(A and B) Representative whole-cell current traces recorded from erythrocytes of a control (A) and a patient (B) with Na-gluconate in the pipette and NaCl in the bath solution (left panel) and after isoosmotic replacement of NaCl by n-methyl-d-glucamine–chloride (NMDG-Cl) in the bath (right panel). (C and D) Mean current-voltage (I-V) relationships (± SEM) for control (C) (n = 3–4) and patients’ erythrocytes (D) (n = 5–6) recorded as in A and B with NaCl bath solution (open circles) and after isoosmotic replacement of NaCl by NMDG-Cl (closed triangles), CaCl₂ (open diamonds), or KCl (open triangles) in the bath. In patients’ erythrocytes (D), the reversal potential of the I-V curve shifted by –30 ± 4, –20 ± 2, and +11 ± 2 mV (mean ± SEM; n = 5–6) from that recorded in NaCl solution following substitution with NMDG-Cl, CaCl₂, and KCl, respectively. (E) Histogram recorded by flow cytometry in erythrocytes from controls (black line) and patients (red line) incubated in Ca²⁺-containing NaCl solution, depicting the fluo3 fluorescence intensity as a measure of the steady-state intracellular ([Ca²⁺]_i). (F and G) Histograms (F) and time course (G) of changes in fluo3 fluorescence intensity of control (black line in F; open circles in G) and patients’ erythrocytes (red line in F; close triangles in G) following Ca²⁺ depletion by incubation for 30 minutes in Ca²⁺-free NaCl solution (F, left panel, and G, 0-minute values) and Ca²⁺ repletion (F, middle panel, and G). As a positive control experiment, the histogram in (F, right panel) shows the fluo3 fluorescence of Ca²⁺-permeabilized erythrocytes from controls (black line) and patients (red line) indicating equal fluo3 dye loading of both cell populations under these conditions (mean values ± SEM, 49 ± 8 and 55 ± 8 relative fluorescence units in erythrocytes from controls and patients, respectively; n = 4). The data in G are averaged geometrical means (± SEM; n = 14–16) of the fluorescence distribution. Lines represent exponential fits yielding the time constant of [Ca²⁺]_i repletion (controls, τ = 16.3 ± 3.4 min; patients, τ = 7.1 ± 1.6 min; n = 14–16, P < 0.05; 2-tailed Welch-corrected t test).