Guanine nucleotide-dependent carboxymethylation: A pathway for aldosterone modulation of apical Na+ permeability in epithelia

Abstract

A number of recent reviews have dealt extensively with the characteristics of epithelial Na + channels and their classification according to level of conductance, selectivity for sodium, and sensitivity to amiloride and hormones [1 to 6, this issue]. In the present review, we will focus on Na + transport regulation by aldosterone in terms of the biochemical pathways involved in the hormone's action on apical Na + permeability in epithelia. Active transport of Na + across epithelial tissues is the primary physiological process responsible for maintenance of salt balance in vertebrates. Entry of Na + into cells at the apical membrane occurs passively by electrodiffusion through amiloride-blockable channels. Intracellular Na + concentration remains low due to active extrusion across the basolateral membrane in exchange for K + . Aldosterone is the key hormone for long-term regulation of this process in the distal tubule of the kidney and other responsive high resistance model epithelia such as toad urinary bladder, frog skin, and A 6 cultured cells derived from toad kidney [7, 8]. Its mode of action is complex involving a number of biochemical pathways, and some or possibly all of them ultimately increase the transepithelial Na + transport rate. Aldosterone increases both the apical Na + permeability and the number of basolateral pump sites albeit with different time courses and through distinct pathways [9–12]. The increase of apical Na + permeability in response to aldosterone is due to an increase in the number of open Na + channels in this membrane with virtually no change in channel selectivity or conductance [10, 13]. Stimulation of transport begins after a lag time of 20 to 40 minutes and is maximal within four to six hours of exposure to the hormone. Aldosterone, a steroid hormone, enters cells by diffusion, is bound by an intracellular receptor, migrates to the nucleus, and gives rise to a variety of gene products commonly termed AIPs (aldosterone-induced proteins; cf. Fig. 1), which in turn lead to the various physiological responses. This scheme is supported by observations that increased Na + transport after aldosterone is abolished by inhibitors of protein or RNA synthesis [14]. It is currently unknown whether any of the AIPs represent apical Na + channels per se or subunits of these channels. This seems unlikely however in view of overwhelming electrical and biochemical data consistent with the idea that aldosterone stimulates channels pre-existing in the membrane [13, 15–18]. In addition, membrane targeting, channel assembly from subunits, and insertion of these complex molecules into the membrane, would likely take several hours as observed for the basolateral response where it has been demonstrated that pumps are synthesized and inserted over many hours or days [11, 12]. This would seem less plausible at the apical membrane where the more rapid initial increase in permeability occurs within 30 minutes. However, a polypeptide of M r 70 kD, identified as a component of the Na + channel, was shown to be induced by aldosterone [19]. Whether this polypeptide represents part of the channel or a regulatory protein is presently unknown. The sequence of events leading to increased channel density following synthesis of AIPs is not understood. However, a number of mechanisms modulate or mediate aldosterone's stimulation of apical Na + permeability. (1) Activation of phospholipase A increases phospholipid fatty acid metabolism. (2) Methylation of apical proteins and lipids increases amiloride-sensitive Na + transport. (3) Guanine nucleotides regulate the aldosterone-induced carboxymethylation. These pathways are not mutually exclusive and indeed may represent only a small portion of the complete picture of aldosterone's influence at the apical membrane. Each of these is considered separately below.