Activation of Na-H exchange by intracellular lithium in barnacle muscle fibers.
We internally dialyzed single barnacle muscle fibers (BMF) for 90 min with a dialysis fluid (DF) containing no Na+ and either 0 or 100 mM Li+ and measured intracellular pH (pHi) with a microelectrode. During dialysis, the pH 8.0 artificial seawater (ASW) contained neither Na+ nor HCO3-. After we halted dialysis with a Li(+)-free/low-pH DF and allowed pHi to stabilize at approximately 6.8, adding 440 mM Na(+)-10 mM HCO3- to the ASW caused pHi to recover rapidly and stabilize at 7.32. In contrast, when the DF contained 100 mM Li+, pHi stabilized at 7.49. In fibers dialyzed to a pHi of approximately 7.2, Li+ stimulated a component of acid extrusion that was dependent on Na+ but not affected by SITS. Thus Li+ activates a Na(+)-dependent acid-extrusion mechanism other than the well characterized Na(+)-dependent Cl-HCO3 exchanger. To study the Li(+)-activated mechanism, we minimized Na(+)-dependent Cl-HCO3 exchange by raising pHDF to 7.35 and pretreated BMFs with SITS. We found that dialysis with Li+ elicits a Na(+)-dependent pHi increase that is largely blocked by amiloride, consistent with the hypothesis that Li+ activates a latent Na-H exchanger even at a normal pHi. In the absence of Li+, the Na-H exchanger is relatively inactive at pHi 7.35 (net acid-extrusion rate, Jnet = 9.5 microM/min) but modestly stimulated by reducing pHi to 6.8 (Jnet = 64 microM/min). In the presence of Li+, the Na-H exchanger is very active at pHi values of both 7.35 (Jnet = 141 microM/min) and 6.8 (Jnet = 168 microM/min). Thus Li+ alters the pHi sensitivity of the Na-H exchanger. Because the Na-H exchanger is only approximately 6% as active as the Na(+)-dependent Cl-HCO3 exchanger in the absence of Li+ at a pHi of approximately 6.8, we suggest that the major role of the Na-H exchanger may not be in pHi regulation but in another function such as cell-volume regulation.
Davis
B A
BA
Department of Cellular and Molecular Physiology, Yale University School of Medicine, New Haven, Connecticut 06510.
Hogan
E M
EM
Boron
W F
WF
D000143:Acids / Q000494:pharmacology; D000818:Animals; D001639:Bicarbonates / Q000494:pharmacology; D002352:Carrier Proteins / Q000378:metabolism*; D003956:Dialysis; D006863:Hydrogen-Ion Concentration; D007425:Intracellular Membranes / Q000378:metabolism*; D008094:Lithium / Q000378:metabolism / Q000494:pharmacology*; D009132:Muscles / Q000378:metabolism*; D012964:Sodium / Q000494:pharmacology; D017923:Sodium-Hydrogen Exchangers; D001468:Thoracica
D000143:Acids; D001639:Bicarbonates; D002352:Carrier Proteins; D017923:Sodium-Hydrogen Exchangers; D008094:Lithium; D012964:Sodium