Attenuating effect of melatonin on pyridoxal-stimulated release of adrenomedullary catecholamines in the rat

Aims: To investigate the ability of melatonin (MEL) to suppress adrenomedullary catecholamine (CAT) release in the rat, with pyridoxal (PL) being used as an adrenomedullary stimulus and liver and gastrocnemius muscle glycogenolysis acting as indices of CAT release. Main methods: MEL (1-4 mg/kg, i.p.) and PL (300 mg/kg, i.p.) were administered separately or together to male Sprague-Dawley rats (275-300 g), and blood samples for the assay of plasma glucose and CATS were periodically collected for up to 3 h after PL Immediately thereafter, the liver and gastrocnemius muscle were surgically removed and used for the assay of glycogen. The role of adrenoceptors in PL-induced glycogenolysis was examined by parallel experiments in which idazoxan (IDX. 1 mg/kg), propranolol (PRO. 2 mg/kg) or metoprolol (MET, 2 mg/kg) were administered alongside MEL In addition, MEL (4 mg/kg) was co-administered with taurine (TAU, 2.4 mmol/kg), a known adrenomedullary membrane stabilizer. Key findings: MEL attenuated the release of adrenomedullary CATS and accompanying liver and gastrocnemius muscle glycogenolysis due to PL in a dose-dependent manner. A co-treatment with MEL and an adrenoceptor blocker had a greater attenuating effect on PL-induced glycogenolysis and hyperglycemia than MEL but without impinging on the CAT levels seen with MEL alone. Evidence of maximal inhibitory action by MEL on PL-induced plasma CAT elevation was suggested by the about equal levels of plasma CATS after treatments with MEL and with MEL plus TAU. Significance: The present study demonstrates the modulatory effect of MEL of exogenous origin on adrenomedullary CAT secretion when present in supraphysiological concentrations. (c) 2009 Elsevier Inc. All rights reserved.