was expressed (mean [±SD] 0.30 ± 0.04 pmol/μg protein), and C16:0 sulfatide was found to be the dominant isoform. No expression of sulfatide was detected in αTC1-9 cells. We conclude that a major mechanism by which the predominant sulfatide isoform in β-cells, C16:0 sulfatide, inhibits glucose-induced insulin secretion is by reducing the K_ATP channel sensitivity to the ATP block.