Characterization of voltage operated R-type Ca2+ channels in modulating somatostatin receptor subtype 2- and 3-dependent inhibition of insulin secretion from INS-1 cells

Somatostatin (SST) inhibits Ca2+ entry into pancreatic B-cells via voltage-operated Ca2+ channels (VOCCs) of L-type, leading to the suppression of insulin secretion. Activation of R-type channels increases insulin secretion. However, the role of R-type Ca2+ channels (CaV2.3) in mediating the effects of SST on insulin secretion has not been so far investigated. Here, we identify the SST-receptor subtypes (SSTR) expressed on insulin-producing INS-1 cells by RT-PCR and by functional assays. The role of R-type channels in regulating [Ca2+]i in response to SST-treatment was detected by cell fluorescence imaging and patch-clamp technique. INS-1 expressed SSTR2 and SSTR3 and agonists (ag.) selective for these receptors reduced 10 nM exendin-4/20 mM glucose-stimulated insulin secretion. Surprisingly, SST and SST2-ag. transiently increased [Ca2+]i. Subsequently, these agonists led to a decrease in [Ca2+]i below the basal levels. In contrast, SST3-ag. failed to induce a transient peak of [Ca2+]i. Instead, a persistent minor suppression of [Ca2+]i was detected from 25 min. R-type channel blocker SNX-482 altered [Ca2+]i in SST- and SST2-ag.-treated cells. Notably, the inhibition of insulin secretion by SST and SST2-ag., but not SST3-ag. was attenuated by SNX-482. Taken together, SST and SSTR2 regulate [Ca2+]i and insulin secretion in INS-1 cells via R-type channels. In contrast, the R-type calcium channel does not mediate the effects of SST3-ag. on insulin secretion. We conclude that R-type channels play a major role in the inhibition of insulin secretion by somatostatin in INS-1 cells.