Effects of Medium pH on <i>p</i><i>-</i>Aminohippurate Transport in Rat Kidney Fragments

Abstract

Addition of many oxidizable substrates to medium often enhances p-aminohippurate (PAH) transport by incubating renal tissue. Since the oxidation of various substrates by rat kidney tissue may change medium pH and this pH change has not been considered by many in conclusions, we followed the effects of medium pH on the magnitude and kinetics of in vitro PAH transport. Two previous studies employing rat kidney slices reported different optimal medium pH for PAH accumulation, i.e. pH 6.5 and pH 8.0. By repeating these studies over a wide range of medium pH, we confirmed the presence of enhanced steady state accumulation relative to pH 7.4 at two separate pH ranges, one at pH 6.7–6.9 (+15%), and the other at pH 7.7–7.9 ( + 22%). PAH influx, as measured by early tissue accumulation, was not different at pH 7.4 and 7.8; while at pH 6.8, it actually decreased. It was only later in the incubation that accumulation was greater at pH 6.8 and 7.8. Probably due to the small stimulation in steady state accumulation at either pH, we could not discern, by the methodology currently available, if this was secondary to augmented influx, decreased efflux, or a combination of both in the latter part of incubation. We conclude that the contribution secondary to medium pH changes must be considered when evaluating the effects of various substrates on PAH accumulation by kidney slices.

The Mechanism of Acetate Stimulation of PAH Transport in Rat Kidney Fragments

“…We observed that when renal tissue is incubated for 90 min in phosphate-buffered medium con taining acetate, pH increases gradually to >7.8. We had previously found that PAH accumulation is significantly augmented at medium pH > 7.8 compared to medium pH 7.4 [2], Since the degree of enhancement to PAH accumulation in renal fragments in duced by 10 mM acetate (20-30%) is of the same magnitude as that produced by alkalo sis (medium pH > 7.8) alone (+20%), we hy pothesized that the in vitro effect of acetate on PAH uptake might be mediated in large part through the induced alkalosis [2], The present studies strengthen this hypo thesis. As previously reported [6], maximum tissue PAH uptake was reached by 30 min, and was maintained through 90 min when no substrate was present ( fig.…”

“…Recently, two observations were made on kidney tissue incubating in vitro that may help answer this question, at least in rats. First, the pH of phosphate-buffered medium consistently rises from 7.4 to greater than 7.8 when renal cortical tissue is incubated in 10 mM acetate [2], Second, PAH accumulation is signifi cantly higher in 'alkalotic medium' (pH>7.8) than at pH 7.4 [3. 4], It was pos tulated [2] that acetate-stimulated PAH ac cumulation was secondary to the rise in medium pH rather than a direct effect from substrate metabolism.…”

“…First, the pH of phosphate-buffered medium consistently rises from 7.4 to greater than 7.8 when renal cortical tissue is incubated in 10 mM acetate [2], Second, PAH accumulation is signifi cantly higher in 'alkalotic medium' (pH>7.8) than at pH 7.4 [3. 4], It was pos tulated [2] that acetate-stimulated PAH ac cumulation was secondary to the rise in medium pH rather than a direct effect from substrate metabolism. The present in vitro studies were undertaken to expand the pre vious studies in order to determine more precisely whether the elevation in pH in duced by acetate, lactate, and pyruvate is responsible, in whole or in part, for the effects of these substrates on PAH transport by incubating rat renal fragments.…”