Chloride channels regulate HIT cell volume but cannot fully account for swelling-induced insulin secretion

Insulin-secreting pancreatic islet beta -cells possess anion-permeable Cl- channels (I-Cl,I-islet) that are swelling-activated, but the role of these channels in the cells is unclear. The Cl- channel blockers 4,4 ' -diisothio cyanostilbene-2,2 ' -disulfonic acid (DIDS) and niflumic acid were evaluate d for their ability to inhibit I-Cl,I-islet in clonal beta -cells (HIT cell s). Both drugs blocked the channel, but the blockade due to niflumic acid w as less voltage-dependent than the blockade due to DIDS. HIT cell volume in itially increased in hypotonic solution and was followed by a regulatory vo lume decrease (RVD). The addition of niflumic acid and, to a lesser extent, BIBS to the hypotonic solution potentiated swelling and blocked the RVD. I n isotonic solution, niflumic acid produced swelling, suggesting that islet Cl- channels are activated under basal conditions. The channel blockers gl yburide, gadolinium, or tetraethylammonium-Cl did not alter hypotonic-induc ed smelling or volume regulation. The Na/K/2Cl transport blocker furosemide produced cell shrinkage in isotonic solution and blocked cell swelling nor mally induced by hypotonic solution. Perifused HIT cells secreted insulin w hen challenged with hypotonic solutions. However, this could not be complet ely attributed to I-Cl,I-islet-mediated depolarization, because secretion p ersisted even when Cl- channels were fully blocked. To test whether blocker -resistant secretion occurred via a distal pathway, distal secretion was is olated using 50 mmol/l potassium and diazoxide. Under these conditions, glu cose-dependent secretion was blunted, but hypotonically induced secretion p ersisted, even with Cl- channel blockers present. These results suggest tha t beta -cell swelling stimulates insulin secretion primarily via a distal I -Cl,I-islet-independent mechanism, as has been proposed for K-ATP-independe nt glucose- and sulfonylurea-stimulated insulin secretion. Reverse transcri ptase-polymerase chain reaction of HIT cell mRNA identified a CLC-3 transcr ipt in HIT cells. In other systems, CLC-3 is believed to mediate smelling-i nduced onwardly rectifying Cl- channels. This suggests that the proximal ef fects of smelling to regulate cell volume may be mediated by CLC-3 or a clo sely related Cl- channel.