Na + /H + exchangers isoforms

Although a uniform nomenclature for Na /H exchanger isoforms has not yet been adopted, we will refer to the amiloride-sensitive type of Na" /H exchanger that is present in the basolateral membrane of epithelia (apical membrane of placental syncytiotrophoblast) and also widely distributed in non-epithelial cells as the sensitive-type. The relatively amiloride-resistant isoform present in apical mem-... 

Recently, Tse et al. [73] and Orlowski et al. [74] have cloned a third isoform of Na /H exchanger (named NHE-3). The inferred 832-amino acid sequence of rabbit NHE-3 is 41% identical with NHE-1, 44% identical with NHE-2, and has a similar secondary structure. In contrast to NHE-1 and NHE-2, NHE-3 is only expressed in epithelia in intestine and kidney. Moreover, administration of glucocorticoids, which stimulates transport activity of the apical Na /H" exchanger in rabbit intestine, increased levels of NHE-3 transcripts but did not affect NHE-1 or NHE-2 [75]. Taken together, these results suggest that NHE-3 may encode a resistant-type Na /H exchanger of epithelia. A fourth Na /H exchanger isoform (NHE-4) is preferentially expressed in stomach [74]. 

Tse, C.M., Brant, S.R., Walker, S., Pouyssegur, J., Donowitz, M. (1992). Cloning and sequencing of a rabbit cDNA encoding an intestinal and kidney specific Na+/H+ exchanger isoform. J. Biol. Chem. 267,9340-9346. 

Ng LL, Quinn PA, Paulene A, Baker F, Carr SJ. Red cell Na/LI countertransport and Na/H exchanger isoforms in human proximal tubules. Kidney Int 2000 58(1) 229-235. 

T. Shimohama, Y. Suzuki, C. Noda, H. Niwano, K. Sato, T. Masuda, K. Kawahara, T. Izumi, Decreased expression of Na+/H+ exchanger isoform 1 (NHE1) in non-infarced myocardium after acute myocardial infarction, Jpn Heart Ji, 273-82 (2002). 

Barr KJ, Garrill A, Jones DH, Orlowski J, Kidder GM 1998. Contributions of Na+/H+ exchanger isoforms to preimplantation development of the mouse. Mol Reprod Dev50(2) 146-153. 

Biochemical studies of plasma membrane Na /H exchangers have been directed at two major goals (1) identification of amino acids that are involved in the transport mechanism and (2) identification and characterization of the transport pro-tein(s). To date, most studies have been performed on the amiloride-resistant form of Na /H" exchanger that is present in apical or brush border membrane vesicles from mammalian kidney, probably because of the relative abundance of transport activity in this starting material. However, some studies have also been performed on the amiloride-sensitive isoform present in non-epithelial cells. 

Next, Reilly et al. [65] localized the Na /H exchanger gene product in renal epithelial cells where the distributions of the kinetic isoforms was well-established. The strategy was based on the observation that the resistant- and sensitive-types are restricted to the apical and basolateral membranes, respectively, in confluent LLC-PK]/Clone 4 cells [8]. Thus, if proteins encoded by the cloned cDNAs localized to the apical membrane this would indicate that they represent the resistant-type. Localization to the basolateral membrane would prove they were the sensitive-type and presence on both membranes would suggest that the two functional isoforms had identical primary structures. Na exchanger proteins were localized by... 

Saxena R, Saksa BA, Fields AP, Ganz MB. Activation of Na/H exchanger in mesangial cells is associated with translocation of PKC isoforms. Am J Physiol 1993 265 F53-F60. 

Three other forms of the Na+/H+ exchanger have subsequently been cloned. Screening of a rabbit kidney proximal tubule library with an NHE-1 probe (using low stringency hybridization) has led to the identification of two other forms of Na+/H+ exchanger (Tse et al., 1992) that are entirely (NHE-3) or predominantly (NHE-2) expressed in the intestine and kidney. A fourth form of Na+/H+ exchanger (NHE-4) has been identified in the gastrointestinal tract (Orlowski et al., 1992). These isoforms are less sensitive than NHE-1 to amiloride derivatives. 

The properties of other isoforms of the Na+/H+ exchanger have been less thoroughly studied but they may be different from those of NHE-1. For instance, the NHE-1 isoform of mammalian fibroblasts is insensitive to cAMP, whereas the P-NHE-1 isoform of trout erythrocytes is sensitive to the effector. Conversely, the isoform which is present in the apical membrane of epithelial cells of the small intestine and kidney (NHE-2 or NHE-3) is inhibited rather than activated by phorbol esters. 

Using a variety of cell lines, D2R has been shown to couple to numerous G-proteins including Got i, Gai2, Gai3, Ga0 and Gaz. Depending on the cell type and isoform expressed, D2R activation can lead to the inhibition of AC and cAMP production, activation of potassium channels, inhibition of L-type calcium channels, stimulation of PLC activity and calcium mobilization, potentiation of Ca2+-evoked arachidonic acid release, stimulation of Na+/H+ exchange, and regulation of PI hydrolysis (Di Marzo et al., 1993 Mercier et al., 2001). 

The red blood cell, which is a convenient model, shows a cell-to-plasma lithium ratio of 0.3-0.6, whereas the Nernst equation would predict a 1.6 ratio. When red blood cells are loaded with hthium in vitro its extrusion is accomplished by a Na /LP countertransporter (SLC), the physiological role of which is unclear, but some believe it represents a mode of operation of the Na /H exchanger. Interestingly, a recent paper suggested that red cell SLC may be a marker of the activity of Na /H exchanger-3 the isoform expressed in the kidney proximal tubule rather than the ubiquitous Na /H exchanger-1 isoform [5]. 

In addition to the NaVK -ATPase, there is also a ubiquitous Na -H exchanger (often referred to as an antiporter) that actively pumps H out of the ICF in exchange for Naft This exchanger is critical for maintaining intracellular pH homeostasis and volume in many ceU types. At least six different isoforms of this transmembrane protein have been identified, and the regulation and tissue distribution of these differ. Of particular importance is the role of this exchanger for acid-base regulation in renal tubular cells as discussed later in this chapter. 

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