CONTROL OF EPITHELIAL NA+ TRANSPORT BY APICAL NA+ ENTRY

在肾上皮细胞中，钠的载体运输是一个两步过程。

In kidney epithelial, vectorial sodium transport is a two step process. First, sodium entry occurs trough amiloride-blockable ion channels located in the apical membrane of sodium transporting cells. Second, the sodium is active transported out of the cells via a basolateral Na+/K+-ATPase. The net effect of these two processes, which occur in series, is the vectorial transport of sodium from the luminal compartment to the basolateral compartment. Previous data suggest that the rate of apical Na+ entry regulates the activity of these transporter proteins. Thus, the nA+ reabsorptive capacity of distal nephron segments increases in response to increased load and decreases in espouse to decreased load. This form of regulation is responsible from many clinically important effects on salt balance, including the maintenance of glomerular-tubular balance, the phenomenon of diuretic resistance, the development of post-obstructure diuresis and the occurrence of downstream tubular atrophy in glomerular disease. Moreover, the control of renal sodium excretion is involved in the pathogenesis of diverse disease states such as hypertension, nephrotic syndrome, and congestive heart failure. Currently, the mechanisms by which the rate of na+ entry regulates Na+ transporter activity are poorly understood. The proposed studies will address this issue at a molecular level using a well characterized model of collecting duct epithelial cells, the A6 cell line grown on porous supports. Published preliminary data in t his cell culture model demonstrates that blockage of Na+ entry result in striking on regulation of both the apical Na+ channel and the basolateral Na+/K+-ATPase. In addition, stimulation of aplical Na+ etry up- regulates both pathways. Recently cDNA encoding for three proteins, alpha, beta, and gamma EnaC (epithelial Na+ channel), has been identified and cloned. All of the activity of low conductance, highly sodium selective, amiloride-sensitive epithelial sodium channel from cortical collecting duct can be accounted for by these channel proteins. Homologues of these proteins have been described in A6 cells and have been termed XeNaC (Xenopus Na+ channel). Using antibody and northern blot probes to the XeNaC channel, and to the alpha and beta subunits of the Na+/K+-ATPase, the effect of apical Na+ entry on the transcriptional, translational and post-translational regulation of these transporter proteins will be described.