title: Late sodium current (INaL) in pancreatic β-cells.
abstract: Recent evidence of beneficial effects of ranolazine (RAN) in type II diabetes motivates interest in the role of the late sodium current (INaL) in glucose-stimulated insulin secretion. In the present work, we characterize INaL and its function in rat INS-1E cells and human islets cells. INaL was identified as steady-state current blocked by 10 μM RAN (IRAN) or 0.5 μM tetrodotoxin (TTX) (ITTX). Veratridine (VERA, 40 μM) was used as INaL enhancer. Baseline INaL 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. ITTX and IRAN 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 INaL coupling with Na(+)-activated K(+) current (IKNa); expression of IKNa channels (Slick/Slack) was confirmed by transcript analysis and Western blot. RAN or TTX blunted cytosolic Ca(2+) response to depolarization. Long-term incubation in high (33 mM) glucose (CHG) constitutively enhanced INaL. 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) INaL was expressed in insulin-secreting cells and coupled to IKNa; INaL affected cytosolic Ca(2+) but, unless enhanced, barely contributed to glucose-stimulated insulin secretion (GSIS); and (2) sustained hyperglycemic stress enhanced INaL, which contributed to the attending increase of glucose-independent insulin "leak" and GSIS impairment.
authors.affiliation: Department of Biotechnology and Biosciences, University of Milano-Bicocca, Piazza della Scienza 2, 20126, Milan, Italy.
authors.forename: Riccardo
authors.initials: R
authors.lastname: Rizzetto
authors.forename: Marcella
authors.initials: M
authors.lastname: Rocchetti
authors.forename: Luca
authors.initials: L
authors.lastname: Sala
authors.forename: Carlotta
authors.initials: C
authors.lastname: Ronchi
authors.forename: Alice
authors.initials: A
authors.lastname: Villa
authors.forename: Mara
authors.initials: M
authors.lastname: Ferrandi
authors.forename: Isabella
authors.initials: I
authors.lastname: Molinari
authors.forename: Federico
authors.initials: F
authors.lastname: Bertuzzi
authors.forename: Antonio
authors.initials: A
authors.lastname: Zaza
mesh_terms: D000818:Animals; D002460:Cell Line; D005947:Glucose / Q000494:pharmacology; D006801:Humans; D007004:Hypoglycemic Agents / Q000494:pharmacology; D007328:Insulin / Q000378:metabolism; D000078790:Insulin Secretion; D050417:Insulin-Secreting Cells / Q000187:drug effects / Q000378:metabolism*; D008564:Membrane Potentials; D009419:Nerve Tissue Proteins / Q000037:antagonists & inhibitors / Q000235:genetics / Q000378:metabolism; D026902:Potassium Channel Blockers / Q000494:pharmacology; D015221:Potassium Channels / Q000187:drug effects / Q000235:genetics / Q000378:metabolism; D000081033:Potassium Channels, Sodium-Activated; D012333:RNA, Messenger / Q000378:metabolism; D051381:Rats; D012964:Sodium / Q000378:metabolism*; D013997:Time Factors; D061567:Voltage-Gated Sodium Channel Blockers / Q000494:pharmacology; D061566:Voltage-Gated Sodium Channels / Q000187:drug effects / Q000235:genetics / Q000378:metabolism*
chemical_list: D007004:Hypoglycemic Agents; D007328:Insulin; C480493:Kcnt2 potassium channel, rat; D009419:Nerve Tissue Proteins; D026902:Potassium Channel Blockers; D015221:Potassium Channels; D000081033:Potassium Channels, Sodium-Activated; D012333:RNA, Messenger; D061567:Voltage-Gated Sodium Channel Blockers; D061566:Voltage-Gated Sodium Channels; C000626167:kcnt1 protein, rat; D012964:Sodium; D005947:Glucose