Efflux studies allow further characterisation of the noradrenaline and 5-hydroxytryptamine transporters in rat lungs

The aim of the present study was to further characterise the noradrenaline and 5-hydroxytryptamine [5-HT] transporters in rat lungs by examining the efflux of noradrenaline and 5-HT, respectively. Lungs from rats were isolated and perfused via the pulmonary artery. After loading the tissue with H-3-5-HT or H-3-noradrenaline the efflux of the relevant amine from the lungs was examined for 15-25 min. The rate constant for efflux of H-3-5-HT increased by 81% when Na+ ions were removed from the perfusion solution; increased gradually when a selective 5-HT transporter inhibitor, 200 nM citalopram, was added to the perfusion solution for the final 6 min of efflux; and increased markedly and rapidly when substrates of the 5-HT transporter, tryptamine (18 mu M) and 7-methyl-tryptamine (12 mu M), were added for the final 6 min of efflux. These effects of the substrates were abolished by 1 mu M citalopram, but were not significantly affected by 1 mu M desipramine, a selective uptake, inhibitor. On the other hand, the previously described substrate-induced increase in the rate of efflux of noradrenaline was significantly reduced by desipramine but was unaffected by citalopram. The results show that efflux of 5-HT is mediated only by the 5-HT transporter, with no significant contribution of uptake(1), and efflux of noradrenaline from rat lungs is mediated only by uptake, and not by the 5-HT transporter. The effects of dopamine on the efflux of noradrenaline over a concentration range of 100-600 nM were investigated and the results showed that 50% of the maximal increase in the rate of efflux occurred at a concentration of 275 nM. This value did not differ from the K-m for uptake of dopamine. This result implies that the only factor affecting the substrate-induced increase in noradrenaline efflux is the affinity of the substrate for uptake(1). The efflux of noradrenaline was also examined in the absence and presence of two concentrations of desipramine (0.35 and 1.5 mu M). Analysis of these results showed that uptake(1) contributed approximately 81% and diffusion 19% to the total efflux of noradrenaline and that 90% of the total noradrenaline efflux was subject to reuptake by uptake(1) into the pulmonary endothelial cells.