Biphasic L‐arginine uptake by the isolated guinea‐pig heart

Abstract

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. © 1995 The Physiological Society