Pancreatic muscarinic acetylcholine receptors play an important role in stimulating insulin and glucagon secretion from islet cells. To study the potential role of the M 3 muscarinic receptor subtype in cholinergic stimulation of insulin release, we initially examined the effect of the muscarinic agonist, oxotremorine-M (Oxo-M), on insulin secretion from isolated pancreatic islets prepared from wild-type (WT) and M 3 receptor؊deficient mice (M3 ؉/؊ and M3 ؊/؊ mice). At a stimulatory glucose level (16.7 mmol/l), Oxo-M strongly potentiated insulin output from islets of WT mice. Strikingly, this effect was completely abolished in islets from M3 ؊/؊ mice and significantly reduced in islets from M3 ؉/؊ mice. Additional in vitro studies showed that Oxo-M؊mediated glucagon release was also virtually abolished in islets from M3 ؊/؊ mice. Consistent with the in vitro data, in vivo studies showed that M3 ؊/؊ mice displayed reduced serum insulin and plasma glucagon levels and a significantly blunted increase in serum insulin after an oral glucose load. Despite the observed impairments in insulin release, M3؊/؊ mice showed significantly reduced blood glucose levels and even improved glucose tolerance, probably due to the reduction in plasma glucagon levels and the fact that M3 ؊/؊ mice are hypophagic and lean. These findings provide important new insights into the metabolic roles of the M 3 muscarinic receptor subtype. Diabetes 53

Cell coupling is important for the normal function of the beta-cells of the pancreatic islet of Langerhans, which secrete insulin in response to elevated plasma glucose. In the islets, electrical and metabolic communications are mediated by gap junctions. Although electrical coupling is believed to account for synchronization of the islets, the role and significance of diffusion of calcium and metabolites are not clear. To address these questions we analyze two different mathematical models of islet calcium and electrical dynamics. To study diffusion of calcium, we use a modified Morris-Lecar model. Based on our analysis, we conclude that intercellular diffusion of calcium is not necessary for islet synchronization, at most supplementing electrical coupling. Metabolic coupling is investigated with a recent mathematical model incorporating glycolytic oscillations. Bifurcation analysis of the coupled system reveals several modes of behavior, depending on the relative strength of electrical and metabolic coupling. We find that whereas electrical coupling always produces synchrony, metabolic coupling can abolish both oscillations and synchrony, explaining some puzzling experimental observations. We suggest that these modes are generic features of square-wave bursters and relaxation oscillators coupled through either the activation or recovery variable.

Glucose-induced membrane potential and Ca(2+) oscillations in isolated pancreatic beta-cells occur over a wide range of frequencies, from >6/min (fast) to <1/min (slow). However, cells within intact islets generally oscillate with periods of 10-60 s (medium). The phantom bursting concept addresses how beta-cells can generate such a wide range of frequencies. Here, we explore an updated phantom bursting model to determine how heterogeneity in a single parameter can explain both the broad frequency range observed in single cells and the rarity of medium oscillations. We then incorporate the single-cell model into an islet model with parameter heterogeneity. We show that strongly coupled islets must be composed of predominantly medium oscillating single cells or a mixture of fast and slow cells to robustly produce medium oscillations. Surprisingly, we find that this constraint does not hold for moderate coupling, and that robustly medium oscillating islets can arise from populations of single cells that are essentially all slow or all fast. Thus, with coupled phantom bursters, medium oscillating islets can be constructed out of cells that are either all fast, all slow, or a combination of the two.