In excitation–contraction coupling (ECC), when the skeletal muscle action potential (AP) propagates into the transverse tubules, it modifies the conformational state of the voltage-gated calcium channels (CaV1.1). CaV1.1 serves as the voltage sensor for activation of calcium release from the sarcoplasmic reticulum (SR); however, many questions about this function persist. CaV1.1 α1 subunits contain four distinct homologous domains (I–IV). Each repeat includes six transmembranal helical segments; the voltage-sensing domain (VSD) is formed by S1–S4 segments, and the pore domain is formed by helices S5–S6. Because, in other voltage-gated channels, individual VSDs appear to be differentially involved in specific aspects of channel gating, here we thus hypothesized that not all the VSDs in CaV1.1 contribute equally to calcium-release activation. Yet, the voltage-sensor movements during an AP (the physiological stimulus for the muscle fiber) have not been previously measured in muscle. Reorientation of VSDs I–IV in CaV1.1 during an AP should generate a small but measurable electrical current. Still, neither the voltage-sensor charge movement during the AP nor the contribution of the individual VSDs to voltage-gated calcium release have been previously monitored. Here, we electrically monitor VSD movements using an AP voltage-clamp technique applied to muscle fibers. We introduce AP-fluorometry, a variant of the functional site-directed fluorescence, to track the movement of each VSD via a cysteine substitution on the extracellular region of S4 of each VSD and its labeling with a cysteine-reacting fluorescent probe, which served as an optical reporter of local rearrangements. Independent optical recordings of AP and calcium transients were performed to establish the temporal correlation between AP, AP-elicited charge movement, VSDs conformational changes, and calcium release flux. Our results support the hypothesis that not all VSDs in CaV1.1 contribute to ECC.
This work is supported by National Institutes of Health grants R37AR055099, R01AR075726 (to M.F. Schneider), R01NS103777 (to R.A. Banniester), and T32AR007592 (to Q. Banks).