Electrogenic sodium-bicarbonate cotransport in human ciliary muscle cells

Abstract

We investigated membrane voltage and intracellular pH (pHi) in cultured human ciliary muscle cells using a cell line (H7CM) and primary-cultured human ciliary muscle cells. 1) Resting potential was 58.9 +/- 1.0 mV in H7CM cells and 61.9 +/- 1.4 mV in primary cultures. The following data are from H7CM cells, but results from primary cultures were basically similar. 2) In HCO3(-)-CO2-buffered solution, removal of extracellular sodium resulted in a depolarization [change in membrane resistance (delta V) = 31.3 +/- 2.8 mV] that was less marked in the absence of HCO3(-)-CO2 (delta V = 0.5 +/- 2.6 mV) and reduced by 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid (DIDS) (delta V = 19.3 +/- 1.9 mV). 3) Removal of extracellular HCO3(-)-CO2 led to a depolarization (delta V = 13.2 +/- 0.8 mV) that was abolished in the absence of extracellular sodium and inhibited by DIDS. 4) Intracellular alkalinization led to a depolarization (delta V = 24.7 +/- 2.3 mV), and intracellular acidification resulted in a hyperpolarization (delta V = 9.4 +/- 1.1 mV) that was inhibited by DIDS and dependent on extracellular HCO3(-)-CO2 and sodium. 5) pHi backregulation after an acid load occurred in both the presence and absence of extracellular bicarbonate but not in the absence of extracellular sodium. Our data are consistent with an electrogenic Na(+)-HCO3- cotransport in human ciliary muscle cells, which is activated by intracellular acidification.

Cloning, Tissue Distribution, Genomic Organization, and Functional Characterization of NBC3, a New Member of the Sodium Bicarbonate Cotransporter Family

“…However, the characteristics of Na ϩ -dependent HCO 3 Ϫ transport vary between different muscle cell preparations. Specifically, the Na(HCO 3 ) n cotransporter in cardiac Purkinje fibers and ventricular myocytes (17, 24) and in certain smooth muscle cells (11, 26 -29) appears to be electroneutral in contrast to the electrogenicity reported in other cell types (13,25,34). Furthermore, stilbene-insensitive Na ϩ -dependent HCO 3 Ϫ transport has been reported in various smooth muscle cells (27,28).…”

“…The relative contribution of each process varies with cell type, the metabolic requirements of the cell, and local environmental conditions. Intracellular pH regulatory processes that have been characterized functionally in skeletal, smooth muscle, and cardiac cells include: Na ϩ /H ϩ exchange (1-3, 9 -22), Na(HCO 3 ) n cotransport (12,13,16,17,21,(23)(24)(25)(26)(27)(28)(29), Na ϩ -dependent (12,18,19,26,30,31) and -independent (19,24,26,29,32,33) Cl Ϫ /base exchange, and lactate/ proton cotransport (3).…”

The anion transport inhibitor DIDS increases rat hepatocyte K+ conductance via uptake through the bilirubin pathway.

“…Inhibition of basolateral Clconductance is also reported from bovine retinal pigment epithelium (Joseph & Miller, 1991). In human ciliary muscle cells, DIDS leads to a considerable depolarization of cell membranes, probably via inhibition of electrogenic Na'-HCO3-cotransport (Stahl et al 1992). In isolated late distal tubules of salamander kidney, 50/M of luminal DIDS induces a slowly developing hyperpolarization of the basolateral membrane, which was attributed to indirect effects via inhibition of apical Cl--HC03-exchange (Stanton, 1988).…”

“…Consequently, changes in membrane voltage may have considerable impact on the anion transport under investigation. In most transport studies using DIDS, however, membrane voltages were not measured, although distinct voltage effects of the compound have been reported (Inoue, 1985(Inoue, , 1986 Stanton, 1988;Wang, Wang, Silbernagl & Oberleithner, 1988;Gallemore & Steinberg, 1989;Hughes, Adorante, Miller & Lin, 1989;Joseph & Miller, 1991;Stahl, Lepple-Wienhues, Kuppinger, Tamm & Wiederholt, 1992).…”

Change of apparent stoichiometry of proximal-tubule Na(+)-HCO3- cotransport upon experimental reversal of its orientation.

“…In certain preparations, the stoichiometry has been reported to be 1:1 (7) or has not been clearly established (8)(9)(10). These results suggest either that there may be more than one Na+-HCOj transport protein or that there is a single protein whose stoichiometry may change.…”