Relaxation of the Voltage Sensing Modules of Excitation-Contraction (EC) Coupling in Mammalian Skeletal Muscle

Voltage-sensing modules (VSMs) of similar molecular structure underlie voltage-sensitivity in ion channels, the voltage-sensor of EC coupling (the DHPR), and voltage-sensitive phosphatases (VSPs). Upon sustained depolarization (or “antipolarization” of VSPs) all VSMs enter a functionally disabled inactivated or relaxed state. This is accompanied in every case by a change in voltage dependence of the sensor charge movement, including a large negative shift of its central voltage VT. In frog muscle, voltage-driven transitions of the EC coupling VSM elicit charge movements that result in two charge distributions with different VT: Q1 in the normally polarized fiber; Q2 when the membrane is held depolarized. Here we show in mammalian muscle —single fibers of mouse FDB— a similar interconversion of voltage-sensor charge. In 2 mouse strains VT at rest was ∼-20 mV, but shifted to ∼-80 mV upon sustained depolarization. The change was a conversion between two distributions (Q1 and Q2) with distinct but fixed VT‘s. The observations require a minimum model with 4 states: C (cis), T (trans), CR (cis relaxed), TR (trans relaxed). The transitions C->T and CR->TR are fast and mobilize respectively Q1 and Q2. C->T opens the RyR, but CR->TR does not. The transitions to and from relaxed states are slow and voltage-independent. Low extracellular [Ca2+] promoted the C->CR and T->TR transitions, while high [Ca2+] opposed it. Observations of similar antagonism between inactivation of Ca2+ and K+ channels and the pore occupancy by ions led to the notion that inactivation involves pore collapse. DHPR pore opening, however, is not required for proper operation of the EC coupling VSM. The effects of extracellular Ca2+ on voltage-dependent inactivation must be reinterpreted in view of these findings. (Support: ANII, CSIC and NIGMS).