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.