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 (pH(i)) 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 pH(i) to stabilize at approximately 6.8, adding 440 mM Na+-10 mM HCO3- to the ASW caused pH(i) to recover rapidly and stabilize at 7.32. In contrast, when the DF contained 100 mM Li+, pH(i) stabilized at 7.49. In fibers dialyzed to a pH(i) 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 pH(DF) to 7.35 and pretreated BMFs with SITS. We found that dialysis with Li+ elicits a Na+-dependent pH(i) increase that is largely blocked by amiloride, consistent with the hypothesis that Li+ activates a latent Na-H exchanger even at a normal pH(i). In the absence of Li+, the Na-H exchanger is relatively inactive at pH(i) 7.35 (net acid-extrusion rate, J(net) = 9.5-mu-M/min) but modestly stimulated by reducing pH(i) to 6.8 (J(net) = 64-mu-M/min). In the presence of Li+, the Na-H exchanger is very active at pH(i) values of both 7.35 (J(net) = 141-mu-M/min) and 6.8 (J(net) = 168-mu-M/min). Thus Li+ alters the pH(i) 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 pH(i) of approximately 6.8, we suggest that the major role of the Na-H exchanger may not be in pH(i) regulation but in another function such as cell-volume regulation.