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The multi-domain protein PIST (protein interacting specifically with Tc10) interacts with the SSTR5 (somatostatin receptor 5) and is responsible for its intracellular localization. Here, we show that PIST is expressed in pancreatic beta-cells and interacts with SSTR5 in these cells. PIST expression in MIN6 insulinoma cells is reduced by somatostatin (SST). After stimulation with SST, SSTR5 undergoes internalization together with PIST. MIN6 cells over-expressing PIST display enhanced glucose-stimulated insulin secretion and a decreased sensitivity to SST-induced inhibition of insulin secretion. These data suggest that PIST plays an important role in insulin secretion by regulating SSTR5 availability at the plasma membrane.

Glutamate has been implicated as an intracellular messenger in the regulation of insulin secretion in response to glucose. Here we demonstrate by measurements of cell capacitance in rat pancreatic beta-cells that glutamate (1 mM) enhanced Ca2+-dependent exocytosis. Glutamate (1 mM) also stimulated insulin secretion from permeabilized rat beta-cells. The effect was dose-dependent (half-maximum at 5.1 mM) and maximal at 10 mM glutamate. Glutamate-induced exocytosis was stronger in rat beta-cells and clonal INS-1E cells compared to beta-cells isolated from mice and in parental INS-1 cells, which correlated with the expressed levels of glutamate dehydrogenase. Glutamate-induced exocytosis was inhibited by the protonophores FCCP and SF6847, by the vacuolar-type H+-ATPase inhibitor bafilomycin A(1) and by the glutamate transport inhibitor Evans Blue. Our data provide evidence that exocytosis in beta-cells can be modulated by physiological increases in cellular glutamate levels. The results suggest that stimulation of exocytosis is associated with accumulation of glutamate in the secretory granules, a process that is dependent on the transgranular proton gradient.

We have recently isolated and cloned a novel endogenous peptide from pig intestine, NK-lysin (NKL). In the present study we show that NKL (1-100 nM) potently and reversibly stimulates insulin secretion in rat pancreatic islets and in the beta-cell line HIT T15. This effect of NKL was not accompanied by changes in cytoplasmic free calcium concentration. The stimulatory activity of NKL on insulin release was also observed in permeabilized islets under Ca2+-clamped conditions. Preincubation of HIT T15 cells with NKL for 1 h or 24 h did not influence cell viability. Possible mechanisms of insulinotropic activity of NKL are discussed.

The respiratory burst induced by phorbol myristate acetate in mouse macrophages was inhibited by ultra-low doses (10(-15)-10(-13) M) of an opioid peptide [D-Ala2]methionine enkephalinamide. The effect disappeared at concentrations above and below this range. The inhibition approached 50% and was statistically significant (P < 0.001). Increasing the time of the opioid incubation with cells brought about a shift in the maximal effect to lower concentrations of the opioid (from 10(-13) to 5 x 10(-15) M) and led to a decrease in the value of the effect, fully in accord with the previously proposed adaptation mechanism of the action of ultra-low doses.