Figure 4. Modulation of N-VDCCs in Xenopus oocytes by Na⁺ depletion and its Gβγ dependency.

A, time course of changes in I_Ba (left panel) and in facilitation (right panel), caused by 7min incubation in Na⁺-free solution (○) or high-Na⁺ solution (•), in a representative experiment. I_Ba was: in high-Na⁺ solution, 849 ± 97 nA at t= 1min and 862 ± 122 nA at t= 7min (n= 4); in Na⁺-free solution, 845 ± 98 nA at t= 1min and 1075 ± 99 nA at t= 7min (n= 4). In each cell, changes in I_Ba with time were monitored relative to I_Ba measured 1min after the beginning of voltage clamp. B, the effect of Na⁺ depletion on I_Ba amplitude (left panel) and facilitation (right panel) is altered by overexpression of Gβγ (▴; 5 ng Gβ RNA oocyte⁻¹). ○, channel expressed alone. n= 11–15 oocytes in each group. C, summary of the effects of 7min incubation of oocytes in low- or high-Na⁺ solutions, in oocytes that expressed N-VDCCs alone (control), with m-cβARK, or with Gβγ. Left panel summarizes the changes in amplitude of I_Ba. **P < 0.01 compared to control group. Right panel summarizes the changes in facilitation. *P < 0.05 for the difference between 1st and 7th minute. D, t_90% of activation shows correlation with the change in I_Ba (a) and in Δ_facilitation (b) after 7min of Na⁺ depletion. This correlation is absent in oocytes that coexpress m-cβARK (c and d). The value of P was calculated using the Pearson correlation test. E, summary of the effects of depletion on t_90% of activation in oocytes that expressed N-VDCCs alone (control) or with m-cβARK. In the control group, only results from cells with more than a 20% increase in I_Ba after 7min of Na⁺ depletion (n= 12) are summarized. ***P < 0.001.