Figure 5.Cation dependence of NBC A. Solution pulse protocol. This solution protocol is used to test whether K+, Li+, or choline+ are capable of stimulating a HCO3 - -dependent current from rkNBC oocytes. Oocytes were voltage clamped at –60 mV and bathed in ND96 for 5 min before switching to test cation/non-HCO3 - ringer for 5 min. The Bath solution was then switched to the corresponding 1.5% CO2/10 mM HCO3 - solution for 2 min, e.g., Li+-ND96 to 1.5% CO2/10 mM HCO3 -/96 mM Li+) and returned to non-HCO3 - Ringer for 2 min. An I-V relation was recorded before and after each solution change. HCO3 - -stimulated current for each cation was taken as the difference between the non-HCO3 - and HCO3 - I-V responses. Cation solutions order was randomized. B. rkNBC I-V response of cations. HCO3 - subtracted I-V response curves from the current sweeps show that only Na+ (black) stimulates a strong HCO3 - -dependent current. Extracellular K+ (red) and choline+ (green) have I-V relations that lie on the voltage axis, indicating no transport. Li+ (blue) shows only a slight current response of a maximal about 3% of the Na+ response over the voltage range tested. Modified from [25].