REGULATION OF BODY WATER AND ELECTROLYTES CONCENTRATIONS BY RENAL NA/K/2CL TRANSPORTER (NKCC2)

已知膜转运蛋白基因在肾小管中的表达丰度在生长和衰老过程中都会发生变化。它的水平受到体内环境化学或荷尔蒙条件的变化的明显调节。体液电解质的生理病理变化可能直接影响离子转运蛋白mRNAs的表达，与食物摄入、体内代谢和各种疾病有关。为了研究可能调节该基因表达的因素，我们制备了两种模型动物：一种是钾负荷新生大鼠，另一种是心肌梗死大鼠。首先，研究了7-14日龄新生大鼠急性输注外源性钾后肾脏排钾能力的变化。超量钾排泄开始时间为1.7天。ROMK1表达的时程与额外排钾的时程平行增加。ROMK2和Kir7.1表达的时程不同。根据这些结果，推测可能的钾分泌限速步骤是位于集合管顶膜的ROMK1通道。其次，分别采用核糖核酸酶保护实验和Western blotting方法研究了心肌梗死大鼠NHE1mRNA和蛋白表达的时间依赖性变化。奇怪的是，NHE1蛋白在梗死后5h显著下降，而NHE1mRNA在同一时期变化不大。通过N+/H+逆向转运体减少钠内流可能通过减少Na+/K+ATPase的ATP消耗来保护心肌细胞。心肌梗死后电解质的继发性变化对Na/K/2Cl转运体(NKCC2)mRNA表达的调节仍未解决。

It is known that abundance of the membrane transporter gene expression in kidney tubules changes during the period of either growing or aging. Its level is provably regulated by changes in the chemical or hormonal conditions of internal environment. Physiological and pathological changes in body fluid electrolytes, which are related with food intake, internal metabolism, and various diseases, may directly affect expression of the ion transporter mRNA.In order to investigate the factors which may regulate the gene expression, we have prepared two model animals : one is the potassium-loaded newborn rat and the other one is the rat with cardiac infarction. First, development of the renal potassium excretion capacity was studied in the newborn rats of 7-14 days which were acutely infused with the exogenous potassium. The onset of the extra potassium excretion was determined as 1.7 day. Time-course of the ROMK1 expression was in parallel increased with that of the extra potassium excretion. Time-courses of the ROMK2 and Kir7.1 expression were different. According to these results, a possible rate limiting step for potassium secretion is hypothesized as ROMK1 channel at the apical membrane of collecting ducts. Second, time-dependent changes in the expression of NHE1 mRNA and protein were investigated by using either RNase protection assay or Western blotting, respectively, in the rat with cardiac infarction. It seems to be strange that NHE1 protein decreased significantly at 5 h after the infarction, whereas NHE1 mRNA varied little in the same period. Decrease in sodium influx through a N+/H+ antiporter may protect the cardiac cell by decreasing ATP consumption of Na+/K+ ATPase. Regulation of the Na/K/2Cl transporter (NKCC2) mRNA expression by the secondary changes in electrolytes after cardiac infarction remains unsolved.