Stereoselectivity of extraneuronal uptake of catecholamines in guinea‐pig trachealis smooth muscle cells

1 The extraneuronal uptake of the (‐)‐ and (+)‐isomers of three catecholamines, isoprenaline, adrenaline and noradrenaline, were compared in guinea‐pig trachealis smooth muscle cells, by a fluorescence microphotometric method. 2 Preliminary experiments showed that the initial rates of uptake of the (‐)‐isomers were greater than those of the (+)‐isomers in tissues incubated in 25 μm adrenaline or noradrenaline or 50 μm isoprenaline. 3 More detailed experiments showed that the Km values of the (+)‐isomers of the amines for extraneuronal uptake were not significantly different from each other, but the Km value of (‐)‐isoprenaline < (‐)‐adrenaline < (‐)‐noradrenaline. Thus, the order of the ratios of the Km values for the (+):(‐)‐isomers was isoprenaline > adrenaline > noradrenaline. 4 The results showed that there is stereoselectivity of the extraneuronal uptake of catecholamines, but it is greatest for isoprenaline (4.9 fold), less for adrenaline (2.5 fold) and almost negligible (1·6 fold) for noradrenaline.