The uptake of amino acids, in particular leucine, by isolated perfused testes of rats.

The uptake of amino acids by the isolated rat testis perfused with Krebs-Ringer solution with albumin has been studied using the single-passage, multiple-tracer technique with [14C]-mannitol as the reference tracer. When the perfusate contained no added amino acids, the uptake of [3H]-Leu was between 60% and 80% of the uptake of mannitol at all times after injection of the bolus; there was a small but significant uptake of some other amino acids studied (Ala, Gly, Glu, and Asp); and with Ala, Glu, and Asp, uptake increased slightly in increasing times after injection. There was no significant uptake of Arg. The uptake of Leu could be decreased by the inclusion of nonradioactive Leu in the perfusate, and the Km and Vmax of the transport were 0.067 mM and 19.5 nmole/(g x min), respectively. The Km value is similar to that for transport into brain and much less than the values obtained in other tissues for the related amino acid Phe, which is transported by the same L system. The transport of L-Leu in the testis was also inhibited by L-Phe or D-Phe, D-Leu, and by the synthetic amino acid Bch, the characteristic marker for the L system, but was only slightly reduced if the perfusate was free of sodium, as is expected for the L system. By autoradiography after fixation by perfusing with glutaraldehyde, the transport of Leu could be localized to the endothelial cells of the larger vessels of the testicular microvasculature.