The substrate specificity of uptake2 in the rat heart.

Experiments were carried out with hearts isolated from reserpine- and pargyline-pretreated rats; both noradrenaline-metabolizing enzymes and uptake1 were inhibited. Initial rates of extraneuronal uptake were measured after perfusion lasting for 2 min, either in the absence or in the presence of 100 mumol/l O-methyl-isoprenaline, a potent inhibitor of uptake2. The ID50 (i.e., the concentration of unlabelled substance that halves the rate of uptake of a tracer concentration of 3H-(+/-)-isoprenaline) was determined for a variety of agents. Two types of stereoselective preference of (-)-isomers were observed: for isoprenaline and adrenaline (but not for noradrenaline)--and also for dobutamine. The stereoselective preference for the (-)-isomers of isoprenaline and adrenaline is also evident from fluorimetric determination of initial rates of uptake of unlabelled isomers. Experiments with various tritiated compounds indicate that uptake2 has a broad substrate spectrum: uptake2 is not restricted to 3H-catecholamines and 3H-phenethylamines, but extends to resorcinols (3H-orciprenaline), imidazoline derivatives (3H-clonidine), 3H-histamine and 3H-5-hydroxytryptamine (3H-5-HT). Determinations of the Vmax of uptake2 revealed a correlation between the ID50 and the Vmax: the higher the ID50, the higher the Vmax. These results indicate that uptake2 is a carrier-mediated process.