Folinic acid It is generally considered that elevated

It is generally considered that elevated blood glucose due to insulin resistance is an important factor in islet compensation to high fat feeding [27]. However, we have shown recently that insulin resistance induced in mice by hydrocortisone injection provokes significant islet Folinic acid in the absence of hyperglycaemia [20]. Interestingly, this response was significantly curtailed in mice with knock-out of either GLP-1 or GIP receptors, with strong evidence for key involvement of local islet incretin production in this process. Such observations provide evidence for functional significance of a growing body of data, including those from the present study, indicating significant alpha cell production of both GLP-1 and GIP [15], [28], [29]. Interestingly in hydrocortisone treated mice, GIP appeared particularly important as numbers of alpha cells mainly producing GIP were markedly increased and animals lacking functional GIP receptors showed a particularly severe loss of islet expansion to hydrocortisone-induced insulin resistance [20]. To further evaluate the role of GIP in islet biology, the present study has utilised GIPRKO mice [30] to evaluate the possibility of a prominent role of GIP in the compensatory islet response to high fat feeding. These animals exhibited decreased non-fasting glycaemia and a strong trend towards reduced insulin and glucagon levels together with evidence of enhanced insulin sensitivity in hypoglycaemic tests compared to both normal and high fat fed C57BL/6 mice. However, others [31] have shown GIPRKO mice to have unaltered insulin sensitivity and the present study also failed to reveal significant differences in the responses of C57BL/6 and GIPRKO mice to feeding high fat diet. Compared with normal diet controls, GIPRKO mice also exhibited a significant increase of non-fasting blood glucose and enhancement of body fat stores to levels similar to C57BL/6 mice. This is somewhat surprising given that other studies have reported GIPRKO mice to be strongly protected from diet-induced obesity and insulin resistance [32], [33]. However, major factors, in addition to differences in the GIPRKO model employed [30], [32], [33], are likely to concern the duration of the present study which was considerably protracted at 45weeks. Further, Miyawaki and colleagues [32] reported observations using high fat and control diets, with similar energy densities (3.57kcal/g). In contrast, the energy density of high fat diet used in the present study was twice as high as control diet (6.13kcal/g vs 3.10kcal/g). This tends to suggest that the positive effect of GIPR deletion on insulin resistance [32], [34] is less important over time than the negative effect of losing the beneficial actions of GIP on pancreatic beta cells [7], [35]. For example, a high fat diet low in carbohydrate may be less detrimental to beta cell function in the longer term than consumption of standard chow rich in carbohydrate as revealed by classical studies in genetically diabetic mice [36]. Further studies are needed to test this hypothesis but the present work has clearly shown that elevation of circulating insulin, increase of pancreatic insulin stores and positive adaptive changes in islet morphology are greatly diminished in GIPRKO mice fed a high fat diet for 45weeks. This occurs despite similar effect of high fat feeding on insulin sensitivity as observed in C57BL/6 mice. Further, high fat feeding induced a markedly decreased pancreatic GLP-1 content and evoked a non-functional compensatory increase of pancreatic GIP in GIPRKO mice. Thus deletion of GIPR compromised the normal increases of islet number, islet size, beta and alpha cell areas, with abolition of increased beta cell proliferation but a notable enhancement of beta cell apoptosis, giving a significantly decreased cellular Ki67/TUNEL ratio less than unity. Indeed, unlike C57BL/6 high fat fed mice which displayed a Ki67/TUNEL ratio of 1.5, all other groups were undergoing beta cell loss, corresponding to well-known age-related deterioration in the function of insulin secreting cells [37]. Further studies investigating activated caspase 3 levels in beta cells would be useful to assess the extent of apoptosis induction.