Essential role of the imidazoline moiety in the insulinotropic effect but not the KATP channel-blocking effect of imidazolines;: a comparison of the effects of efaroxan and its imidazole analogue, KU14R

Aims/hypothesis: Imidazolines are a class of investigational antidiabetic drugs. It is still unclear whether the imidazoline ring is decisive for insulinotropic characteristics. Materials and methods: We studied the imidazoline efaroxan and its imidazole analogue, KU14R, which is currently classified as an imidazoline antagonist. The effects of both on stimulus secretion-coupling in normal mouse islets and beta cells were compared by measuring K-ATP channel activity, plasma membrane potential, cytosolic calcium concentration ([Ca2+](c)) and dynamic insulin secretion. Results: In the presence of 10 mmol/l but not of 5 mmol/l glucose, efaroxan ( 100 mu mol/l) strongly enhanced insulin secretion by freshly isolated perifused islets, whereas KU14R ( 30, 100 or 300 mu mol/l) was ineffective at both glucose concentrations. Surprisingly, the insulinotropic effect of efaroxan was not antagonised by KU14R. K-ATP channels were blocked by efaroxan (IC50 8.8 mu mol/l, Hill slope - 1.1) and by KU14R (IC50 31.9 mu mol/l, Hill slope - 1.5). Neither the K-ATP channel-blocking effect nor the depolarising effect of efaroxan was antagonised by KU14R. Rather, both compounds strongly depolarised the beta cell membrane potential and induced action potential spiking. However, KU14R was clearly less efficient than efaroxan in raising [Ca2+](c) in single beta cells and whole islets at 5 mmol/l glucose. The increase in [Ca2+](c) induced by 10 mmol/l glucose was affected neither by efaroxan nor by KU14R. Again, KU14R did not antagonise the effects of efaroxan. Conclusions/interpretation: The presence of an imidazole instead of an imidazoline ring leads to virtually complete loss of the insulinotropic effect in spite of a preserved ability to block K-ATP channels. The imidazole compound is less efficient in raising [Ca2+](c); in particular, it lacks the ability of the imidazoline to potentiate the enhancing effect of energy metabolism on Ca2+-induced insulin secretion.