The contribution of diffusion to the entry of catecholamines into guinea-pig trachealis smooth muscle cells

1. In smooth muscle or cartilage preparations of guinea-pig trachea incubated in 0.1 μmol/l 3H-isoprenaline, extraneuronal uptake inhibitors (corticosterone, normetanephrine, phenoxybenzamine) caused only partial inhibition (64-75%) of the formation of 3-O-methylisoprenaline. Thus, isoprenaline appeared to be exposed to catechol-O-methyltransferase by diffusional entry as well as by extraneuronal uptake in both the smooth muscle and cartilage regions of the trachea. 2. Fluorescence microphotometric measurements of catecholamine uptake in trachealis smooth muscle cells, when tissues were incubated in 200 or 1,200 μmol/l isoprenaline, adrenaline or noradrenaline in the absence and presence of extraneuronal uptake inhibitors (corticosterone, normetanephrine, phenoxybenzamine), showed that the contribution of diffusional entry to the uptake of the amines into the cells fitted with the order of their lipophilicities, viz. isoprenaline ≫ adrenaline > noradrenaline. 3. A kinetic analysis of the uptake of isoprenaline into the trachealis smooth muscle cells was carried out in the absence and presence of 100 μmol/l corticosterone. The kinetic analysis (a) showed that the corticosterone-resistant component of total uptake was not saturable, supporting the view that it represented diffusional entry of isoprenaline into the cells, and (b) provided Km and Vmax values (112 μmol/l and 101 F/min, respectively) for the saturable extraneuronal uptake of isoprenaline into the cells. 4. The study provided evidence for marked diffusional entry of the lipophilic amine isoprenaline into guinea-pig trachealis smooth muscle cells. The diffusional entry of adrenaline was much less and that of noradrenaline negligible. Thus the diffusional entry of these amines should be taken into account in studies on their extraneuronal uptake, even when the measurements of amine uptake relate to microscopically identified cell types