Figure 4

Estimation of transport stoichiometry for NBCe2-C using the delta current method. a) NBCe2-C expressing cells were voltage clamped at -50 mV. A series of 400 ms voltage-clamp pulses range from -108 to +48 mV with increment of 12 mV (containing a pulse to 0 mV during this protocol) was applied and whole-cell current responses were recorded. Patch pipette solution contained 10 mM Na⁺ and 25 mM HCO₃ ⁻ (Solution b in Table 1). Bath solution also contained 10 mM Na⁺ and 25 mM HCO₃ ⁻ (Bath solution D in Table 1) (left panel). Enhancing Na⁺ concentration from 10 to 25 mM in the bath solution (Bath solution E in Table 1) increased voltage-dependent current (central panel). Net current (ΔI) through NBCe2-C induced by changing [Na⁺]_o is obtained by subtracting the current traces at [Na⁺]_o = 10 mM from the current traces at [Na⁺]_o = 25 mM (right panel). b) Current-voltage (I-V) relations of steady-state current (mean of 80 ms current trace toward the end of each voltage pulse) in bath solutions containing 10 mM and 25 mM Na⁺. c) I-V relation of ΔI. d) Estimation of transport stoichiometry ratio q with Eq. 7 (n = 5).