The role of neuronal and extraneuronal uptake in the inactivation of 3H-(--)noradrenaline in the rabbit aorta determined by a method relating with amine diffusion in the tissue.

Apparent coefficients of diffusion for 3H-noradrenaline and 14C-sorbitol were determined in the different layers of the rabbit aorta; uptake1 and uptake2 were inhibited. For 3H-noradrenaline, a value of 6.3 . 10(-7) cm2 . s-1 was obtained in intact aorta and a similar one in intima-media, while that in adventitia was three times higher than in intact aorta. The apparent coefficient was the same for 3H-noradrenaline and 14C-sorbitol in adventitia. However, the values were slightly higher for 3H-noradrenaline than for 14C-sorbitol in intact aorta and in intima-media. Thus, the distribution of sorbitol did not exactly reflect the distribution of noradrenaline in these two preparations, even when both uptake mechanisms were inhibited. When either uptake1 or uptake2 was intact, about 60% of the 3H-noradrenaline (0.12 mumol . 1(-1) was inactivated on its way through the vessel wall (intact aorta). This indicated that each mechanism was equieffective in removing noradrenaline from the extracellular fluid either in the adventitia (uptake1) or intima-media (uptake2). Similarly, when noradrenaline left the extracellular space by diffusion, about the same amount of noradrenaline was inactivated neuronally and extraneuronally. However, under classical incubation conditions, neuronal inactivation of 3H-noradrenaline was three to five times higher than extraneuronal one. This difference reflected the rapid access of noradrenaline to the neurones rather than the capacity of the neuronal and extraneuronal uptake sites to remove the amine from the extracellular fluid in the different layers of the aortic wall.