Transepithelial short-circuit current (Iscc) and intracellular free Ca2+ (Ca2+i) was studied in monolayers of cultured human sweat duct cells (CSDCs) in the presence or absence of HCO3- (and CO2) in the bathing solutions. Addition of HCO3- (and CO2) increased the control Iscc by more than 50%. The effect of HCO3- (and CO2) on Iscc was confined to the serosal bath. The HCO3- (and CO2) effect was also studied during stimulation with the cholinergic agonist methacholine (MCh), which in CSDC induces a complex response consisting of an initial Iscc and Ca2+i spike, which is independent of extracellular Ca2+, followed by regular Iscc and Ca2+i oscillations, which are absent during Ca(2+)-free bathing conditions. The sustained Iscc and Ca2+i oscillations, but not the initial Iscc and Ca2+i spike were abolished by the removal of extracellular HCO3- (and CO2). It is concluded that the Ca2+ influx and the Iscc in CSDCs are critically influenced by the presence of extracellular HCO3- (and CO2) in the bathing solutions.