Modulation of Na+-H+ exchange activity by intracellular Na+, H+, and Li+ in IMCD cells.
In the present study, cultured inner medullary collecting duct (IMCD) cells loaded with the pH-sensitive dye 2',7' bis(carboxyethyl)-carboxyfluorescein were used to study the interaction of H+, Na+, and Li+ with the intracellular face of the Na+-H+ exchanger. The interaction of Na+ with the external face of the exchanger followed simple saturation kinetics with an apparent affinity for Na+ of 19.5 +/- 2 mM (n = 3) at an extracellular pH of 7.4. Extracellular H+ and Li+ inhibit Na+o-H+i exchange, mainly due to interference with Na+ binding to an external site or the same enzyme intermediate species in the turnover cycle of the Na+-H+ exchanger. The interaction of H+ with the internal face of the exchanger does not follow simple saturation kinetics. The calculated Hill coefficient for H+ interaction with intracellular sites was 1.92. Na+ and Li+ also interact with the intracellular face of the Na+-H+ exchanger. This interaction of Li+ and Na+ with internal sites results in inhibition of Na+-H+ exchange activity in the forward direction (Na+ influx/H+ efflux). The inhibitory effect of intracellular Na+ and Li+ is pHi dependent and most apparent under physiological conditions, i.e., at an intracellular pH of 7.1, an extracellular pH of 7.4, and an extracellular Na+ of 140 mM. This study shows the importance of intracellular Na+ concentration in determining exchanger activity.
Wall
S M
SM
Veterans Administration Medical Center, West Los Angeles, California.
Kraut
J A
JA
Muallem
S
S
D000584:Amiloride / Q000494:pharmacology; D000818:Animals; D002352:Carrier Proteins / Q000378:metabolism*; D002478:Cells, Cultured; D006863:Hydrogen-Ion Concentration; D007679:Kidney Medulla / Q000187:drug effects / Q000378:metabolism*; D007684:Kidney Tubules / Q000378:metabolism*; D007685:Kidney Tubules, Collecting / Q000187:drug effects / Q000378:metabolism*; D007700:Kinetics; D008094:Lithium / Q000378:metabolism*; D008297:Male; D051381:Rats; D011919:Rats, Inbred Strains; D012964:Sodium / Q000378:metabolism* / Q000494:pharmacology; D017923:Sodium-Hydrogen Exchangers
D002352:Carrier Proteins; D017923:Sodium-Hydrogen Exchangers; D000584:Amiloride; D008094:Lithium; D012964:Sodium