Functional properties of the rat Na/H exchanger NHE-2 isoform expressed in Na/H exchanger-deficient Chinese hamster ovary cells.
The primary structure and functional expression of the rat Na/H exchanger (NHE) NHE-2 isoform has recently been reported (Wang, Z., Orlowski, J., and Shull, G. E. (1993) J. Biol. Chem. 268, 11925-11928). To further characterize some of its functional properties, biochemical and pharmacological analyses were performed on exchanger-deficient Chinese hamster ovary cells (AP-1) that had been stably transfected with a full-length NHE-2 cDNA. Transport activity for NHE-2 was assayed by measuring amiloride-inhibitable 22Na+ influx following an acute intracellular acid load. Pharmacological analyses revealed that NHE-2 had a relatively high affinity for amiloride and some of its analogues. The most potent analogue was 5-(N-ethyl-N-isopropyl)amiloride (EIPA) (K0.5 = 79 nM), followed by 5-(N,N-dimethyl)amiloride (DMA) (K0.5 = 250 nM), amiloride (K0.5 = 1.4 microM), and benzamil (K0.5 = 320 microM). Nonamiloride compounds known to inhibit the activity of other Na/H exchanger isoforms also inhibited NHE-2 with the following order of potency: clonidine (K0.5 = 42 microM) > harmaline (K0.5 = 330 microM) approximately cimetidine (K0.5 = 330 microM). Biochemical analyses showed that the extracellular Na+ dependence of NHE-2 followed simple, saturating Michaelis-Menten kinetics with an apparent affinity constant for Na+ (KNa) of 50 mM. In contrast, intracellular H+ appeared to activate NHE-2 by a positive cooperative mechanism with an apparent half-maximal activation value of pK 6.90. Other cations, such as extracellular Li+ and H+, acted as competitive inhibitors of 22Na+ influx by NHE-2, with apparent Ki values of 3.0 mM and 10 nM, respectively. In contrast, extracellular K+ had no effect on the transport activity of NHE-2. These results indicated that the rat NHE-2 cDNA encodes a functional Na/H exchanger isoform with distinct properties compared to rat NHE-1 and -3.
Yu
F H
FH
Department of Physiology, McGill University, Montréal, Québec, Canada.
Shull
G E
GE
Orlowski
J
J
D000584:Amiloride / Q000031:analogs & derivatives / Q000494:pharmacology*; D000818:Animals; D001693:Biological Transport, Active / Q000187:drug effects; D016466:CHO Cells; D002352:Carrier Proteins / Q000096:biosynthesis / Q000187:drug effects / Q000378:metabolism*; D002927:Cimetidine / Q000494:pharmacology; D002999:Clone Cells; D003000:Clonidine / Q000494:pharmacology; D006224:Cricetinae; D006246:Harmaline / Q000494:pharmacology; D006863:Hydrogen-Ion Concentration; D007700:Kinetics; D051381:Rats; D012964:Sodium / Q000378:metabolism*; D017923:Sodium-Hydrogen Exchangers / Q000235:genetics / Q000378:metabolism*
D002352:Carrier Proteins; C513110:Slc9a2 protein, rat; D017923:Sodium-Hydrogen Exchangers; C069139:growth factor-activatable Na-H exchanger NHE-1; D000584:Amiloride; D002927:Cimetidine; D012964:Sodium; D006246:Harmaline; D003000:Clonidine