Regulation Of Renal Tubular Transport Research Articles

Roles of Akt and SGK1 in the Regulation of Renal Tubular Transport.

A serine/threonine kinase Akt is a key mediator in various signaling pathways including regulation of renal tubular transport. In proximal tubules, Akt mediates insulin signaling via insulin receptor substrate 2 (IRS2) and stimulates sodium-bicarbonate cotransporter (NBCe1), resulting in increased sodium reabsorption. In insulin resistance, the IRS2 in kidney cortex is exceptionally preserved and may mediate the stimulatory effect of insulin on NBCe1 to cause hypertension in diabetes via sodium retention. Likewise, in distal convoluted tubules and cortical collecting ducts, insulin-induced Akt phosphorylation mediates several hormonal signals to enhance sodium-chloride cotransporter (NCC) and epithelial sodium channel (ENaC) activities, resulting in increased sodium reabsorption. Serum- and glucocorticoid-inducible kinase 1 (SGK1) mediates aldosterone signaling. Insulin can stimulate SGK1 to exert various effects on renal transporters. In renal cortical collecting ducts, SGK1 regulates the expression level of ENaC through inhibition of its degradation. In addition, SGK1 and Akt cooperatively regulate potassium secretion by renal outer medullary potassium channel (ROMK). Moreover, sodium-proton exchanger 3 (NHE3) in proximal tubules is possibly activated by SGK1. This review focuses on recent advances in understanding of the roles of Akt and SGK1 in the regulation of renal tubular transport.

Guanylyl cyclase-C receptor mRNA distribution along the rat nephron

Guanylin (GN) and uroguanylin (UGN) are two recently identified peptides that have been shown to affect water and electrolyte transport in both the intestine and the kidney. Mechanistically, the effects of both peptides are thought to be mediated by intracellular cGMP which results from ligand binding to a plasma membrane guanylyl cyclase-C (GC-C) receptor. To date, the specific intrarenal site(s) of GN and UGN action have not been established. To begin to address this issue, the present studies utilized semi-quantitative RT-PCR to assess the distribution of GC-C mRNA in specific microdissected segments of the rat nephron. GC-C mRNA expression was highest in the cortical collecting tubule, followed by the proximal convoluted tubule, medullary thick ascending limb and collecting tubule, and thin limbs of Henle’s loop. Expression levels were significantly lower in all other segments tested, including the glomerulus. The renal tubular expression pattern for cGMP-dependent protein kinase II (cGK-II) mRNA, which is activated in response to GN/UGN-dependent cGMP accumulation, was similar to that for GC-C. Notably, both GN and UGN mRNAs were also expressed along the nephron. The highest levels of expression for both peptides were detected in the medullary collecting tubule. Lower, but comparable levels of GN and UGN expression also occurred in the cortical collecting tubule, cortical and medullary thick ascending limb, and thin limbs of Henles loop. In the proximal convoluted tubule, GN mRNA expression was also quite high, while UGN mRNA was almost undetectable. The presence of renal GC-C and cGK-II in the kidney are consistent with a proposed endocrine function for GN and UGN. In addition however, the present data suggest that intrarenally synthesized GN and UGN may also contribute to the regulation of renal tubular transport.

Studies on Renal Aldosterone-binding Proteins

Abstract 3H-Aldosterone-macromolecular complexes were isolated from the nuclear and cytosol fractions of rat kidneys by precipitation with (NH4)2SO4 and by chromatography on Sephadex G-50 and G-75. These complexes were recovered after injection of 3H-aldosterone into adrenalectomized rats and after incubation of labeled steroid with renal nuclear extracts and cytosol fractions in vitro. The nuclear aldosterone binding substances were identified as proteins by the following criteria: (a) chemical analysis of Tris-CaCl2 extracts of purified nuclear fractions; (b) dissociation of the complexes by wide spectrum proteolytic enzymes but not by nucleases; and (c) loss of 3H-aldosterone-binding activity when the complexes were treated with the sulfhydryl reagents, p-hydroxymercuribenzoate and 2-(2-aminoethyl)-2-thiopseudourea dihydrochloride. The cytosol aldosterone-binding substances were identified as proteins by the following criteria: (a) dissociation of the complexes by wide spectrum proteolytic enzymes but not by nucleases, lipase, or phospholipase; (b) loss of 3H-aldosterone-binding activity on treatment with p-hydroxymercuribenzoate. Estradiol-17β,6α-methylprednisolone, deoxycorticosterone, and 9α-fluorocortisol impaired the formation of the 3H-aldosterone-macromolecular complexes in the nuclear and cytosol fractions in direct proportion to their potencies in the regulation of renal tubular transport of sodium. In the in vitro system, spirolactone inhibited the formation of the 3H-aldosterone complex at concentration ratios that inhibit the action of aldosterone in the rat in vivo. The isomer of aldosterone, 17-isoaldosterone, was not bound to the proteins in nuclear extracts, and at a 10-fold concentration ratio did not inhibit the formation of 3H-aldosterone complexes in nuclear extracts. In addition, 17-isoaldosterone (1.4 x 10-7 m) had no effect on active sodium transport in the isolated toad bladder system and at 1 µg/150 g of body weight had no effect on the urinary Na+:K+ concentration ratios in the adrenalectomized rat. Thus, we have isolated and partially purified renal nuclear proteins which have many of the properties to be expected of the specific mineralocorticoid receptors.