Swelling-induced changes in cytosolic [Ca2+] in insulin-secreting cells: arole in regulatory volume decrease?

Exposure of insulin-secreting cells to hypotonic solutions causes cell swel ling followed by regulatory volume decrease (RVD). We have previously demon strated that RVD is due to activation of a Cl- conductance. The present stu dy investigates whether changes in cytosolic [Ca-2+] play a role in these r esponses. Hypotonic swelling of RINm5F insulinoma cells caused a marked inc rease in cytosolic [Ca2+] This effect was abolished by omission of extracel lular Ca2+, by the Ca2+ channel blockers D600 or Gd3+ and by 4,4 ' -dithioc yanatostilbene-2,2 ' -disulphonic acid (DIDS), an inhibitor of the volume-s ensitive anion. RVD was markedly impaired in the absence of extracellular C a2+, but not by D600 nor by Gd3+. RVD was also inhibited by the maxi-K+ (BK Ca) channel blockers tetraethylammonium (TEA) and iberiotoxin (IbTx), where as the K-ATP channel blocker tolbutamide was ineffective. Cell swelling was accompanied by activation of a K+ conductance which was sensitive to TEA a nd IbTx but not to tolbutamide. It is concluded that cell swelling causes a ctivation of the volume-sensitive anion channel, leading to depolarization and Ca2+ entry via voltage-gated Ca2+ channels. RVD is a Ca2+-dependent pro cess, requiring low 'resting' levels of intracellular [Ca2+]. However, the swelling-induced increase in cytosolic [Ca2+] is not required for RVD to oc cur. RVD depends upon simultaneous activation of Cl and K+ channels. We sug gest that the BKCa channel is the major K+ conductance involved in RVD. (C) 2001 Elsevier Science Ireland Ltd. All rights reserved.