Figure 2.

Extracellular Ca²⁺ blocks STIM1- and 2-APB–activated Orai3 Na⁺ currents with similar sensitivity. (A, B, D, and E) Inhibition of STIM1- or 2-APB–activated Orai3 Na⁺ currents by Ca²⁺_o. In each case, the cell was voltage clamped to a constant potential of −100 mV and 200-ms sweeps were collected at 2 kHz. The mean current during each sweep is plotted against time. (C) Predicted topology of a single Orai3 subunit with the acidic residues (TM1 and TM3) highlighted. (F) Dose–response relationships of Ca²⁺ blockade. Ca²⁺ blockade of Na⁺ currents through 2-APB–activated WT and E165A Orai3 channels occurred with a similar affinity as STIM1-activated Orai3 channels. E81D substitution increases the K_i of block in both STIM1- and 2-APB–activated modes (also see Table 1). Block was quantified by measuring the Na⁺ current immediately after application of a DVF solution supplemented with the indicated [Ca²⁺]_o. Each dashed line is a least-squares fit of the Hill equation block = max/[1 + (K_i/[Ca])ⁿ], where max is the predicted maximal blockade at saturating Ca²⁺ concentrations. Error bars represent SEM.