Selective Enhancement of Nutrient-Induced Insulin Secretion by ATP-Sensitive K⁺ Channel-Blocking Imidazolines

Abstract

The contribution of ATP-sensitive K ϩ channel (K ATP channel)-dependent and -independent signaling to the insulinotropic characteristics of imidazolines was explored using perifused mouse islets and ␤-cells. Up to a concentration of 100 M efaroxan had no insulinotropic effect in the presence of a basal glucose concentration, but enhanced the effect of a stimulatory concentration of glucose or nonglucidic nutrients (ketoisocaproate plus glutamine). The secretion by a non-nutrient (40 mM KCl) was not enhanced. At 500 M, efaroxan stimulated insulin secretion when glucose was basal. Likewise, at 0.1 to 10 M RX871024 [2-(imidazolin-2-yl)-1-phenylindole] showed a purely enhancing effect, but at 100 M it elicited a strong KCl-like secretory response in the presence of basal glucose. At 0.1 and 1 M RX871024 did not significantly depolarize the ␤-cell membrane. However, at a purely enhancing drug concentration (10 M RX871024 or 100 M efaroxan) K ATP channel activity was strongly reduced, the membrane was depolarized, and the cytosolic Ca 2ϩ concentration was elevated in the presence of basal glucose. Insulin secretion by sulfonylurea receptor (SUR)1 knockout (KO) islets, which have no functional K ATP channels, was not increased by efaroxan (100 or 500 M) or by 10 M RX871024 but was increased by 100 M RX871024. The imidazolines phentolamine and alinidine (100 M) were also ineffective on SUR1 KO islets. It is concluded that a significant K ATP channel block is compatible with a purely enhancing effect of the imidazolines on nutrient-induced insulin secretion. Only RX871024 has an additional, nondepolarizing effect, which at a high drug concentration is able to elicit a K ATP channel-independent secretion.The insulinotropic effect of the prototypical imidazoline phentolamine was originally believed to be due to an antagonism at ␣ 2 -adrenoceptors of the ␤-cell (Robertson and Porte, 1973; Efendic et al., 1975). The observation that the insulinotropic effect of this compound was not shared by other antagonists at ␣ 2 -adrenoceptors but was shared by other compounds with imidazoline moieties (Ostenson et al., 1988;Schulz and Hasselblatt, 1989) led to the hypothesis that the insulinotropic effect was mediated by a ␤-cell-specific subtype of the imidazoline receptor (Chan et al., 1994). In other tissues, at least two nonadrenergic imidazoline binding sites have been identified, and consequently the hypothetical ␤-cell subtype was named I 3 receptor (Eglen et al., 1998;Morgan, 1999). In contrast to the I 1 and I 2 sites where binding occurs with nanomolar affinity, binding to I 3 sites requires micromolar concentrations of the imidazoline (Rustenbeck et al., 1997).The demonstration that phentolamine and other imidazolines block ATP-sensitive K ϩ channels (K ATP channels) in pancreatic ␤-cells offered an explanation for their insulinotropic property (Plant and Henquin, 1990;Chan and Morgan, 1990). However, it remained unclear how the effect on the K ATP channel was related to the nonadrenergic imidazoline binding sites on...

Mechanisms of antihyperglycaemic action of efaroxan in mice: time for reappraisal of α2A-adrenergic antagonism in the treatment of type 2 diabetes?

“…As the latter is mostly due to the open probability of K ATP channels [32,33], the strong reduction of membrane conductance can be ascribed to a K ATP channel-blocking effect. Although the most straightforward explanation would be that all these effects rely on a single non-enantiospecific mechanism involving imidazoline effects on K ATP channels, the contribution of more than one mechanism, as well as of more distal events in the pathway of insulin exocytosis, is not excluded by our results [17][18][19]34]. With regard to insulin release from isolated islets, such equipotency of enantiomers has likewise been ascribed to the imidazoline SL84.0418 [35], but our results clearly contradict reports (and even a patent claim for (−)-enantiomers of imidazoline derivatives), which suggest distinct superiority of (−)-efaroxan over (+)-efaroxan [25,26,36].…”

“…However, this idea was soon left behind when the insulinotropic and glucoselowering activity of individual imidazolines seemed to deviate from their α 2 -antagonistic properties [11][12][13][14]. Accordingly, non-adrenergic mechanisms of imidazoline action were discovered, which included promotion of insulin release via interaction with the pore-forming subunit of the ATP-dependent K + (K ATP ) channel [15][16][17][18] as well as via modulation of downstream events relevant to the exocytosis of insulin [19][20][21][22]. While this line of research provided considerable mechanistic insight, it was largely restricted to isolated islets and beta cells with little attention given to the question of how the picture obtained in vitro translates into the pharmacology of imidazolines in vivo.…”

Dissection of mechanisms that account for imidazoline-induced lowering of blood glucose in mice

“…Together with the finding of non-imidazoline α-antagonists devoid of insulinotropic activity Hasselblatt, 1988, 1989), this resulted in the idea that insulin release was mediated by an imidazoline-preferring site, sometimes referred to as the "I 3 -receptor" (Efendic et al, 2002;Morgan and Chan, 2001). The finding was made that imidazolines can block K ATP channels on β-cells, apparently due to interaction with the poreforming subunit Kir6.2 (regarded as the correlate of the I 3 -receptor) (Hatlapatka et al, 2009;Monks et al, 1999;Morgan and Chan, 2001;Proks and Ashcroft, 1997;Rustenbeck et al, 1995Rustenbeck et al, , 1999Szollosi et al, 2010). This provided a plausible alternative explanation for promotion of insulin release, as further supported by evidence that idazoxan is a less effective K ATP channel blocker than the insulinotropic imidazolines phentolamine and efaroxan (Chan and Morgan, 1990;Östenson et al, 1989;Rustenbeck et al, 1999;Shepherd et al, 1996).…”

Evidence that the multiflorine‐derived substituted quinazolidine 55P0251 augments insulin secretion and lowers blood glucose via antagonism at α2‐adrenoceptors in mice

“…In addition, many α 2A ‐adrenoceptor antagonists are known to modulate insulin release not only via adrenoceptors, but also via one (or even more) non‐adrenergic mechanism(s). While non‐adrenergic action(s) obviously include direct interaction with K ATP channels on β cells, the potency of a given α 2A ‐adrenoceptor antagonist as K ATP channel blocker is not necessarily proportional to its potency at the adrenoceptor, which presumably contributes to variability among the pharmacological profiles of individual compounds …”