Transport ofl-cystine in isolated perfused proximal straight tubules

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Abstract
Unidirectional fluxes ofl-35S-cystine and intracellular35S activity were measured in isolated perfused segments of rabbit proximal straight tubule. The absorptive (lumen-to-both) flux ofl-35S-cysteine showed a tendency toward saturation within the concentration limits imposed by the low solubility of cystine (0.3 mmol·l−1). In contrast, for the bath-to-lumen fluxes, there was a linear relation between the bathing solution concentration ofl-35S-cystine and the rate of35S appearance in the lumen. Nonlinear fitting of both sets of unidirectional flux data gave a maximal cystine transport rate (J max) of 1.45±0.27 (SEM) pmol min−1 mm−1, a Michaelis constant (K m) of 0.20±0.07 mmol·l−1, and an apparent permeability coefficient of 0.27±0.11 pmol min−1 mm−1 (mmol·l−1)−1 (approximately 0.06 μm/s). The35S concentration in the cell exceeded that in the lumen by almost 60-fold during the lumen-to-bath flux, and exceeded the bathing solution concentration by 4.7-fold during the bath-to-lumen flux. Thus cystine was accumulated by the cells across either membrane, but over 77% of the intracellular activity was in the form of cysteine. Although the presence of luminall-lysine or cycloleucine inhibited the absorptive flux of cystine, neither amino acid affected the bath-to-lumen flux.