Bisphenol-A disruption of the endocrine pancreas and blood glucose homeostasis.

The link between endocrine disruptors and altered blood glucose homeostasis has been recently suggested. Epidemiological studies have correlated levels of phthalates, dioxins and persistent organic pollutants with alterations of blood glucose homeostasis in humans. Environmentally relevant doses of the ubiquitous endocrine disruptor bisphenol-A (BPA) have profound effects on mice endocrine pancreas--an essential tissue involved in glucose metabolism. BPA exerts rapid non-genomic effects on insulin releasing beta-cells and glucagon releasing alpha-cells within freshly isolated islets of Langerhans. In vivo, a single BPA injection of 10 microg/kg rapidly increases plasma insulin and concomitantly decreases glycaemia. When mice were treated with BPA 100 microg/kg/day for 4 days, the environmental oestrogen produced an increase in beta-cell insulin content along with a post-prandial hyperinsulinaemia and insulin resistance. The results reviewed here demonstrate that doses well below the current lowest observed adverse effect level considered by the US-EPA, disrupt pancreatic beta-cell function producing insulin resistance in male mice. Therefore, this altered blood glucose homeostasis by BPA exposure may enhance the risk of developing type II diabetes.

Nongenomic actions of estrogens and xenoestrogens by binding at a plasma membrane receptor unrelated to estrogen receptor alpha and estrogen receptor beta.

The molecular mechanism used by environmental chemicals to exert their hormone-like actions is still only partially resolved. Although it generally is accepted that xenoestrogens act at the genomic level by binding to intracellular estrogen receptors, we have shown here that they trigger nongenomic effects in pancreatic beta cells. Both xenoestrogens and the circulating hormone, 17beta-estradiol, bind with high affinity to a common membrane binding site unrelated to the intracellular estrogen receptors ERalpha and ERbeta. This binding site is shared by dopamine, epinephrine, and norepinephrine and has the pharmacological profile of the gammaadrenergic receptor. This study provides an outline of the membrane receptor involved in rapid xenoestrogen actions.

Non-genomic actions of 17beta-oestradiol in mouse pancreatic beta-cells are mediated by a cGMP-dependent protein kinase.

1. Intracellular calcium concentration ([Ca2+]i) was measured in mouse whole islets of Langerhans using the calcium-sensitive fluorescent dye Indo-1. 2. Application of physiological concentrations of 17beta-oestradiol in the presence of a stimulatory glucose concentration (8 mM) potentiated the [Ca2+]i signal in 83 % of islets tested. Potentiation was manifested as either an increase in the frequency or duration of [Ca2+]i oscillations. 3. The effects caused by 17beta-oestradiol were mimicked by the cyclic nucleotide analogues 8-bromoguanosine-3',5'-cyclic monophosphate (8-Br-cGMP) and 8-bromoadenosine-3',5'-cyclic monophosphate (8-Br-cAMP). 4. Direct measurements of both cyclic nucleotides demonstrated that nanomolar concentrations of 17beta-oestradiol in the presence of 8 mM glucose increased cGMP levels, yet cAMP levels were unchanged. The increment in cGMP was similar to that induced by 11 mM glucose. 5. Patch-clamp recording in intact cells showed that 8-Br-cGMP reproduced the inhibitory action of 17beta-oestradiol on ATP-sensitive K+ (KATP) channel activity. This was not a membrane-bound effect since it could not be observed in excised patches. 6. The action of 17beta-oestradiol on KATP channel activity was not modified by the specific inhibitor of soluble guanylate cyclase (sGC) LY 83583. This result indicates a likely involvement of a membrane guanylate cyclase (mGC). 7. The rapid decrease in KATP channel activity elicited by 17beta-oestradiol was greatly reduced using Rp-8-pCPT-cGMPS, a specific blocker of cGMP-dependent protein kinase (PKG). Conversely, Rp-cAMPS, which inhibits cAMP-dependent protein kinase (PKA), had little effect. 8. The results presented here indicate that rapid, non-genomic effects of 17beta-oestradiol after interaction with its binding site at the plasma membrane of pancreatic beta-cells is a cGMP-dependent phosphorylation process.

Effects of hexose pentaacetates on electrical activity and cytosolic Ca2+ in mouse pancreatic islets.

Electrical activity of beta-cells and cytosolic Ca2+ concentration ([Ca2+]i) were monitored in mouse pancreatic islets exposed to the pentaacetate esters of alpha-D-glucose, beta-D-galactose and beta-L-glucose, all tested at 1.7 mM concentration. In the presence of 5 mM D-glucose, alpha-D-glucose pentaacetate induced electrical activity and increased [Ca2+]i, whilst beta-D-galactose pentaacetate failed to do so. The electrical and cationic response to the D-glucose ester occurred with a delay of between 5 and 10 min, the ester-induced increase in [Ca2+]i being suppressed in the absence of extracellular Ca2+. As a rule, beta-L-glucose pentaacetate also failed to evoke biophysical responses in the islets exposed to 5 mM D-glucose. However, in the presence of 10 mM L-leucine the L-glucose ester induced electrical activity. These findings, which parallel the insulinotropic action of the same esters in rat pancreatic islets reinforce the view that the positive insulinotropic action of selected hexose pentaacetates cannot be attributed to the catabolism of their acetate moiety but, instead, involves a dual mode of action linked to both the metabolism of their carbohydrate moiety and a direct effect of the ester itself upon a yet unidentified receptor system. Furthermore, this study provides the first evidence that the latter direct effect results in the induction of both electrical activity and [Ca2+]i oscillations.