Uptake and metabolism of catecholamines in the awake dog

The contributions of uptake, metabolism and excretion to the removal of circulating dopamine (D) have been studied in comparison to those of adrenaline (A) and noradrenaline (NA). In two experiments radioactive catecholamines were infused during 80 min in an awake dog. In the first experiment [14C]D and D, L-[3H]A was used and in the second experiment these catecholamines were infused together with D, L-[14C]NA. Renal excretion of 14C-radioactivity was almost equal in both experiments, as was the case with the accumulation of 14C-components in plasma, demonstrating that the uptake of D was comparable to that of NA. The removal of [14C]D, [14C]NA and [3H]A, by uptake was 50, 50 and 13.5% respectively after 1 h. The conversion by metabolism was 46, 46 and 81%. Renal excretion was 3.5, 2 and 0.5%. Thus only 0.5, 2 and 5% was left in the extracellular fluid (ECF). In a report on similar experiments in anaesthetized dogs much higher levels of unchanged NA in plasma were measured. Probably this is due to anaesthesia inhibiting uptake. In the pulmonary circulation 14C-radioactivity was extracted at a constant rate during infusion which can mainly be attributed to extraneuronal uptake of [14C]D and to neuronal uptake of [14C]NA. Besides extraneuronal uptake of [3H]A in the lung expiration of [3H]water may contribute to the pulmonary extraction.