Lee D B, Walling M W, Corry D B
Am J Physiol. 1986 Jul;251(1 Pt 1):G90-5. doi: 10.1152/ajpgi.1986.251.1.G90.
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.
在短路条件下，对完整的大鼠空肠上皮中的无机磷酸盐（Pi）转运进行了体外测量。跨上皮净吸收性Pi通量随细胞外钠浓度（[Na]）升高至144 mM呈线性增加。相比之下，跨黏膜边界Pi内流表现出双相性的钠依赖性。随着[Na]从0升高至100 mM，Pi内流增加。当[Na]进一步升高至144 mM时，Pi内流出现意外降低。跨黏膜边界内流与跨上皮吸收通量之间这种分离的原因尚不清楚。然后我们研究了细胞外pH变化对Pi内流的影响。pH从7.4降至6.0与Pi内流增加150%相关，这一观察结果与报道的肠刷状缘膜囊泡（BBMV）Pi摄取与囊泡外pH之间的反比关系一致。然而，与BBMV数据不同的是，当pH从7.4升高至8.5时，完整上皮中的黏膜Pi内流仅出现较小幅度的增加（50%）。在Pi内流的优化条件下，即[Na] = 90 mM且pH 6.0时，测量了1,25 - 二羟维生素D3 [1,25(OH)2D3]对Pi内流的影响。总内流可分解为一个可饱和的、钠依赖性成分和一个不可饱和的、钠非依赖性成分。1,25(OH)2D3刺激了Pi内流的可饱和成分。
