Na+/H+-exchanger

Prototypic inhibitor of the family of Na/H exchangers and of epithelial Na+ channels. 

Additional cellular events linked to the activity of blood pressure regulating substances involve membrane sodium transport mechanisms Na+/K.+ ATPase Na+fLi countertransport Na+ -H exchange Na+-Ca2+ exchange Na+-K+ 2C1 transport passive Na+ transport potassium channels cell volume and intracellular pH changes and calcium channels. 

Na+/Ca2+ Exchangers. Figure 5 Chemical structures of amiloride derivatives and their IC50 on NCX and NHX activity. Chemical structure of the two classes of amiloride derivatives and their inhibitory concentrations on NCX and Na+/H+ exchanger activity (Reproduced from Annunziato L, Pignataro G, Di Renzo GF (2004) Pharmacol Rev 56 633-654). 

Na+/H+ Exchangers. Figure 1 Prototypical structure and transmembrane disposition of NHE. The amino (N) and carboxy termini of the molecule are indicated. 

Transduction mechanism Inhibition of adenylyl cyclase stimulation of tyrosine phosphatase activity stimulation of MAP kinase activity activation of ERK inhibition of Ca2+ channel activation stimulation of Na+/H+ exchanger stimulation of AM PA/kainate glutamate channels Inhibition of forskol in-stimulated adenylyl cyclase activation of phos-phoinositide metabolism stimulation of tyrosine phosphatase activity inhibition of Ca2+ channel activation activation of K+ channel inhibition of AM PA/ kainate glutamate channels inhibition of MAP kinase activity inhibition of ERK stimulation of SHP-1 and SHP-2 Inhibition of adenylyl cyclase stimulation of phosphoinositide metabolism stimulation of tyrosine phosphatase activation of K+ channel inhibi-tion/stimulation of MAP kinase activity induction of p53 and Bax Inhibition of adenylyl cyclase stimulation of MAP kinase stimulation of p38 activation of tyrosine phosphatase stimulation of K+ channels and phospholipase A2 Inhibition of adenylyl cyclase activation/ inhibition of phosphoinositide metabolism inhibition of Ca2+ influx activation of K+ channels inhibition of MAP kinase stimulation of tyrosine phosphatase... 

Silver RB, Mackins CJ, Smith NCE, Koritchneva IL, Lefkowitz K, Lovenberg TW> Levi R Coupling of histamine H3 receptors to neuronal Na+/H+ exchange a novel protective mechanism in myocardial ischemia, Proc Natl Acad Sci USA 2001 98 2855. Silver RB, Poonwasi KS, Seyedi N, Wilson SJ, Lovenberg TW, Levi R Decreased intracellular calcium mediates the histamine H3-receptor-induced attenuation of norepinephrine exocytosis from cardiac sympathetic nerve endings. Proc Natl Acad Sci USA 2002 99 501. 

Primarily using isolated plasma membrane vesicles as an experimental preparation, the functional properties of Na /H exchangers have been elucidated. The important kinetic properties include (1) stoichiometry (one-for-one) (2) reversibility (3) substrate specificity (monovalent cations Na, H, Li, NH4, but not K, Rb, Cs, choline) (4) modes of operation (Na -for-H, Na -for-Na Li " -for-Na, Na -for-NH4 ) (5) existence of an internal site for allosteric activation by (6) reversible inhibition by amiloride (Af-amidino-5-amino-6-chloropyr-azine carboxamide) and its 5-amino-substituted analogs and (7) competitive nature... 

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-... 

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. 

Kinetic studies indicated that external H interacted with the renal brush border Na /H exchanger (resistant-type) at a single site with apparent pKa 7.3-7.5. Because the imidazolium ring of histidine is the principal group that is titratable in this... 

Effects of group-specific reagents on transport activity and regulation of plasma membrane Na /H exchangers... 

Group Na /H exchanger type Reagent Effect" Protectant" Ref. 

Ganapathy et al. [18] showed that the human placental brush border Na" /H exchanger (a sensitive-type) was also inactivated by DEPC, and amiloride protected against inhibition. DEPC also inactivated the sensitive-type Na /H exchanger in thymic lymphocytes [19]. 

Further evidence that carboxyl groups are important for transport activity was provided by Igarashi and Aronson [22], Friedrich et al. [23], and Kinsella et al. [24] using the carboxyl group-specific reagent, A,A -dicyclohexylcarbodiimide (DCCD). DCCD irreversibly inactivated the brush border Na /H exchanger in rabbit and... 

Kulanthaivel et al. [28] found that the apical Na /H exchanger in human placenta (sensitive-type) was sensitive to phenylarsine oxide, a reagent specific for dithiols that are situated in close proximity (vicinal dithiols). Moreover, the effect of phenylarsine oxide was to decrease without affecting apparent affinity for... 

Na oi and was partially blocked by amiloride but not by cimetidine. Since these investigators also found that amiloride and cimetidine bound competitively with Na" at the external transport site of the placental brush border Na /H exchanger, they concluded that the vicinal dithiol groups are necessary for transport function but are located at a site distinct from the external transport site. 

Igarashi and Aronson [22] found that the renal brush border Na /H exchanger (resistant-type) was inhibited 40% by 1 mM NEM, and inhibition was not blocked by 1 mM amiloride. Haggerty et al. [13] reported that both the apical and basolat-eral Na /H exchangers in LLC-PKi cells were inactivated by 0.5mM NEM, although the apical Na /H exchanger was more sensitive to inhibition (70% inhibition compared to 20% inhibition of the basolateral transport activity). 

Taken together, these results indicate that similar to other proton-translocating membrane proteins, both types of Na /H exchangers contain critical sulfhydryl groups that are involved in the transport mechanism. These sulfhydryl groups do not appear to be present at the external transport site but may be involved in switching from an inactive to an activated state. 

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