Contraluminal para-aminohippurate (PAH) transport in the proximal tubule of the rat kidney - I. Kinetics, influence of cations, anions, and capillary preperfusion

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 ofH-paraaminohippurate from the capillary perfusate at different concentrations and contact times was measured and saturation type behaviour was found with a K of 0.08±0.01 (SE) mmol/l, J of 1.1±0.1 pmol·s·cm and r, 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 and 25 mmol/l Hepes in isotonic solutions reduced PAH uptake at high transport rates. Addition of 5 mmol/l Ca, Mg, Mn, Ba, Cd to isotonic Na-gluconate solution did not influence PAH uptake except for Mg and Mn which inhibited uptake at small transport rates only. Preperfusion of the peritubular capillaries with rat serum, Na gluconate (Ca-+Mg-free), Na gluconate (Ca-+Mg-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-, Ca- and Mg-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-, Ca-, Mg-free preperfusion - with serum or solutions which contained Na and Ca or Mg 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. © 1987 Springer-Verlag.