A volume-activated anion conductance in insulin-secreting cells

Abstract

The whole-cell patch-clamp recording technique was used to measure volume-activated currents in K+-free solutions in RINm5F and HIT-T15 insulinoma cells and in dispersed rat islet cells. Cell swelling, induced by intracellular hypertonicity or extracellular hypotonicity, caused activation of an outwardly rectifying conductance which could be subsequently inactivated by hypertonic extracellular solutions. The conductance required adenosine 5'-triphosphate (ATP) in the pipette solution but was Ca2+ independent. Na+ and Cl- substitution studies suggested that the swelling-activated current is Cl- selective with a halide permeability sequence of Br > Cl > I. The conductance was reversibly inhibited by the anion channel inhibitors 4,4'-diisothiocyanatostilbene-2,2'-disulphonic acid (DIDS) and by 5-nitro-2-(3-phenylpropylamino) benzoic acid (NPPB). Further evidence for a volume-activated anion conductance was provided by studies of volume regulation in insulin-secreting cells. When RINm5F cells were exposed to a hypotonic medium, the initial cell swelling was followed by a regulatory volume decrease (RVD). This RVD response was also inhibited by DIDS and by NPPB. These data therefore provide evidence for a volume-activated anion conductance in insulin-secreting cells which could be involved in the RVD following osmotic stress. A possible role for the conductance in hypotonically induced insulin release is also discussed.

Tolbutamide potentiates the volume-regulated anion channel current in rat pancreatic beta cells

“…This result may seem inconsistent with a previous report from this laboratory indicating that electrical activity in response to a high concentration of tolbutamide was resistant to inhibition by DIDS [18]. However, it should be noted that DIDS is a relatively weak inhibitor of the VRAC, particularly the inward current component [7]. This could explain why DIDS failed to inhibit the effects of a high concentration of tolbutamide but was effective in inhibiting electrical activity evoked by a lower concentration of the sulphonylurea in the present study.…”

“…Since direct recordings of K ATP channel activity indicate that these channels are closed at glucose concentrations of 5 mmol/l and above [5], it is likely that sulphonylureas can exert additional ionic effects on the beta cell. The aim of the present study was to test the hypothesis that sulphonylureas influence the glucose-and volume-regulated anion channel (VRAC) current previously described in beta cells [6,7].…”

Chloride Channels Regulate HIT Cell Volume but Cannot Fully Account for Swelling-Induced Insulin Secretion

“…The discovery of a novel anion-selective channel in pancreatic islet ␤-cells by Kinard and Satin (5) and Best et al (6) suggests that some of the previously observed effects of anions on islet function may be mediated by an anion-permeable ion channel. This anion-permeable pancreatic islet ␤-cell Cl Ϫ channel (I Cl,islet ) is an outwardly rectifying Cl Ϫ channel that is activated by cell swelling (induced by the application of hypotonic solution) or by a rise in intracellular cAMP (5).…”

“…I Cl,islet is inhibited by the well-known Cl Ϫ channel blocker 4,4Ј-diisothiocyanatostilbene-2,2Ј-disulfonic acid (DIDS) or by 100 mol/l glyburide (5). The channel may also be under metabolic control, either by virtue of its sensitivity to intracellular ATP (5,6) or because of ␤-cell swelling associated with increased glucose metabolism (7).…”

“…Due to the relatively depolarized reversal potential of I Cl,islet (-30 mV) (5), its activation under physiological conditions would be expected to depolarize ␤-cells by producing a net inward current mediated by a net efflux of anions (5,6). Thus, I Cl,islet activation after increased glucose metabolism would lead to ␤-cell depolarization, the activation of voltage-activated Ca 2ϩ channels, increased Ca 2ϩ influx, and enhanced Ca 2ϩ -dependent insulin secretion.…”