D-chiro-inositol attenuates epinephrine-stimulated hepatic glucose output in the isolated perfused liver independently of insulin

D-chiro-Inositol (DCI) is a cyclic sugar alcohol that evokes both antidiabetic and insulin sensitizing effects. Pharmacological administration of DCI has been shown to lower blood glucose in rat models of diabetes mellitus and enhance insulin sensitivity in humans with polycystic ovary syndrome (PCOS). We hypothesised that the antidiabetic effects of DCI could be due to inhibition of hepatic glucose output (HGO). To test this hypothesis, we perfused isolated rat livers either with buffer, myo-inositol, DCI, or insulin, and investigated their respective effects on the stimulation of HGO by epinephrine. We found that perfusion with 200 μM DCI attenuated epinephrine-stimulated HGO by 35% over 30 min as compared to the buffer control perfusion (p=0.05). By comparison, perfusion with 1 nM insulin attenuated epinephrine-stimulated HGO by 57% (p<0.0001). The glucose-lowering effects by DCI occurred independently of insulin and were specific to the DCI stereoisomer as 200 μM myo-inositol had no effect. These findings suggest that DCI could evoke its antidiabetic effects in vivo by inhibition of HGO. Further identification of the protein targets involved could open up new avenues to regulate hyperglycaemia with wider implications for the treatment of hepatic insulin resistance in PCOS.

Both p110alpha and p110beta isoforms of phosphatidylinositol 3-OH-kinase are required for insulin signalling in the hypothalamus

Both insulin and leptin action in the brain are considered to involve activation of phosphoinositide 3-kinase (PI3K), although the roles of different PI3K isoforms in insulin signalling in the hypothalamus are unknown. In the present study, we characterised the roles of these isoforms in hypothalamic insulin and leptin signalling and investigated the cross-talk of both hormones. To evaluate PI3K levels in the hypothalamus, PI3K was immunoprecipitated using an antibody directed against the p85 subunit, and then total PI3K activity was measured in the presence of novel isoform-selective pharmacological inhibitors of each isoform of PI3K. Subsequently, these inhibitors were administered into the lateral ventricle of male Sprague-Dawley rats, followed by vehicle, insulin, leptin or both hormones 45 min later. PI3K activity was determined by immunohistochemical detection of phosphorylated AKT (S473). In a separate study, the effects of the inhibitors on the anorexigenic action of insulin and leptin were determined. Hypothalamic insulin signalling was specifically mediated by the combined actions of the class Ia isoforms p110alpha and p110beta. Total hypothalamic PI3K activity was inhibited 65% by a p110alpha inhibitor, and 35% by a p110beta inhibitor, with a combination of inhibitors being equally effective as the broad-spectrum PI3K inhibitor wortmannin. Individual i.c.v. administration of p110alpha and p110beta inhibitors partly prevented insulin-induced phosphorylated AKT (S473) in the arcuate nucleus, whereas simultaneous application completely blocked insulin action. Unlike insulin, leptin did not induce phosphorylated AKT in the hypothalamus, as detected by immunohistochemistry, and the anorectic effects of leptin were not affected by pre-treatment with a combination of p110alpha and p110beta inhibitors. The enhanced anorectic effect of a combined i.c.v. application of both insulin and leptin could be prevented by pre-treatment with the combination of p110alpha and p110beta inhibitors. The data suggest that p110alpha and p110beta isoforms of PI3K are necessary to mediate insulin action in the hypothalamus. The role of PI3K in leptin action is less clear, but it may be involved by means of an insulin-dependent sensitisation of leptin action.

Atypical antipsychotic drugs induce derangements in glucose homeostasis by acutely increasing glucagon secretion and hepatic glucose output in the rat

AIMS/HYPOTHESIS

Use of the second-generation antipsychotic drugs (SGAs) results in the development of obesity and a type 2 diabetes-like syndrome. We hypothesised that, in addition to the insulin resistance associated with the obesity, the SGAs might have acute effects on glucose metabolism that could contribute to the derangements in glucose metabolism.

METHODS

We investigated the effects of therapeutically relevant levels of three different antipsychotic medications (haloperidol, quetiapine and clozapine) on glucose tolerance, measures of insulin resistance and hepatic glucose production, and on insulin and glucagon secretion in rats.

RESULTS

We found that these drugs induce impaired glucose tolerance in rats that is associated with increased insulin secretion (clozapine>quetiapine>haloperidol) but is independent of weight gain. However, Akt/protein kinase B activation is normal, and at these levels of drug there was no effect on insulin action in fat cells or soleus muscle, and no effect on insulin sensitivity as evaluated by insulin tolerance tests. We show that clozapine induces increased glucose levels following pyruvate and glycerol challenges, indicating an increase in hepatic glucose output (HGO). Increased HGO would in turn increase insulin release and would explain the apparent phenotype mimicking insulin resistance. We provide evidence that this effect could at least in part be mediated by a stimulation of glucagon secretion.

CONCLUSIONS/INTERPRETATION

Our findings indicate that SGAs can cause acute derangements in glucose metabolism that are not caused by a direct induction of insulin resistance but act via an increase in glucagon secretion and thus stimulation of hepatic glucose production.