Effects of L-bromotetramisole on phosphate transport by the proximal renal tubule: failure to demonstrate a direct involvement of alkaline phosphatase

Several recent studies have suggested that the alkaline phosphatase situated in the brush border membrane of the proximal convoluted tubule is positively related to inorganic phosphate reabsorption through this segment of the nephron. We examined this hypothesis by studying the influence of an inhibitor of alkaline phosphatase, L-bromotetramisole, upon phosphate transport through the isolated rabbit tubule, phosphate transport through brush border membrane vesicles of rats, and phosphate efflux from these vesicles. Results were compared with those obtained with D-bromotetramisole which is inactive on alkaline phosphatase.The microperfusion studies on isolated tubules did not show any significant influence of L-bromotetramisole on phosphate transport which slightly increased by 0.22 ± 0.78 pmol ∙ mm−1 ∙ min−1 (NS).The experiments performed on brush border membrane vesicles indicated that at pH 7.5, both L- and D-bromotetramisole at a concentration of 0.1 mM slightly but significantly increased phosphate input from 1800 ± 149 to 1999 ± 157 and 1982 ± 168 pmol/mg protein per 30-s incubation (P < 0.01 and P < 0.005, respectively). At pH 8.5, the same tendency was observed but was significant with the D isomer only. Finally, the effect of an increasing phosphate transport was accentuated when both isomers were tested at 1.0-mM concentration. Phosphate efflux from brush border membrane vesicles was not influenced by L- or D-bromotetramisole at 0.1 mM. At 1.0 mM, however, both isomers accelerated this release.These results do not support the hypothesis of a direct action of alkaline phosphatase upon phosphate transport through the entire proximal convoluted tubule, nor upon input or output of this electrolyte through the brush border membrane vesicles. The two forms of bromotetramisole have variable effects upon phosphate transport which are unrelated to the alkaline phosphatase activity.