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Abstract

In order to study the characteristics of contraluminal phosphate transport the stopped flow microperfusion technique [13] has been applied. By measuring the time-dependent decrease of interstitial ³³P_i concentration at different starting concentrations a simple diffusion kinetics with a permeability coefficient of 7.5±1.0 · 10⁻⁸ cm² s⁻¹ was found. Such a kinetic was so far only observed with 2-deoxy-d-glucose. This substance, however, is transported in addition by facilitated diffusion as was seen by paraaminohippurate, methylsuccinate and sulfate. The contraluminal transport of phosphate was inhibited by H₂-DIDS (5 mmol/l). It was, however, not influenced by omission of Na⁺ from the perfusates, by addition of sulfate (150 mmol/l), methylsuccinate (50 mmol/l), arsenate (50 mmol/l), the Hg-compound mersalyl (5 mmol/l), high and low phosphate diet and pH changes between 6.0 and 8.0. The data indicate that phosphate, which is reabsorbed from the lumen by a Na⁺-dependent transport system, leaves the cell by a rather unspecific contraluminal diffusion pathway.