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Background G protein‐coupled receptors (GPCRs) play crucial roles in regulating islet function, with Gαs‐ and Gαq‐coupled receptors being linked to the stimulation of insulin secretion. We have quantified the mRNA expression of 384 non‐olfactory GPCRs in islets isolated from lean and obese organ donors to determine alterations in islet GPCR mRNA expression in obesity. Methods RT‐qPCR was used to quantify GPCR mRNAs relative to five reference genes (ACTB, GAPDH, PPIA, TBP, and TFRC) in human islets isolated from lean (BMI = 22.6 ± 0.5) and obese (BMI = 32.0 ± 0.8) donors. Results Overall, 197 and 256 GPCR mRNAs were detected above trace level in islets from lean and obese donors, respectively, with 191 GPCR mRNAs being common to the lean and obese groups. 40.9% (n = 157) and 27.1% (n = 104) of the mRNAs were expressed at trace level whilst 7.8% and 6.3% were absent in islets from lean and obese donors, respectively. Hundred and seventeen GPCR mRNAs were upregulated at least twofold in islets from obese donors, and there was >twofold downregulation of 21 GPCR mRNAs. Of particular interest, several receptors signalling via Gαs or Gαq showed significant mRNA upregulation in islets from obese donors (fold increase: PTH2R: 54.0 ± 14.6; MC2R: 34.3 ± 11.5; RXFP1: 8.5 ± 2.1; HTR2B: 6.0 ± 2.0; GPR110: 3.9 ± 1.2; PROKR2: 3.9 ± 0.7). Conclusions Under conditions of obesity, human islets showed significant alterations in mRNAs encoding numerous GPCRs. The increased expression of Gαs‐ and Gαq‐coupled receptors that have not previously been investigated in β‐cells opens up possibilities of novel therapeutic candidates that may lead to the potentiation of insulin secretion and/or β‐cell mass to regulate glucose homeostasis.

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Aim This study investigated whether therapeutically relevant concentrations of fluoxetine, which have been shown to reduce plasma glucose and glycated haemoglobin independent of changes in food intake and body weight, regulate beta‐cell function and improve glucose homeostasis. Methods Cell viability, insulin secretion, beta‐cell proliferation and apoptosis were assessed after exposure of MIN6 beta cells or isolated mouse and human islets to 0.1, 1 or 10 μmol/L fluoxetine. The effect of fluoxetine (10 mg/kg body weight) administration on glucose homeostasis and islet function was also examined in ob/ob mice. Results Exposure of MIN6 cells and mouse islets to 0.1 and 1 μmol/L fluoxetine for 72 hours did not compromise cell viability but 10 μmol/L fluoxetine significantly increased Trypan blue uptake. The dose of 1 μmol/L fluoxetine significantly increased beta‐cell proliferation and protected islet cells from cytokine‐induced apoptosis. In addition, 1 μmol/L fluoxetine induced rapid and reversible potentiation of glucose‐stimulated insulin secretion from islets isolated from mice, and from lean and obese human donors. Finally, intraperitoneal administration of fluoxetine to ob/ob mice over 14 days improved glucose tolerance and resulted in significant increases in beta‐cell proliferation and enhanced insulin secretory capacity. Conclusions These data are consistent with a role for fluoxetine in regulating glucose homeostasis through direct effects on beta cells. Fluoxetine thus demonstrates promise as a preferential antidepressant for patients with concomitant occurrence of depression and diabetes.

Elevations in intracellular Ca2+ in electrically permeabilized islets of Langerhans produced rapid insulin secretory responses from β-cells, but the Ca2+-induced secretion was not maintained and was irrespective of the pattern of administration of elevated Ca2+. Ca2+-insensitive β-cells responded normally to activators of protein kinase C or cAMP-dependent kinase with increased insulin secretion. The loss of secretory responsiveness to Ca2+ was paralleled by a reduction in Ca2+-induced protein phosphorylation. This was caused by a reduction in Ca2+/calmodulin-dependent protein kinase II (CaMK II) activity in the desensitized cells, as assessed by measuring the phosphorylation of a CaMK II-specific exogenous substrate, autocamtide-2. The Ca2+-induced reductions in kinase activity and protein phosphorylation were not dependent on the activation of Ca2+-dependent protein kinases and were not caused by the activation of phosphoprotein phosphatases or of Ca2+-activated proteases. The concomitant reductions in CaMK II activity and Ca2+-induced insulin secretion suggest that the activation of CaMK II is required for normal insulin secretory responses to increased intracellular Ca2+ concentrations.