The complex effects of cannabinoids on insulin secretion from rat isolated islets of Langerhans.

Recent interest in the endocrine pancreas has revealed the presence of a functional endocannabinoid system in pancreatic islets, however, the effects of endocannabinoids and cannabinoid CB receptor activation on downstream signalling and on insulin release still remains unclear. In the current study, a variety of purported cannabinoid CB receptor agonists and antagonists were evaluated for their effects on insulin secretion. In fresh rat isolated islets, the endocannabinoid anandamide caused a glucose-dependent, concentration-dependent inhibition of insulin release, with two populations of islets being identified based on their sensitivity to anandamide. Methanandamide (a non-hydrolysable analogue of anandamide) elicited similar inhibition of insulin secretion, comparable to the responses obtained with anandamide-sensitive islets, suggesting that the islet responsiveness may be due to differences in local metabolism of anandamide. The antagonists O-2050 (CB1) and AM630 (CB2) failed to reveal the involvement of cannabinoid receptors in the inhibitory activity of anandamide on insulin release. Inhibition of fatty acid amide hydrolase (FAAH) with URB597 did not alter basal or glucose-induced insulin secretion, suggesting that endogenous islet endocannabinoids do not affect insulin release, or that islet FAAH content is low. URB597 also failed to affect the inhibitory actions of anandamide on insulin release in fresh isolated islets. However, in islets following overnight culture, anandamide caused augmentation of basal and glucose-mediated insulin release. The effects of cannabinoid agents on insulin secretion described in this study does not identify a precise mode of action but points to important modulation which may be dependent on local metabolism and prevailing cellular conditions.

Imidazoleacetic acid-ribotide: an endogenous ligand that stimulates imidazol(in)e receptors.

We identified the previously unknown structures of ribosylated imidazoleacetic acids in rat, bovine, and human tissues to be imidazole-4-acetic acid-ribotide (IAA-RP) and its metabolite, imidazole-4-acetic acid-riboside. We also found that IAA-RP has physicochemical properties similar to those of an unidentified substance(s) extracted from mammalian tissues that interacts with imidazol(in)e receptors (I-Rs). ["Imidazoline," by consensus (International Union of Pharmacology), includes imidazole, imidazoline, and related compounds. We demonstrate that the imidazole IAA-RP acts at I-Rs, and because few (if any) imidazolines exist in vivo, we have adopted the term "imidazol(in)e-Rs."] The latter regulate multiple functions in the CNS and periphery. We now show that IAA-RP (i) is present in brain and tissue extracts that exhibit I-R activity; (ii) is present in neurons of brainstem areas, including the rostroventrolateral medulla, a region where drugs active at I-Rs are known to modulate blood pressure; (iii) is present within synaptosome-enriched fractions of brain where its release is Ca(2+)-dependent, consistent with transmitter function; (iv) produces I-R-linked effects in vitro (e.g., arachidonic acid and insulin release) that are blocked by relevant antagonists; and (v) produces hypertension when microinjected into the rostroventrolateral medulla. Our data also suggest that IAA-RP may interact with a novel imidazol(in)e-like receptor at this site. We propose that IAA-RP is a neuroregulator acting via I-Rs.

Expression of 25-hydroxyvitamin D3-1alpha-hydroxylase in pancreatic islets.

A number of studies have suggested that Vitamin D has a potential role in the development/treatment of diabetes. These effects may be mediated by circulating levels of 1alpha,25(OH)(2)D(3), but local production of 1alpha,25(OH)(2)D(3), catalysed by the enzyme 25-hydroxyvitamin D(3)-1alpha-hydroxylase (1alpha-OHase), is also likely to be important. RT-PCR analyses demonstrated that both isolated rat islets and MIN6 cells (mouse insulin-secreting cell line, characteristic of beta cells) expressed 1alpha-OHase mRNA. The transcript in both cell types was similar to that seen in HKC-8 cells (a renal cell line, which expresses 1alpha-OHase). Western blot analysis and immunolocalisation identified 1alpha-OHase protein in MIN6 cells and human pancreatic tissue. In addition, suspensions of rat islets were able to convert [3H]-25-hydroxyvitamin D(3) to [3H]-1alpha,25(OH)(2)D(3), demonstrating 1alpha-OHase activity. Both cell systems expressed the Vitamin D receptor and 1alpha,25(OH)(2)D(3) (50nM) evoked a rapid rise in [Ca(2+)](i) in MIN6 cells. This data clearly demonstrates islets are able to produce 1alpha,25(OH)(2)D(3) and respond rapidly to treatment with 1alpha,25(OH)(2)D(3). Therefore, we would postulate that local production of 1alpha,25(OH)(2)D(3) maybe an important autocrine link between Vitamin D status and pancreatic function.