Isolated renal tubule fragments prepared from adult Sprague-Dawley rats were used to study the cellular uptake of hypoxanthine. This uptake was rapid, reaching a steady state after 30 min of incubation. Analysis of the intracellular pool during the initial uptake and at the steady state revealed a concentration gradient of hypoxanthine consistent with active transport, although only one-third of the transported hypoxanthine remained unmetabolized. The remainder of the transported hypoxanthine was converted to inosine and inosinic acid, but detectable conversion to uric acid was not noted. A kinetic analysis of uptake revealed that two systems for cellular entry of hypoxanthine existed with Km1 = 0.005 and Km2 = 0.80 mM. Hypoxanthine uptake at physiologic concentrations was oxygen, sodium, and temperature dependent, but the addition of metabolic fuels and alteration of the medium pH over the range of from 6.1 to 7.4 had no effect. Adenine, guanine, and inosine inhibited the uptake of hypoxanthine via the low-Km system which mediates the majority of uptake at physiologic levels. Xanthine, uric acid, and probenecid inhibited uptake via the high-Km system, but did not affect uptake via the low-Km system. The data indicate that hypoxanthine at physiologic levels is transported into the renal tubule cell via a system different from that for other oxypurines.
John W. Foreman, Stanton Segal