THE IMIDAZOLINE SITE INVOLVED IN CONTROL OF INSULIN-SECRETION - CHARACTERISTICS THAT DISTINGUISH IT FROM I-1-SITE AND I-2-SITE

1 The nature of the binding site mediating the insulin secretagogue ac tivity of certain imidazoline compounds remains unclear and the pharma cology of the I-1- and I-2-imidazoline sites, described in many tissue s, does not correlate with the observed responses to imidazolines in i slets. In the present paper, we describe further results which support the concept that the islet imidazoline site may represent a novel sub type of imidazoline receptor. 2 Culture of rat isolated islets in the presence of imidazoline secretagogues (either efaroxan or phentolamine ) resulted in loss of responsiveness on subsequent re-exposure to thes e agents. However, culture of islets with either idazoxan or UK14,304 (imidazoline ligands that do not stimulate insulin secretion) did not lead to any loss of response when the islets were subsequently exposed to efaroxan. By contrast, islets cultured with UK14,304 (a potent alp ha(2)-adrenoceptor agonist), displayed loss of sensitivity to noradren aline, consistent with down-regulation of alpha(2)-adrenoceptors. 3 In order to characterize the imidazoline site further, radioligand bindi ng studies were performed in membranes from RINm5F insulinoma cells us ing [H-3]-RX821002, an imidazoline insulin secretagogue that does not interact significantly with imidazoline sites in other tissues. [H-3]- RX821002 labelled alpha(2)-adrenoceptors with high affinity (2.01 +/- 0.7 nM) but also labelled a second, non-adrenoceptor site with much lo wer affinity. 4 Under conditions of alpha(2)-adrenoceptor blockade (in the presence of adrenaline), efaroxan displaced [H-3]-RX821002 bindin g to the low affinity site, in a dose-dependent manner. Competition st udies employing additional imidazoline compounds of varying secretagog ue activity revealed that the pharmacological profile of the low affin ity site correlates well with that observed in secretion experiments. 5 The results obtained from the down-regulation experiments with isola ted islets and from the radioligand binding studies suggest that the l ow affinity [H-3]-RX821002 binding site may represent the functional r eceptor responsible for the secretagogue activity of imidazoline compo unds in the endocrine pancreas and that it has a pharmacological profi le distinct from those of I-1- and I-2-sites.