Effects of Imidazoline Compounds on Cytoplasmic Ca2+ concentration and ATP-sensitive K+ channels in Pancreatic B-cells

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Abstract
Phentolamine, an α-adrenoceptor-blocking agent with an imidazoline structure, induces an increase in the cytoplasmic Ca2+ concentration of pancreatic B-cells. This effect occurs at a concentration (32 μM) at which phentolamine is able to enhance glucose-induced insulin secretion. The increase in cytoplasmic Ca2+ concentration caused by phentolamine is additive to the one elicited by a maximally effective concentration of tolbutamide (100 μM). Imidazoline-binding sites in insulin-secreting HIT cells can also be occupied by the guanidinium compound guanabenz which was found to be a potent and reversible blockers of ATP-dependent K+-channels in B-cells. In contrast to phentolamine, guanabenz blocks the ATP-dependent K+-channels only in the inside-out mode, but not in cell-attached mode of the patch-clamp technique In conclusion, imidazolines and structurally related compounds block ATP-dependent K+-channels b binding to the cytoplasmic face of the plasma membrane, and may have effects on other ion channel which contribute to the elevation of cytoplasmic Ca2 concentration and, consequently, to insulin release.

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