Specific and unspecific removal of circulating noradrenaline in pulmonary and systemic vascular beds in man

In order to study the extent and degree of specificity of the removal of circulating noradrenaline (NA) from the blood stream, during a single passage through the vascular beds of the lungs and of the forearm musculature of unanaesthetized, healthy young men, the following radiometric method was used: A misture of 3H-NA with 14C-inulin and 125I-labelled human serum albumin was injected proximally to the bed under study; six serial blood smaples were drawn distally to it, with about 15 s interval, and the disappearance of 3H-NA relatively to that of 14C-inulin and 125I-albumin was determined. 3H-NA was found to be removed from the blood stream in both these vascular beds. However, in the forearm the removal of 3H-NA was much greater, and apparently to a large extent due to specific trapping in the tissues, since the clearance of 3H-NA over the vascular bed of the forearm markedly exceeded that of 14C-inulin. During passage through the lungs removal of 3H-NA was less marked, and apparently largely unspecific, since it was initially essentially equal to that of 14C-inulin. This suggests that removal of circulating ergones of small molecular size, from the blood stream passing through the lungs, may occur initially by unspecific filtration/diffusion. Their subsequent fate would depend on whether the extraluminal tissues, supplied by the lung circulation, possess specific mechanisms for concentrating and/or inactivating each particular compound.