Phosphate transport across rat jejunum: influence of sodium, pH, and 1,25-dihydroxyvitamin D3.

Inorganic phosphate (Pi) transport in intact, rat jejunal epithelium was measured in vitro under short-circuited conditions. Transepithelial net absorptive Pi flux increased linearly with increases in extracellular sodium concentration ([Na]) up to 144 mM. Transmucosal border Pi influx, in contrast, displayed a biphasic Na dependency. Pi influx increased as [Na] was raised from 0 to 100 mM. A further increase in [Na] to 144 mM caused unanticipated reduction in Pi influx. The reason for this dissociation between transmucosal border influx and transepithelial absorptive flux is not clear. We then examined the effect of changes in extracellular pH on Pi influx. Reduction in pH from 7.4 to 6.0 was associated with 150% increase in Pi influx, an observation consistent with the reported reciprocal relation between intestinal brush-border membrane vesicles (BBMV) Pi uptake and extravesicular pH. In contrast to BBMV data, however, a smaller increase (50%) in mucosal Pi influx was noted in intact epithelium when pH was increased from 7.4 to 8.5. Under optimized conditions for Pi influx, i.e., [Na] = 90 mM and pH 6.0, the effect of 1,25-dihydroxyvitamin D3 [1,25(OH)2D3] on Pi influx was measured. The total influx could be resolved into a saturable, Na-dependent and a nonsaturable, Na-independent component. 1,25(OH)2D3 stimulated the saturable component of Pi influx.