Figure 2.

Restoration of retrograde coupling after insertion of α_1S residues 720–764 into the Musca II-III loop (chimera GFP-SkLMS₄₅). (A) Representative whole-cell Ca²⁺ currents recorded from dysgenic myotubes expressing GFP-α_1S (Left), GFP-SkLM (Center), and GFP-SkLMS₄₅ (Right). After a prepulse to inactivate T type currents (24), macroscopic Ca²⁺ currents were elicited by 200-ms step depolarizations from a holding potential of −80 mV to the indicated test potentials. Current amplitudes were normalized by linear cell capacitance and are expressed as pA/pF. (B) Representative immobilization-resistant intramembrane charge movements measured at +40 mV after blocking Ca²⁺ currents with 0.5 mM Cd²⁺ and 0.1 mM La³⁺, recorded from cells expressing the same three constructs shown above. (C) Average maximal Ca²⁺ conductance (G_max, Left) and charge movement (Q_max, Right) and ratios of G_max/Q (Center) for GFP-α_1S, GFP-SkLM, and GFP-SkLMS₄₅. The asterisk indicates a statistically significant (P < 0. 001) difference in average G_max from the other two constructs after comparison by an unpaired two-sample t test. No asterisk indicates lack of statistically significant difference (P > 0.05). (Bars = mean ± SEM of 12–20 recordings.)