Stimulus-secretion coupling of hypotonicity-induced insulin release in BRIN-BD11 cells

Abstract

The stimulus-secretion coupling for hypotonicity-induced insulin release was investigated in BRIN-BD11 cells. A 50 mM decrease in extracellular NaCl caused a twofold increase in insulin release. The release of insulin evoked by hypotonicity progressively decreased in an exponential manner. The response to extracellular hypotonicity displayed a threshold value close to 20 mOsmol/L and a maximal response at about 70 mOsmol/ L. Hypotonicity also caused a rapid increase in cell volume followed by a regulatory volume decrease (RVD), cell membrane depolarization with induction of spike activity, and a rise in cytosolic Ca2+ concentration. 5-Nitro-2-(3-phenylpropylamino)benzoate inhibited the secretory response to hypoosmolarity, failed to affect the early increase in cell volume but prevented the RVD, and suppressed the hypotonicity-induced plasma membrane depolarization. Insulin release provoked by hypotonicity was inhibited by verapamil, absence of Ca2+, thapsigargin, furosemide, tributyltin, and diazoxide. On the contrary, tolbutamide augmented modestly insulin release recorded in the hypoosmolar medium. Last, a rise in extracellular K+ concentration, while augmenting basal insulin output, failed to affect insulin release in the hypoosmolar medium. Thus, the insulin secretory response to hypotonicity apparently represents a Ca2+-dependent process triggered by the gating of volume-sensitive anion channels with subsequent depolarization and gating of voltage-sensitive Ca2+ channels.

Contrasting Effects of Glycerol and Urea Transport on Rat Pancreatic β-Cell Function

“…All the above effects are clearly reminiscent of the effects of hypo-osmotic cell swelling on these parameters described earlier, and suggest that ionic effects are indeed implicated in swelling-induced β-cell activation. Such ionic effects of cell swelling probably explain the finding that swellinginduced insulin release from BRIN-BD11 cells was sensitive to removal of extracellular Ca 2+ [1]. This does not of course preclude the possibility of additional, possibly distal effects of cell swelling on the exocytotic pathway.…”

“…However, this study was designed to investigate the effects of these permeant osmolytes on β-cell volume and activation, rather than to reflect possible effects of circulating glycerol or urea on β-cell function. Such high concentrations of osmolytes were used to in order to generate an osmotic gradient comparable to that produced by the removal of 50 mM NaCl from the extracellular medium, a manoeuvre previously shown to induce cell swelling and VRAC activation [1]. The finding that such isosmotic addition of urea or glycerol causes rapid β-cell swelling implies that the cell membrane is highly permeable to these osmolytes.…”

“…Indeed, several studies have demonstrated that exposure of insulin-secreting cells to hypotonic extracellular solutions causes cell swelling and depolarization of the cell membrane potential, leading to electrical activity and insulin release [1][2][3][4][5][6][7]. We have suggested that activation of the volume-regulated anion channel (VRAC) plays a central role in these responses 256 [8].…”

Expression of the Electrogenic Na^+-HCO₃^--Cotransporter NBCe1 in Tumoral Insulin-Producing BRIN-BD11 Cells

“…BRIN-BD11 cells were grown as described elsewhere [5]. Some experiments were also conducted in INS-1 [6] and MIN6 cells [7].…”

Ca2+-dependent desensitization of insulin secretion by strong potassium depolarization

“…The ATP dependency may involve the ATPase of the insulin granules, the activity of which is required for the granule acidification. The link between Na ϩ and H ϩ concentration is given by the Na ϩ /H ϩ exchange and the Na ϩ -dependent HCO 3 Ϫ /Cl Ϫ exchange at the ␤-cell plasma membrane (22,32).…”