Swelling-induced changes in cytosolic [Ca 2+] in insulin-secreting cells: a role in regulatory volume decrease?

Abstract

Exposure of insulin-secreting cells to hypotonic solutions causes cell swelling followed by regulatory volume decrease (RVD). We have previously demonstrated that RVD is due to activation of a Cl − conductance. The present study investigates whether changes in cytosolic [Ca 2+] play a role in these responses. Hypotonic swelling of RINm5F insulinoma cells caused a marked increase in cytosolic [Ca 2+]. This effect was abolished by omission of extracellular Ca 2+, by the Ca 2+ channel blockers D600 or Gd 3+and by 4,4′-dithiocyanatostilbene-2,2′-disulphonic acid (DIDS), an inhibitor of the volume-sensitive anion. RVD was markedly impaired in the absence of extracellular Ca 2+, but not by D600 nor by Gd 3+. RVD was also inhibited by the maxi-K + (BK Ca) channel blockers tetraethylammonium (TEA) and iberiotoxin (IbTx), whereas the K ATP channel blocker tolbutamide was ineffective. Cell swelling was accompanied by activation of a K + conductance which was sensitive to TEA and IbTx but not to tolbutamide. It is concluded that cell swelling causes activation of the volume-sensitive anion channel, leading to depolarization and Ca 2+ entry via voltage-gated Ca 2+ channels. RVD is a Ca 2+-dependent process, requiring low ‘resting’ levels of intracellular [Ca 2+]. However, the swelling-induced increase in cytosolic [Ca 2+] is not required for RVD to occur. RVD depends upon simultaneous activation of Cl − and K + channels. We suggest that the BK Ca channel is the major K + conductance involved in RVD.