A quantitative description of the diffusion of noradrenaline in the media of blood vessels following its release from sympathetic varicosities

A quantitative model is provided which describes how noradrenaline (NAd), released from varicosities at the adventitial surface of an artery, either diffuses into the media of the vessel to reach the intimal surface, diffuses into the volume of solution surrounding the artery, or is removed by the uptake 1 process in the varicosities. These predictions are then compared with experimental evaluations of the extent of changes in NAd to be found at the adventitial and intimal surfaces of the rat-tail artery, during and after trains of impulses, as determined using amperometry. In the model of the blood vessel there is a sequential decrease in the diffusion constant of NAd from the surrounding solution, to the adventitia, to the media, to the endothelium, to rise again in the lumen of the vessel; there is also an uptake 1 NAd pump in the varicosities described by Michaelis–Menten kinetics. This model is shown to provide a quantitative account of the spatial and temporal changes in NAd observed following trains of impulses at different frequencies of stimulation (5–40 Hz) for different periods of times (10–40 s). Changes in the spatio-temporal distribution of NAd observed following block of the uptake 1 NAd pump were also successfully predicted by the model. It is concluded that, within the context of the model, there is no need to evoke special mechanisms of buffering at the sympathetic varicosities, nor distinctions on the basis that only secreting varicosities utilize the uptake 1 mechanism, in order to describe the dynamics of NAd distribution in arteries during nerve activity.