Figure 4.

All three TPC subtypes from rabbit are Na⁺ selective when activated by PI(3,5)P₂. HEK293 cells were transiently transfected with cDNA coding for OcTPC1-EGFP, OcTPC2-EGFP, or OcTPC3-EGFP. (A,B) Representative I–V curves derived from voltage ramps for OcTPC1 in an inside-out (IO) patch (A) and OcTPC2 in a whole-endolysosome (WL) patch (B) sequentially exposed to 1 µM PI(3,5)P₂ in the normal bath solution containing mainly K⁺ (red traces) and then in a Na⁺-based bath solution (blue traces). (C) Representative current traces of OcTPC3 detected in an outside-out (OO) patch containing 5 µM PI(3,5)P₂ in the pipette solution and sequentially exposed to normal Tyrode’s solution followed by equiosmotic solutions that contained 150 mM NaCl, 150 mM KCl, 105 mM CaCl₂, and 150 mM CsCl. Note the lake of tail currents in KCl, CaCl₂, and CsCl. (D) Representative current traces of OcTPC3 detected in an IO patch exposed to 1 µM PI(3,5)P₂ in normal Tyrode’s solution. After stepping to +100 mV for 4 s, the voltage was switched to 0, 40, 80, 120, and 160 mV for 100 ms before returning to 0 mV. For clarity, only the last 0.1 s of the +100-mV step is shown together with the tail currents at post-step potentials of 0 to 160 mV followed by that at 0 mV. For (C,D), dashed lines indicate zero current. (E) I–V relationship of currents at the post-step potentials normalized to that of the absolute value at 0 mV for patches recorded as in (D). Data points are means ± SEM of n = 5 patches fit by linear regression.