In isolated rat pancreatic β-cells, the nitric oxide (NO) donor NOC-7 at 1 μM reduced the amplitude of the oscillations of cytosolic Ca²⁺ concentration ([Ca²⁺]_c) induced by 11.1 mM glucose, and at 10 μM terminated them. In the presence of N ^G-nitro-l-arginine (l-NNA), however, NOC-7 at 0.5 and 1 μM increased the amplitude of the [Ca²⁺]_c oscillations, although the NO donor at 10 μM still suppressed them. Aqueous NO solution also had a dual effect on the [Ca²⁺]_c oscillations. The soluble guanylate cyclase inhibitor LY-83583 and the cGMP-dependent protein kinase inhibitor KT5823 inhibited the stimulatory effect of NO, and 8-bromo-cGMP increased the amplitude of the [Ca²⁺]_c oscillations. Patch-clamp analyses in the perforated configuration showed that 8-bromo-cGMP inhibited whole cell ATP-sensitive K⁺ currents in the isolated rat pancreatic β-cells, suggesting that the inhibition by cGMP of ATP-sensitive K⁺ channels is, at least in part, responsible for the stimulatory effect of NO on the [Ca²⁺]_c oscillations. In the presence ofl-NNA, the glucose-induced insulin secretion from isolated islets was facilitated by 0.5 μM NOC-7, whereas it was suppressed by 10 μM NOC-7. These results suggest that NO facilitates glucose-induced [Ca²⁺]_c oscillations of β-cells and insulin secretion at low concentrations, which effects are mediated by cGMP, whereas NO inhibits them in a cGMP-independent manner at high concentrations.

β-Cell swelling induces membrane depolarization, which has been suggested to be caused at least partly by the activation of cation channels. Here, we show the identification of the cation channels. In isolated mouse pancreatic β-cells, the exposure to 30% hypotonic solution elicited an increase in cytosolic Ca²⁺ concentration ([Ca²⁺]_c). The [Ca²⁺]_c elevation was partially inhibited by ruthenium red, a blocker of several Ca²⁺-permeable channels including transient receptor potential vanilloid receptors [transient receptor potential cation channel subfamily V (TRPV)], and by nicardipine, but not by the depletion of intracellular Ca²⁺ stores with thapsigargin and caffeine. The hypotonic stimulation also increased insulin secretion from isolated mouse islets, which was significantly suppressed by ruthenium red. Expression of TRPV2 and TRPV4 was confirmed in mouse pancreatic islets and the MIN6 β-cell line by RT-PCR, Western blot, and immunohistochemical analyses. However, neither 4α-phorbol 12,13-didecanoate nor GSK1016790A, TRPV4 activators, showed any apparent effect on [Ca²⁺]_c in isolated mouse β-cells or in MIN6 cells. In contrast, probenecid, a TRPV2 activator, induced an increase in [Ca²⁺]_c in MIN6 cells, which was attenuated by ruthenium red. Moreover, the [Ca²⁺]_c elevation induced by 30% hypotonic stimulation was significantly reduced by knockdown of TRPV2 with siRNA and by tranilast, a TRPV2 inhibitor. The knockdown of TRPV2 also decreased insulin secretion induced by the hypotonic stimulation. In addition, glucose-stimulated insulin secretion was also significantly reduced in the TRPV2-knockdown MIN6 cells. These results suggest that osmotic cell swelling activates TRPV2 in mouse β-cells, thereby causing membrane depolarization and subsequent activation of voltage-dependent Ca²⁺ channels and insulin secretion.

An involvement of inducible nitric oxide (NO) synthase (NOS) in pancreatic β-cell degeneration during the process of type 1 diabetes has been well discussed. Recently, there is growing evidence for pivotal roles of constitutive NOS (cNOS) in β-cells; the presence of NOS1 and NOS3 in pancreatic β-cells and the effects of low-concentration NO, which is assumed to be derived from cNOS, on β-cell functions have been reported. However, the roles of cNOS-derived NO in β-cells are still under debate. One of the reasons seems to be that NO has multiple biological activities, which are dependent on its concentration. In β-cells, NO has been shown to exert positive and negative regulation of insulin secretion and anti- and pro-apoptotic activities, which is likely to be dependent on concentrations. In this review article, we will describe the current understanding of the roles of NO in pancreatic β-cells, especially focusing on cNOS-derived NO and its differential roles depending on concentrations.

This study investigated a local effect of cooling on the plantar skin blood flow (PSBF) of tetrodotoxin‐treated rats by laser‐Doppler flowmetry. When the air temperature around the left foot was locally cooled from 25 to 10°C, the PSBF of the left foot decreased. The response was inhibited by theα‐adrenoceptor antagonist phentolamine, theα₁‐adrenoceptor antagonist bunazosin, theα₂‐adrenoceptor antagonist RS79948, and bretylium and guanethidine that inhibit noradrenaline release from sympathetic nerves. Adrenalectomy of the rats did not affect the cooling‐induced response. The P2 purinoceptor antagonists suramin and PPADS also significantly suppressed the cooling‐induced reduction of PSBF. However, the inhibitory effect of PPADS on the cooling‐induced response was abolished after the treatment with phentolamine. Intra‐arterial injections of ATPγS, a stable P2 purinoceptor agonist, at 25°C caused a transient decrease in PSBF in a dose‐dependent manner, which was significantly inhibited by phentolamine and guanethidine. These results suggest a novel mechanism for local cooling‐induced reduction of skin blood flowin vivo; moderate cooling of the skin induces the release of ATP, which stimulates presynaptic P2 purinoceptors on sympathetic nerve terminals and facilitates the release of noradrenaline, thereby causing contractions of skin blood vesselsviathe activation ofα₁‐andα₂‐adrenoceptors. British Journal of Pharmacology(2006)148, 579–586. doi:10.1038/sj.bjp.0706765