Altered functions of ion channels in diabetic beta cells

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 Ca(2+) entry into pancreatic B-cells via voltage-operated Ca(2+) 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 Ca(2+) channels (Ca(V)2.3) 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 [Ca(2+)](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 [Ca(2+)](i). Subsequently, these agonists led to a decrease in [Ca(2+)](i) below the basal levels. In contrast, SST3-ag. failed to induce a transient peak of [Ca(2+)](i). Instead, a persistent minor suppression of [Ca(2+)](i) was detected from 25 min. R-type channel blocker SNX-482 altered [Ca(2+)](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 [Ca(2+)](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.

Role of L-type Ca2+ channels in attenuated morphine antinociception in streptozotocin-diabetic rats

The role of L-type Ca2+ channels in morphine antinociception was studied in streptozotocin-induced diabetic and control rats, using [3H]PN200-110 [isopropyl 4-(2,1,3-benzoxadiazol-4-yl)-1,4-dihydro-2,6-dimethyl-5-methoxycabonylpyridine-3-caboxylate] binding (0.005-1.0 nmol/l) and rat whole brain membranes, to determine if the attenuation of morphine antinociception was related to alterations in dihydropyridine-sensitive Ca2+ channel binding characteristics. The tail-flick antinociceptive effect of morphine (4 mg/kg, i.p.) was significantly reduced in diabetic rats in comparison to that in controls. Nimodipine (0.1-3 mg/kg, i.p.) did not produce antinociception but significantly potentiated the morphine response in control rats. Nimodipine (0.3-3 mg/kg, i.p.) reversed the attenuation of morphine antinociception in a dose-dependent manner in diabetic rats. Moreover, insulin (2 U/kg, s.c.) [correction of mu/kg] reversed the attenuated morphine antinociception in streptozotocin-diabetic rats. A significant increase in the Bmax (+41%) of [3H]PN200-110 binding was observed in diabetic rat brain membranes compared to that in control rats. However, there was no change in affinity (Kd) value of [3H]PN200-110. The reduced morphine response in diabetic rats, in accordance with up-regulation of dihydropyridine sites, may be due to an increased Ca2+ influx through L-type channels. These results suggest a functional role of L-type Ca2+ channels in morphine antinociception and the diabetic state may lead to alterations in their density.

Effect of streptozotocin-induced diabetes on the activity of calcium channels in rat dorsal horn neurons

We have previously found that spinal dorsal horn neurons from streptozotocin-diabetic rats, an animal model for diabetes mellitus, show the prominent changes in the mechanisms responsible for [Ca2+]i regulation. The present study aimed to further characterize the effects of streptozotocin-induced diabetes on neuronal calcium homeostasis. The cytoplasmic Ca2+ concentration ([Ca2+]i) was measured in Fura-2AM-loaded dorsal horn neurons from acutely isolated spinal cord slices using fluorescence technique. We studied Ca2+ entry through plasmalemmal Ca2+ channels during potassium (50 mM KCl)-induced depolarization. The K+-induced [Ca2+]i elevation was inhibited to a different extent by nickel ions, nifedipine and omega-conotoxin suggesting the co-expression of different subtypes of plasmalemmal voltage-gated Ca2+ channels. The suppression of [Ca2+]i transients by Ni2+ (50 microM) was the same in control and diabetic neurons. On the other hand, inhibition of [Ca2+]i transients by nifedipine (50 microM) and omega-conotoxin (1 microM) was much greater in diabetic neurons compared with normal animals. These data suggest that under diabetic conditions the activity of N- and L- but not T-type voltage-gated Ca2+ channels substantially increased in dorsal horn neurons.