Cationic amino acid fluxes beyond the proximal convoluted tubule of rat kidney.

To investigate the fluxes of cationic amino acids beyond the proximal convolution, we micropunctured and microperfused superficial tubules of male Wistar rats in vivo et situ. In free-flow micropuncture experiments, the concentrations of endogenous L-arginine+, [Arg], and of intravenously infused L-homoarginine+, [HoArg], were determined by HPLC. Fluorescein isothiocyanate-labeled inulin was detected on-line in the same tubular fluid samples. To determine undirectional fluxes, radiolabeled Arg and inulin were (1) microperfused through short loops of Henle and (2) microinfused into different tubule segments to measure urinary recovery of the radiolabel. At a mean [Arg]plasma of 116 mumol/l, [Arg] was 9.3 mumol/l in the late proximal tubule (LPT), and 35.6 mumol/l in the early distal tubule (EDT) corresponding to fractional deliveries (FD) of 0.055 in LPT and 0.078 in EDT. Fractional urinary excretion (FE) of Arg was 0.00033 (P < 0.05 vs FDEDT). Infusion of HoArg (2.5 or 7.5 mumol/min) led to respective mean [HoArg]plasma values of 1.44 and 3.73 mmol/l, and resulted in respective FDLPT values for HoArg of 0.23 and 0.53, respective FDEDT values of 0.29 and 0.41, and finally, respective FE values for HoArg of 0.25 and 0.58. When short loops of Henle were microperfused with 1 or 50 mmol/l [14C]Arg (+[3H]inulin), fractional recovery (FR) of 14C (relative to inulin) in the EDT was 0.13 and 0.36, respectively. During microinfusion of radiolabeled Arg (1 or 50 mmol/l) and inulin into LPT, the urinary FR of the radiolabel was 0.14, or 0.59, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)