Aims/hypothesis Cell-cell coupling mediated by gap junctions formed from connexin (CX) contributes to the control of insulin secretion in the endocrine pancreas. We investigated the cellular production and localisation of CX36 and CX43, and gap junction-mediated beta cell coupling in pancreatic islets from rats of different ages, displaying different degrees of maturation of insulin secretion. Methods The presence and distribution of islet connexins were assessed by immunoblotting and immunofluorescence. The expression of connexin genes was evaluated by RT-PCR and quantitative real-time PCR. The ultrastructure of gap junctions and the function of connexin channels were assessed by freeze-fracture electron microscopy and tracer microinjection, respectively. Results Young and adult beta cells, which respond to glucose, expressed significantly higher levels of Cx36 (also known as Gjd2) than fetal and newborn beta cells, which respond poorly to the sugar. Accordingly, adult beta cells also showed a significantly higher membrane density of gap junctions and greater intercellular exchange of ethidium bromide than newborn beta cells. Cx43 (also known as Gja1) was not expressed by beta cells, but was located in various cell types at the periphery of fetal and newborn islets. Conclusions/interpretation These findings show that the pattern of connexins, gap junction membrane density and coupling changes in islets during the functional maturation of beta cells.

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A low protein diet has been shown to affect the amount and activity of several enzymes and to decrease insulin secretion by islets isolated from rats fed such a diet. To understand the mechanisms involved in this phenomenon, we investigated the effects of forskolin, a stimulator of adenylyl cyclase, on insulin secretion by pancreatic islets from rats fed a normal (17%; NP) or low (6%; LP) protein diet for 8 wk. Isolated islets were incubated for 1 h in Krebs-bicarbonate solution containing 8.3 mmol glucose/L, with or without 10 micromol forskolin/L. The forskolin-induced insulin secretion was higher in islets from NP rats than in those from LP rats (P<0.05). Western blotting revealed that the amount of the alpha catalytic subunit of protein kinase A (PKAalpha) was 35% lower in islets from LP rats than in islets from NP rats (P<0.05). Moreover, PKAalpha mRNA expression was reduced by 30% in islets from LP rats (P<0.05). Our results indicated a possible relationship between a low protein diet and a reduction in PKAalpha expression. These alterations in PKAalpha may be responsible in part for the decreased insulin secretion by islets from rats fed a low protein diet.

Tumor necrosis factor-a (TNF-a) is known to participate in the wastage syndrome that accompanies cancer and severe infectious diseases. More recently, a role for TNF-a in the pathogenesis of type 2 diabetes mellitus and obesity has been shown. Much of the regulatory action exerted by TNF-a upon the control of energy stores depends on its action on the hypothalamus. In this study, we show that TNF-a activates canonical pro-inflammatory signal transduction pathways in the hypothalamus of rats. These signaling events lead to the transcriptional activation of an early responsive gene and to the induction of expression of cytokines and a cytokine responsive protein such as interleukin-1b, interleukin-6, interleukin-10 and suppressor of cytokine signalling-3, respectively. In addition, TNF-a induces the expression of neurotransmitters involved in the control of feeding and thermogenesis. Thus, TNF-a may act directly in the hypothalamus inducing a proinflammatory response and the modulation of expression of neurotransmitters involved in energy homeostasis.

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