Insulin sensitization induced by oral cicletanine in conscious rabbits

Abstract

The endogenous insulin sensitizing machinery termed the hepatic insulin sensitizing substance (HISS) mechanism has been shown to be nitrergic and linked to sensory fibers in the anterior hepatic plexus. We studied whether this mechanism could pharmacologically be exploited by cicletanine, a cGMP-PDE inhibitor antihypertensive drug, in conscious rabbits. Whole body insulin sensitivity and peripheral glucose uptake were determined by hyperinsulinaemic euglycaemic glucose clamping, and cardiac radiolabelled deoxyglucose (DOG) uptake in neurogenic, achieved by perineurial capsaicin treatment of the anterior hepatic plexus through defunctionalization of hepatic sensory fibers, and metabolic, induced by dietary hypercholesterolemia, insulin resistance models after single oral doses of cicletanine (3, 10 and 30 mg kg⁻¹) or rosiglitazone (3 mg kg⁻¹). The effect of cicletanine on cardiac and vascular tissue NO, cGMP, cAMP was measured by means of spin trapping technique and radioimmunoassay, respectively. Insulin sensitivity and peripheral DOG uptake were significantly increased by 10 and 30 mg kg⁻¹ cicletanine in both healthy and hypercholesterolaemic rabbits, but not in those with neurogenic insulin resistance. Rosiglitazone had no effect in healthy and neurogenic insulin resistant rabbits although it improved insulin sensitivity in hypercholesterolemic animals. The 10 mg kg⁻¹ cicletanine dose induced no change in either cardiac or vascular tissue NO, cGMP or cAMP concentrations. Nevertheless, at a dose of 30 mg kg⁻¹ producing an insulin sensitizing effect of approximately the same amplitude as seen with 10 mg kg⁻¹, the drug significantly increased tissue NO and cGMP concentrations. Oral cicletanine attains its insulin sensitizing effect at doses lower than those necessary to activate the NO-cGMP pathway in the cardiovascular system. This metabolic effect requires functional integrity of hepatic sensory nerves.