Effects Of Naloxone On L-type Calcium Channels In Isolated Cardiomyocytes Of SD Rat

Objective:Naloxone,a specific opioid antagonist,has been used in the treatment of hypovolemic,endotoxemic,and spinal shock。In recent years, A variety of clinical and experimental studies have demonstrated that naloxone may significantly increase resuscitation rate.It has been proposed that endogenous opioids are produced in various shock and cardiac arrest states,and naloxone generates the cardiovascular pressor effect by antagonizing the inhibitory actions of endogenous opioids,resulting in potentiation of the effect of released catecholamines.Beside this opioid antagonistic effect of naloxone,the non-opiate effect on direct cardiac muscle contractility at various animal preparations has also been proposed. Although a non-opiate receptor-mediated effect of naloxone has been proposed,no underlying mechanisms have been defined. The purpose of this study,therefore,was to study the effects of naloxone on L-type calcium channel current in SD rat ventricular myocytes by patch clamp. The aim was to determine the non-opioid effect on direct cardiac muscle contractility.Methods:Single ventricular cell of SD rat was obtained by an enzymatic dissociation method,We used traditional whole-cell patch clamp technique to record the L-type calcium channel current as control, the whole-cell patch clamp recording technique was used to record the change of calcium current. the present study was designed to investigate the effects of naloxone on L-type calcium channel current in SD rat ventricular myocytes,and explore preliminarily its mechanism .We needed more than five sets of data in each group.Results:The results were as follows:1.The membrane potential was clamped at -70mV and ICa was elicited by step voltage command pulse from -40mV to +60mV for 250ms with a 10mV increment . ICa began to appear when the step voltage command pulse of -30mV was performed and reached its peak at 0mV . The reversal potential was at about 40mV. Naloxone 20μg·ml^-1 and 100μg·ml^-1 decreased peak current of I_Ca,L from 341.19±30.30pA to 269.62±22.68pA and to 172.93±21.18pA(n=11,P<0.05).The I-V curve of I_Ca,L was shifted upwards with higer concentration,but the activation voltage,and the reversal petential of I_Ca,L were not altered by naloxone.2.Naloxone 20μg·ml^-1 and 100μg·ml^-1 reduced the peak current of I_Ca,L by 79.02±11.21% and 43.28±15.77%(n=5,P<0.05).3.Naloxone had no significant efect on steady-state activation curve.Conclusion:The results suggest that naloxone inhibites the L-type calcium channels of ventricular myocytes in a concentration-dependent manner. The present study shows that naloxone depresses the contractile force in a concentration-dependent manner. opioid antagonistic effect of naloxone is chief in the cardiovascular pressor effect.naloxone exerts its beneficial hemodynamic effects by acting directly on the heart by potentiating the effects of catecholaminergic stimulation of the myocardium.The potentiation of catecholaminergic stimulation of the myocardium in shock state may increase the Ca²⁺ entry by stimulation of the G-protein cascade resulting in enhancement of Ca²⁺ entry,which may overwhelm the reduced Ca²⁺ entry by a direct non-opiate effect of naloxone.Naloxone can decrease of intra-cellular calcium ion content may be one of the reasons why improve myocardium contraction force and exert antiarrhythmic activity.