Activation of I2-imidazoline receptors by agmatine improved insulin sensitivity through two mechanisms in type-2 diabetic rats

In an attempt to clarify the role of endogenous opioid in peripheral I₂-imidazoline receptors activation for improvement of insulin action, bilateral adrenalectomy was carried out in rats with insulin resistance induced by 4-week fructose-rich chow feeding. Single intravenous (i.v.) injection of agmatine (1 mg/kg) for 30 min increased the plasma β-endorphin-like immunoreactivity (BER) in a way parallel to the reduction of plasma glucose in sham-operated fructose chow-fed rats; this action of agmatine was totally abolished by BU224 at sufficient dosage (1 mg/kg, i.v.) to block I₂-imidazoline receptors. The plasma glucose lowering effect of agmatine was markedly reduced but not totally deleted by adrenalectomy in fructose chow-fed rats. A direct effect of agmatine on glucose homeostasis can thus be considered. The hyperinsulinemic-euglycemic clamp technique was performed to evaluate insulin sensitivity. The effect of agmatine on elevation of the average rate of glucose infusion at the glucose clamp steady state in sham-operated fructose chow-fed rats was lessen in adrenalectomized fructose chow-fed rats, but was completely abolished by BU224. The obtained results suggest that the improvement of insulin sensitivity by agmatine is produced by two mechanisms, stimulation of adrenal gland to enhance β-endorphin secretion and a direct activation of peripheral I₂-imidazoline receptor in tissues, for the amelioration of insulin action.