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

Summary

• 1. 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.

• 2. When either uptake₁ or uptake₂ was intact, about 60% of the ³H-noradrenaline (0.12 μmol·l⁻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.

• 3. 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.