Section Review: Oncologic, Endocrine and Metabolic Adrenoceptors and imidazoline binding sites in the endocrine pancreas as targets for anti-hyperglycaemic drugs

Imidazolines and pancreatic hormone secretion

A range of imidazoline derivatives are known to be effective stimulators of insulin secretion, and this response correlates with closure of ATP-sensitive potassium channels in the pancreatic beta-cell. However, mounting evidence indicates that potassium channel blockade may form only part of the mechanism by which imidazolines exert their effects on insulin secretion. Additionally, it remains unclear whether members of this class of drugs can bind directly to potassium channel components and whether occupation of a single binding site accounts for their functional activity. This review considers recent developments in the field and highlights evidence that does not fit readily with the concept that a single mechanism of action is sufficient to mediate the effects of imidazolines on pancreatic hormone secretion.

Characterisation of new efaroxan derivatives for use in purification of imidazoline-binding sites

The insulin secretagogue activity of certain imidazoline compounds is mediated by a binding site associated with ATP-sensitive K+ (K(ATP)) channels in the pancreatic beta-cell. We describe the effects of a series of structural modifications to efaroxan on its activity at this site. Substitution of amino-, nitro- or azide- groups onto the 5-position of the benzene ring of efaroxan did not significantly affect the functional interaction of the ligand with the islet imidazoline binding site. Modification of the imidazoline ring to an imidazole to generate 2-(2-ethyl-2,3-dihydrobenzo[b]furan-2-yl)-1H-imidazole (KU14R) resulted in loss of secretagogue activity. Indeed, this reagent appeared to act as an imidazoline antagonist since it blocked the secretory responses to imidazoline compounds and also inhibited the blockade of beta-cell K(ATP) channels by efaroxan in patch clamp experiments. Application of KU14R alone resulted in a modest reduction in K(ATP) channel opening, suggesting that it may display weak partial agonism, at least in patch-clamp experiments.

Insulin secretagogues with an imidazoline structure inhibit arginine-induced secretion from isolated glucagon secretion from isolated rat islets of Langerhans

It is well documented that imidazoline compounds such as efaroxan and phentolamine act as potent insulin secretagogues both in vivo and in vitro, an effect which is mediated principally by blockade of ATP-sensitive potassium channels in the pancreatic B-cell. However, little is known about the effects of these drugs on the secretion of other pancreatic hormones and, in the present work, we have investigated the effects of selective imidazoline compounds on glucagon release from isolated rat islets of Langerhans. None of several imidazoline compounds tested (efaroxan, phentolamine, idazoxan, antazoline) affected glucagon secretion from islets incubated with 4 mM glucose. However, when the rate of glucagon release was stimulated by L-arginine (20 mM) efaroxan caused a rapid, sustained and dose-dependent inhibition of the secretory response (EC50 approximately 30 microM). This effect was seen under both static incubation and islet perifusion conditions. Antazoline and phentolamine also inhibited arginine-induced glucagon secretion, whereas idazoxan (an imidazoline which does not affect insulin secretion) failed to alter glucagon release. The inhibitory effects of imidazolines on glucagon release were not secondary to changes in insulin secretion. Taken together, the results indicate that pancreatic A-cells express functional imidazoline receptors which can regulate the secretory activity of the cells.