Are β-receptors involved in the uptake by cardiac sympathetic nerves of infused and released noradrenaline?

ICI-50172 was used to evaluate the role of β-receptors in the re-uptake of noradrenaline in sympathetic transmission, since most previous studies on the role of receptors in this process have been based on the effects of α-receptor antagonists. ³H-dl-noradrenaline was infused intravenously into pithed rats, and 1 hr later the heart was homogenized and the catecholamines were extracted by ion exchange chromatography. Endogenous noradrenaline was estimated fluorimetrically, and radioactivity was measured by scintillation counting. ICI-50172 (10 mg/kg), given intravenously 30 min before the ³H-noradrenaline infusion caused an insignificant reduction of 7% in the uptake of ³H-noradrenaline compared with the significant reduction of 57% produced by an equivalent dose of cocaine. Preganglionic stimulation of cardiac sympathetic nerves significantly depleted radioactive noradrenaline stores by 31%, but ICI-50172 did not increase this loss of radioactivity. On the other hand, endogenous noradrenaline stores were not reduced either by stimulation alone, ICI-50172 alone, or stimulation in the presence of the drug. Since ICI-50172 in a dose sufficient for β-blockade neither reduced the uptake of infused noradrenaline, nor facilitated the depletion of noradrenaline stores by nerve stimulation, it appears that β-receptors do not play a major part in the conservation of liberated transmitter.