BRIN-BD11 cells were used as a model of cultured b cells. The exchange of the salt-balanced incubation medium for a hypotonic solution by reducing NaCl concentration induced an increase in insulin secretion as previously reported in b HC9 cells. The present study aimed at identifying the mechanism(s) responsible for this increase in insulin secretion.

In most cells, hypotonicity induces a rapid swelling which leads to the opening of anion and K⁺ channels resulting in an efflux of these ions accompanied by osmotically-obligated water. This results in a return of the cell volume towards its initial value. These anion channels are usually referred to as Volume Sensitive Anion Channels (VSAC) and are inhibited by chloride channel inhibitors such as NPPB.

In BRIN-BD11 cells, insulin output increased from 15.9 ± 0.8 to 47.3 ± 7.9 mU/30 min per well when changing the standard medium to an hypotonic medium. In the presence of 0.1 mM NPPB, the response to hypotonicity was decreased, with control and experimental values of 18.5 ± 1.0 and 24.3 ± 1.5 mU/30 min. The increase in insulin output was related to the magnitude of the decrease in osmolality.

Since the electrochemical potential for Cl^- is thought to be higher inside than outside the b cells, the hypotonicity-activated VSAC most probably mediate a Cl^- efflux, hence depolarizing the cell membrane. Indeed, preincubation with furosemide, an inhibitor of the main Cl^- entry mechanism (the Na-K-2 Cl^- cotransporter) led to a 22% decrease in insulin output under hypotonic conditions.

The secretory response to hypotonicity was decreased by verapamil (10 mM) and by incubation in a Ca²⁺-deprived buffer containing 0.5 mM EGTA. The latter condition also reduced basal insulin release.

In conclusion, the present data suggest that the increase in insulin release from b cells exposed to a hypotonic medium results from the activation of VSAC which leads to cell depolarization and opening of voltage-dependent calcium channels.