Late sodium current (I-NaL) in pancreatic beta-cells

摘要

Recent evidence of beneficial effects of ranolazine (RAN) in type II diabetes motivates interest in the role of the late sodium current (I-NaL) in glucose-stimulated insulin secretion. In the present work, we characterize I-NaL and its function in rat INS-1E cells and human islets cells. I-NaL was identified as steady-state current blocked by 10 mu M RAN (I-RAN) or 0.5 mu M tetrodotoxin (TTX) (I-TTX). Veratridine (VERA, 40 mu M) was used as I-NaL enhancer. Baseline I-NaL was similar between INS-1E and human islet cells. In INS-1E cells, activated by glucose or tolbutamide, TTX or RAN hyperpolarized membrane potential (V (m)). VERA-induced depolarization was countered by TTX or RAN. I-TTX and I-RAN reversal potentials were negative to Na+ equilibrium one, but they approached it after Na+ substitution with Li+ or when K+ channels were blocked. This revealed I-NaL coupling with Na+-activated K+ current (I-KNa); expression of I-KNa channels (Slick/Slack) was confirmed by transcript analysis and Western blot. RAN or TTX blunted cytosolic Ca2+ response to depolarization. Long-term incubation in high (33 mM) glucose (CHG) constitutively enhanced I-NaL. VERA immediately increased glucose-stimulated insulin secretion. CHG increased glucose-independent secretion instead and abolished the secretory response to glucose. RAN or TTX countered VERA- and CHG-induced changes in insulin secretion. Our study demonstrated that (1) I-NaL was expressed in insulin-secreting cells and coupled to I-KNa; I-NaL affected cytosolic Ca2+ but, unless enhanced, barely contributed to glucose-stimulated insulin secretion (GSIS); and (2) sustained hyperglycemic stress enhanced I-NaL, which contributed to the attending increase of glucose-independent insulin "leak" and GSIS impairment.