Fig. 5.

Decreased Na⁺ current of T⁵⁶³ variant was due to increased sensitivity to extracellular Mg²⁺. A: representative Na⁺ current traces of A⁵⁶³ and T⁵⁶³ variants in the presence of 0 and 0.5 mM extracellular Mg²⁺ are shown in top. The I-V curves of Na⁺ current for A⁵⁶³ and T⁵⁶³ were parallel in the absence of extracellular Mg²⁺ and departed in the presence of 0.5 mM extracellular Mg²⁺ (bottom). B: Na⁺ current of the T⁵⁶³ variant was more sensitive to extracellular Mg²⁺ concentration than that of the A⁵⁶³ variant. Currents at −160 mV from two independent experiments of 18 oocytes/group are expressed as means ± SE. Flufenamic acid at 200 μM was included in the solutions to block the Ca²⁺-inactivated Cl⁻ current induced by the removal of both Ca²⁺ and Mg²⁺ in the extracellular solution. C: effect of extracellular Mg²⁺ on ⁴⁵Ca²⁺ uptake mediated by oocytes expressing A⁵⁶³, T⁵⁶³, or by water-injected control oocytes (water). Mg²⁺ (10 mM) in the extracellular medium led to a stronger reduction in Ca²⁺ uptake in oocytes expressing T⁵⁶³ than those expressing A⁵⁶³ (49.4 ± 2.2 vs. 38.5 ± 2.5%). Data from 3 independent experiments are expressed as means ± SE.