Evidence supporting the existence of presynaptic ?-adrenoceptors in the regulation of endogenous noradrenaline release upon hepatic sympathetic nerve stimulation in the dog liver in vivo

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Abstract
The existence and functional signficance of presynaptic α-adrenoceptors within the liver was investigated in anesthetized dogs. The stimulation-evoked endogenous catecholamine overflow was determined in hepatic venous blood upon hepatic nerve stimulation (12 V, 1–8 Hz, 1 min). Under resting conditions, average plasma catecholamine levels in hepatic venous and aortic blood were 0.064 ng/ml and 0.334 ng/ml, respectively. A frequency-dependent increase (PPPPP<0.01) at 1 Hz. The pretreatment with yohimbine (0.1 mg/kg, i. v.) abolished the inhibitory effects of clonidine both on the catecholamine overflow and the hepatic arterial responses. The results support the existence of a negative feedback mechanism mediated by presynaptic α-adrenoceptors in the local regulation of noradrenaline release from hepatic sympathetic fibers. A functional significance of this process was suggested by an improved correlation found in the presence of clonidine between the catecholamine overflow and the hepatic arterial vascular conductance.

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