Abstract
In the adult kidney, carrier mediated Pi co-transport is highly Na+ specific. Studies in our laboratory showed that Pi uptake (PiU) in renal brush border membrane vesicles (BBMV) from FL kidneys is also stimulated by Na+. In contrast to the adult, we report here that carrier mediated PiU in FL renal BBMV is also energized by a K+ gradient ([K+]o>[K]i). BBMV from 5 FL kidneys were prepared by CaCl2 precipitation and verified by marker enzyme assays, electron microscopy, and PiU into osmoticaly active vesicles. Peak 60s PiU with a 100mM Na+ and K+ gradient was 1968±65 and 1010±133 pMol/mg BBMV protein, respectively (p<.01). PiU for each was greater than diffusion PiU (p<.001 each). The effect of trans-stimulation was studied by preloading BBMV with 5mM Pi. Initial 12s PiU in preloaded BBMV was 1007±181 pMol/mg BBMV protein and was 4-fold higher than PiU in non-loaded BBMV (p<.001). The effect of extravesicular (O) pH was studied by measuring initial 15s PiU in BBMV with constant intravesicular pH 7.4 and varying O pH from 6.0, 7.4 and 8.0. PiU was 700±64, 1983±230 and 411±10 pMol/mg BBMV protein for OpH 6.0, 7.4 and 8.0 respectively. PiU at pHO=pHi was higher than that for OpH 6.0 and OpH 8.0 (p<.01; p<.001). The results demonstrate K+-Pi symport in FL kidney. The co-transport system may represent shared affinity by K+ with Na+ for the cation site on the carrier or a different carrier system.
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