OBJECTIVE— Somatostatin (SST) is secreted by islet δ-cells and by extraislet neuroendocrine cells. SST receptors have been identified on α- and β-cells, and exogenous SST inhibits insulin and glucagon secretion, consistent with a role for SST in regulating α- and β-cell function. However, the specific intraislet function of δ-cell SST remains uncertain. We have used Sst−/− mice to investigate the role of δ-cell SST in the regulation of insulin and glucagon secretion in vitro and in vivo. RESEARCH DESIGN AND METHODS— Islet morphology was assessed by histological analysis. Hormone levels were measured by radioimmunoassay in control and Sst−/− mice in vivo and from isolated islets in vitro. RESULTS— Islet size and organization did not differ between Sst−/− and control islets, nor did islet glucagon or insulin content. Sst−/− mice showed enhanced insulin and glucagon secretory responses in vivo. In vitro stimulus-induced insulin and glucagon secretion was enhanced from perifused Sst−/− islets compared with control islets and was inhibited by exogenous SST in Sst−/− but not control islets. No difference in the switch-off rate of glucose-stimulated insulin secretion was observed between genotypes, but the cholinergic agonist carbamylcholine enhanced glucose-induced insulin secretion to a lesser extent in Sst−/− islets compared with controls. Glucose suppressed glucagon secretion from control but not Sst−/− islets. CONCLUSIONS— We suggest that δ-cell SST exerts a tonic inhibitory influence on insulin and glucagon secretion, which may facilitate the islet response to cholinergic activation. In addition, δ-cell SST is implicated in the nutrient-induced suppression of glucagon secretion.

The pathophysiology of diabetes as a disease is characterised by an inability to maintain normal glucose homeostasis. In type 1 diabetes, this is due to autoimmune destruction of the pancreatic β-cells and subsequent lack of insulin production, and in type 2 diabetes it is due to a combination of both insulin resistance and an inability of the β-cells to compensate adequately with increased insulin release. Animal models, in particular genetically modified mice, are increasingly being used to elucidate the mechanisms underlying both type 1 and type 2 diabetes, and as such the ability to study glucose homeostasisin vivohas become an essential tool. Several techniques exist for measuring different aspects of glucose tolerance and each of these methods has distinct advantages and disadvantages. Thus the appropriate methodology may vary from study to study depending on the desired end-points, the animal model, and other practical considerations. This review outlines the most commonly used techniques for assessing glucose tolerance in rodents and details the factors that should be taken into account in their use. Representative scenarios illustrating some of the practical considerations of designingin vivoexperiments for the measurement of glucose homeostasis are also discussed.

We previously reported that prenatal and suckling exposure to a maternal diet rich in animal fat leads to cardiovascular dysfunction in young adult rat offspring with subsequent development of dyslipidemia and hyperglycemia. We have further investigated glucose homeostasis in adult female offspring by euglycemic-hyperinsulinemic clamp and by dynamic assessment of glucose-stimulated insulin secretion in isolated, perifused pancreatic islet cells. Additionally, given the link between reduced mitochondrial DNA (mtDNA) content and the development of type 2 diabetes mellitus, we have measured mtDNA in organs from young adult animals. Sprague-Dawley rats were fed a diet rich in animal fat or normal chow throughout pregnancy and weaning. Infusion of insulin (5 mU·kg⁻¹·min⁻¹) resulted in a higher steady-state plasma insulin concentration in 1-year-old offspring of fat-fed dams (OHF, n = 4) vs. offspring of control dams (OC, n = 4, P < 0.01). Glucose-stimulated insulin secretion in isolated islets from 9-mo-old OHF was significantly reduced compared with OC ( n = 4, P < 0.05). Transmission electron micrography showed altered insulin secretory granule morphology in OHF pancreatic β-cells. Kidney mtDNA was reduced in 3-mo-old OHF [16S-to-18S gene ratio: OC ( n = 10) 1.05 ± 0.19 vs. OHF ( n = 10) 0.66 ± 0.06, P < 0.05]. At 6 mo, gene chip microarray of OHF aorta showed reduced expression of the mitochondrial genome. Prenatal and suckling exposure to a diet rich in animal fat leads to whole body insulin resistance and pancreatic β-cell dysfunction in adulthood, which is preceded by reduced tissue mtDNA content and altered mitochondrial gene expression.

The effect of cell-to-cell contact on Ca2+ influx and secretory responses in the beta-cell line MIN6 was studied using MIN6 pseudoislets, which are three-dimensional islet-like cell aggregates that develop when MIN6 cells are cultured for 6-8 days on gelatin. The formation of pseudoislets is dependent on the Ca2+-dependent adhesion molecule E-cadherin (E-CAD), since the process can be inhibited by incubation in the absence of Ca2+ or in the presence of an anti-E-CAD antibody. Glucose and alpha-ketoisocaproic acid (KIC) evoked a Ca2+ influx in only a small fraction of the MIN6 monolayer cells, whereas >80% of cell groups within the pseudoislets responded to both nutrients. In contrast, changes in the intracellular free Ca2+ concentration ([Ca2+]i) were observed in all or most monolayer cells or pseudoislet cell groups in response to physical or pharmacological depolarizing stimuli. No significant increase in insulin release was observed from MIN6 monolayer cells in response to nutrient or nonnutrient insulin secretagogues. Conversely, pseudoislets were found to respond significantly to both nutrients and nonnutrients. These results suggest that close cell-to-cell contact improves the functional responsiveness of MIN6 cells and that pseudoislets may therefore serve as a useful research model in the study of beta-cell function.