BIPHASIC L-ARGININE UPTAKE BY THE ISOLATED GUINEA-PIG HEART

L-Arginine is the physiological substrate far the formation of nitric oxide (NO) and accounts for the biological activity of endothelium-der ived relaxing factor. We have studied L-arginine transport in the hear t using a rapid dual-isotope dilution technique. The time course of L- [H-3]arginine uptake (extraction) by the isolated perfused guinea-pig heart was found to occur in two phases. The first phase reached a plat eau in 6.6 +/- 0.6 s and lasted 8.8 +/- 0.7 s, whereas the second phas e developed a plateau after 16.3 +/- 0.8 s. The first phase of maximal uptake (U-max,U-1) accounted for 13.4 +/- 1.4% of the total uptake an d the second (U-max,U-2) for 32.3 +/- 1.8 %. The two phases of uptake were inhibited by unlabelled L-arginine in a dose-dependent manner, wh ich suggests that both phases are carrier mediated. The degree of inhi bition of U-max,U-1 and U-max,U-2 by unlabelled L-arginine was not sig nificantly different. Studies of the kinetics of uptake of these proce sses revealed an apparent K-m,K-1 of 183 +/- 10 mu M with a V-max,V-1 of 50 +/- 10 nmol min(-1) g(-1) for the first phase and K-m,K-2 of 167 +/- 14 mu M with a V-max,V-2 of 93 +/- 13 nmol min(-1) g(-1) for the second phase of uptake. These results suggest a similar affinity for t he receptors of both transport systems, but with different values for V-max (P < 0.05). In contrast, 1 mM unlabelled D-arginine had no effec t on either the first or second phase of uptake of L-[H-3]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 U-max,U-1 was inhib ited by about 60 % while U-max,U-2 was inhibited by only 20 %, which s uggests that there is a difference in the effect of U L-NAME on the tw o phases of L-arginine uptake. The first phase most probably represent s uptake into the capillary wall, i.e. endothelium and smooth muscle, while the second phase represents entry into the extra-endothelial com partment, i.e. the cardiac myocytes and fibroblasts.