Contraluminal para-aminohippurate (PAH) transport in the proximal tubule of the rat kidney

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Abstract
In order to study the characteristics of contraluminal para-aminohippurate transport into proximal tubular cells the stopped flow capillary perfusion method was applied. The disappearance of3H-paraaminohippurate from the capillary perfusate at different concentrations and contact times was measured and saturation type behaviour was found with aK m of 0.08±0.01 (SE) mmol/l,J max of 1.1±0.1 pmol·s−1·cm−1 andr, the final extracellular/intracellular distribution ratio of 0.93±0.03. Omission of Na+ from the capillary test perfusate caused a small reduction of contraluminal PAH uptake at small transport rates (0.1 mmol/l PAH in the test perfusate) but not at high transport rates (1.0 mmol/l PAH in the test perfusate). Change of K+ between 0 and 40 mmol/l and pH between 6.0 and 8.0 did not influence contraluminal PAH uptake. Isotonic replacement of chloride by gluconate, nitrate, sulfate, phosphate, methanesulfonate or increase in bicarbonate to 50 mmol/l did not influence PAH uptake at small transport rates. But isotonic sulfate and phosphate, as well as 50 mmol/l HCO 3 − and 25 mmol/l Hepes in isotonic solutions reduced PAH uptake at high transport rates. Addition of 5 mmol/l Ca2+, Mg2+, Mn2+, Ba2+, Cd2+ to isotonic Na+-gluconate solution did not influence PAH uptake except for Mg2+ and Mn2+ which inhibited uptake at small transport rates only. Preperfusion of the peritubular capillaries with rat serum, Na+ gluconate (Ca2+-+Mg2+-free), Na+ gluconate (Ca2+-+Mg2+-free) plus 10 mmol/l lactate or pyruvate or 0.1 mmol/l 2-oxoglutarate did not influence PAH uptake at small PAH transport rates, but inhibited at high transport rates. Preperfusion of the capillaries for 10 s with Na+-, Ca2+- and Mg2+-free solutions reduced PAH uptake in the presence of Na+ at both transport rates. A second 10 s preperfusion — after the first 10 s Na+-, Ca2+-, Mg2+-free preperfusion — with serum or solutions which contained Na+ and Ca2+ or Mg2+ restored the PAH fluxes to control values. The data are compatible with the hypothesis that contraluminal PAH uptake occurs by a saturable transport mechanism in exchange for other intracellular anions rather than in cotransport with Na+ ions. It was, however, not possible to identify the type of counteranions involved. The large effect of cation replacement on para-aminohippurate transport, which was reported in many previous studies with kidney slices, is not a direct effect on the para-aminohippurate transporter, but is rather caused indirectly via cell metabolism and/or changed ion gradients.