Autoinhibition of noradrenaline release from the rat heart as a function of the biophase concentration. Effects of exogenous ?-adrenoceptor agonists, cocaine, and perfusion rate

Abstract

Rat isolated perfused hearts with the right sympathetic nerves intact were loaded with ³H-(-)-noradrenaline. The nerves were stimulated with trains of 180 pulses at 3 Hz and at 10 min intervals. The overflow of ³H-noradrenaline and ³H-metabolites was determined by liquid scintillation spectrometry. Clonidine (IC₅₀ 17 nM), oxymetazoline (IC₅₀ 63 nM), and α-methylnoradrenaline (apparent IC₅₀ 35 nM, determined in the presence of cocaine and propranolol) decreased the stimulation-evoked overflow of ³H-noradrenaline by 26, 49, and 78%, respectively, but not methoxamine up to 100 μM (propranolol present). Oxymetazoline and α-methyl-noradrenaline did not cause desensitization of the presynaptic adrenoceptors when present at their IC₈₀ for 33 min. At a perfusion rate of 7 ml/min, yohimbine 1 μM enhanced the stimulation-evoked ³H-noradrenaline overflow by 26% in the absence, and by 58% in the presence of cocaine. Phentolamine 1 μM increased it by 69% when the neuronal reuptake was blocked. The increase by the antagonists faded with successive periods of nerve stimulations, and was positively correlated with the biophase concentration of noradrenaline as reflected by the amount of ³H-noradrenaline released into the perfusate per nerve stimulation. At a perfusion rate of 1.8 ml/min (neuronal reuptake blocked), yohimbine 1 μM increased the overflow by 127%. The results indicate that the α ₂-adrenoceptor-mediated autoinhibition in the rat perfused heart depends on the clearance of transmitter from the biophase via neuronal reuptake and diffusion into the vascular space. Reduction of either elimination pathway enhances the biophase concentration of noradrenaline, thus increasing the autoinhibition of release.