Adrenergic influence on rat plasma concentrations of tyrosine and tryptophan.

Isoprenaline given to rats in doses between 0.08 and 10 mg/kg intraperitoneally caused a significant decrease in plasma concentrations of tyrosine and tryptophan. Low doses of adrenaline (0.04 - 0.16 mg/kg, intraperitoneally) caused a 30 per cent decrease in plasma concentrations of tyrosine, while high doses (0.63 - 1.25 mg/kg, intraperitoneally) caused an increase in plasma tyrosine to nearly 200 per cent of the controls. High doses of noradrenaline (0.63 - 2.5 mg/kg, intraperitoneally) caused a similar increase in plasma tyrosine concentration. The decrease in plasma amino acids caused by these catecholamines is inhibited by propranolol, suggesting that this effect is mediated via adrenergic beta-receptors, while the increasing effect is inhibited by phenoxybenzamine, which suggests that this effect is caused by an alpha-adrenergic mechanism.