Figure 4. N-Myc expression shapes the phenotype of native TRPM7/TRPM6 currents.

Whole-cell MagNuM currents were measured in SHEP-21N cells treated with tetracycline (control) or not (N-Myc) and peak outward currents at +80 mV were analyzed. A, inhibition of MagNuM current by 200 μM 2-APB (control, n = 10; N-Myc, n = 7), normalized to current amplitude just prior to 2-APB application. B, representative currents evoked by voltage ramps (I-V curves) were derived from N-Myc-expressing cells before (500 s) and after (700 s) 2-APB application. C, ATP sensitivity of currents (n = 7-13 cells for each point; *, p<0.01). Mg·ATP concentrations were fixed at 1 or 3 mM with an internal free Mg²⁺ concentration of 264 μM. Currents were normalized against the control condition containing no Mg·ATP internally. D, E, current measurements in control and N-Myc-expressing cells perfused with intracellular solutions of defined free Mg²⁺ concentrations as labeled in the graph (control, n = 5-6 for each Mg²⁺ concentration; N-Myc, n = 5-7 for each Mg²⁺ concentration). Free Mg²⁺ concentration in internal solution was clamped to the indicated levels with 10 mM EGTA. F, peak current amplitudes derived from D and E as a function of free Mg²⁺ concentration. Dose-response fit rendered IC₅₀ values of 425 μM and 236 μM for control and N-Myc-expressing cells, respectively.