L-Arginine is the physiological substrate for the formation of nitric oxide (NO) and accounts for the biological activity of endothelium-derived relaxing factor. We have studied L-arginine transport in the heart using a rapid dual-isotope dilution technique. The time course of L-[3H]arginine uptake (extraction) by the isolated perfused guinea-pig heart was found to occur in two phases. The first phase reached a plateau in 6.6 +/- 0.6 s and lasted 8.8 +/- 0.7 s, whereas the second phase developed a plateau after 16.3 +/- 0.8 s. The first phase of maximal uptake (Umax,1) accounted for 13.4 +/- 1.4% of the total uptake and the second (Umax,2) for 32.3 +/- 1.8%. The two phases of uptake were inhibited by unlabelled L-arginine in a dose-dependent manner, which suggests that both phases are carrier mediated. The degree of inhibition of Umax,1 and Umax,2 by unlabelled L-arginine was not significantly different. Studies of the kinetics of uptake of these processes revealed an apparent Km,1 of 183 +/- 10 microM with a Vmax,1 of 50 +/- 10 nmol min-1 g-1 for the first phase and Km,2 of 167 +/- 14 microM with a Vmax,2 of 93 +/- 13 nmol min-1 g-1 for the second phase of uptake. These results suggest a similar affinity for the receptors of both transport systems, but with different values for Vmax (P < 0.05). In contrast, 1 mM unlabelled D-arginine had no effect on either the first or second phase of uptake of L-[3H]arginine by the heart, which suggests that these processes are stereospecific. In the presence of the L-stereoisomer of nitro-arginine-mono-methyl ester (L-NAME), a potent inhibitor of NO synthesis, the Umax,1 was inhibited by about 60% while Umax,2 was inhibited by only 20%, which suggests that there is a difference in the effect of L-NAME on the two phases of L-arginine uptake. The first phase most probably represents uptake into the capillary wall, i.e. endothelium and smooth muscle, while the second phase represents entry into the extra-endothelial compartment, i.e. the cardiac myocytes and fibroblasts.