PHOSPHATE (Pi) UPTAKE IN FETAL LAMB (FL) RENAL BRUSH BORDER MEMBRANES (BBMV)

Abstract

We investigated Pi uptake (PiU) in BBMV in 9 FL and 6 ewes (E) at 100-165 days' gestation. BBMV were prepared by differential centrifugation, and PiU was measured by Millipore filtration. PiU after lh incubation in extravesicular media (EM) with [sucrose] from 0.3-1.0 osm varied inversely with [1/osm] (r-0.94, p<.01), indicating PiU into osmotically active BBMV. Results, x, ±, nMol/mg protein: (a-p<.01; b-p<.005; c-p<.001)

1-Na⁺-gradient ([Na⁺]_o>[Na⁺]_i); 2-Na⁺-dependent [(Na⁺-G)-(O-Na⁺)]. In FL, Na⁺-G PiU varied directly with EM pH from 6.5-8.5 (r-.79, p<.01) and was 1.6-fold greater at pH 6.5 than at pH 8.5. Na⁺-G PiU at 15s in FL remained linear with EM [substrate] 0.1-3.0 mMol (r-.99, p<.001); however, O-Na⁺ PiU was saturated at 0.8 mMol with an apparent Km of 0.39 mMol and Vmax of 48.4 nMol/15s·mg protein. The results suggest significant differences between mechanisms of renal Pi handling in FL compared to E with non-Na⁺-Dep active PiU also present in FL. This may explain in part low renal Pi excretion in early life.

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