Hormonal and osmotic regulation of NaCl transport in renal distal nephron epithelium.

One of the most important factors controlling blood pressure is the total body Na+ content, which depends upon Na+ intake and excretion. The kidney influences body Na+ content by regulating the tubular absorption of the Na+ filtered through the glomeruli. Thus, the regulation of Na+ absorption in the tubules of the kidney plays an important role in controlling blood pressure. More than 99% of the Na+ passing through the glomerulus is reabsorbed in the kidney. About 90% of the filtered Na+ through the glomerulus is reabsorbed in the proximal tubule and the ascending limb of the loop of Henle. The remainder of the Na+ absorption occurs in the distal nephron. This process is regulated by hormones such as aldosterone and antidiuretic hormone (ADH), and also by the osmolality of the plasma. These observations suggest that the regulation of Na+ transport in the distal nephron by hormones and osmolality plays an important role in the control of blood pressure. The distal nephron is composed of two different types of epithelial cells: the principal cell and the intercalated cell. The latter is also composed of two types of cells: alpha and beta intercalated cells. In addition to Na+ absorption, the distal-nephron epithelial cells also participate in K+ and H+ secretion. Na+ absorption is mediated through the principal cell, which also contributes to K+ secretion, whereas H+ is secreted through both types of intercalated cells, alpha and beta, in different ways. There are, in general, two steps in the transepithelial ion movement across the epithelium, including the distal-nephron epithelium. For example, in the case of Na+ absorption, one is the entry step of Na+ across the apical membrane and the other is the extrusion step of Na+ across the basolateral membrane. This means that there are two major regulatory sites of transepithelial Na+ absorption: namely, regulation of the entry and extrusion steps of Na+. It is generally thought that the entry step of Na+ across the apical membrane is the rate-limiting step in the transepithelial Na+ transport and that Na+ channels in the apical membrane play an important role as an entry step of Na+ and are regulated by hormones and plasma osmolality. In this review, we describe the regulatory mechanisms of Na+ absorption in renal distal-nephron epithelium by aldosterone, ADH and osmolality. Further, we will review the regulatory mechanisms of Cl- transport, which also plays an important role in Na+ transport as a major counter ion, and discuss other roles of Cl- in the active regulation of Na+ transport.