Constitutive electrophysiological activity of TRPV6-like current in principal cells: inwardly rectifying Ca²⁺-selective and monovalent conductance. (A) Time course of the current changes recorded at different extracellular Ca²⁺ concentrations ([Ca²⁺]_o). The current was initially recorded in a nominally Ca²⁺-free solution. Increasing [Ca²⁺]_o to 30 mM reversibly evoked a current, which was attenuated in 2.5 mM Ca²⁺-containing solution. When increasing [Ca²⁺]_o from nominally Ca²⁺-free solution to 2.5 mM Ca²⁺-solution, a small inward current was evoked, which was then followed by an almost complete rundown. (B) A monovalent cation current was revealed when switching the normal external solution to a divalent (Ca²⁺ and Mg²⁺) cation-free saline. (C and D) Current–voltage ramps recorded at the time points as indicated in A and B, and current–voltage curves were digitally subtracted from different conditions as indicated. The subtracted currents showed inwardly rectifying at negative potentials. Arrows indicated the negative slopes of current responses at the hyperpolarization step before the applied voltage ramp. Dotted lines indicate the zero current levels. (E) Typical tracings showing the current kinetics at negative potentials in different ionic conditions. Tested pulses were applied from a holding potential at 0 mV to a series of 500-ms steps between 20 and −120 mV in 20-mV intervals. Bathing fluid low chloride Cs-PSS and low chloride Cs-based pipette solutions were used for recording.