The role of action potential changes in depolarization-induced failure of excitation contraction coupling in mouse skeletal muscle

Abstract

Excitation-contraction coupling (ECC) is the process by which electrical excitation of muscle is converted into force generation. Depolarization of skeletal muscle resting potential contributes to failure of ECC in diseases such as periodic paralysis, intensive care unit acquired weakness and possibly fatigue of muscle during vigorous exercise. When extracellular K+ is raised to depolarize the resting potential, failure of ECC occurs suddenly, over a narrow range of resting potentials. Simultaneous imaging of Ca2+ transients and recording of action potentials (APs) demonstrated failure to generate Ca2+ transients when APs peaked at potentials more negative than –30mV. An AP property that closely correlated with failure of the Ca2+ transient was the integral of AP voltage with respect to time. Simultaneous recording of Ca2+ transients and APs with electrodes separated by 1.6mm revealed AP conduction fails when APs peak below –21mV. We hypothesize propagation of APs and generation of Ca2+ transients are governed by distinct AP properties: AP conduction is governed by AP peak, whereas Ca2+ release from the sarcoplasmic reticulum is governed by AP integral. The reason distinct AP properties may govern distinct steps of ECC is the kinetics of the ion channels involved. Na channels, which govern propagation, have rapid kinetics and are insensitive to AP width (and thus AP integral) whereas Ca2+ release is governed by gating charge movement of Cav1.1 channels, which have slower kinetics such that Ca2+ release is sensitive to AP integral. The quantitative relationships established between resting potential, AP properties, AP conduction and Ca2+ transients provide the foundation for future studies of failure of ECC induced by depolarization of the resting potential.

Effects of lowered [Na+]o and membrane depolarization on the Ca2+ transients of fast skeletal muscle fibers. Implications for muscle fatigue

“…Our previous (Figure 5C in [7]) and current data (Figure 4) concur with previously published relationships between AP OS and [K + ] o (Figure 5 in [4]), [Na + ] o (Figure 6 in [6]), and membrane potential (Figure 2E in [26]). Here we have extended those studies to a larger number of conditions.…”

“…The experiments at different membrane potentials (Figures 2 and 3) confirmed our previous observations that depolarization has a dual effect on Ca +2 release triggered by single stimulation in fast frog muscle fibers exposed to physiological [Na + ]o [7]. In spite of ample differences in experimental approaches, a similar voltage dependence of Ca +2 release on membrane potential was also confirmed in murine skeletal muscles using different temperatures and Ca +2 sensors (30 o C and Indo-1 [25]; 22 o C and GCAMP6f [26]). The fact that intact fibers from long muscles (EDL and soleus) were used in these studies reinforces the validity of our approach using cut fibers.…”

“…A high correlation was determined for the data at 115-60 mM Na + o (Figure 7E, see legend for R 2 values), suggesting a causative relationship between both parameters. A similar correlation between the peak of Ca +2 -dependent fluorescence transients and the area of APs was recently described [26].…”

Excitation-contraction coupling in mammalian skeletal muscle: Blending old and last-decade research

“…Physiologically, the activation of the muscle fibre is modulated by the depolarization of the sarcolemma, including the tubular system (Kuffler, 1947;Hodgkin and Horowicz, 1960a). Under resting conditions, the fibre is polarized between -70 (Head, 1993) and -83 mV (Luff and Atwood, 1972;Wang et al, 2022), at 22 and 37 °C, respectively. Upon binding of acetylcholine (ACh) to the motor end plate, the inward sarcolemmal conductance to Na + rapidly increases, bringing about an AP.…”

Voltage-Dependent Ca2+ Release Is Impaired in Hypokalemic Periodic Paralysis Caused by CaV1.1-R528H but not by NaV1.4-R669H

“…DiFranco & Cannon The resting potential of acutely dissociated FDB fibers is typically -40 mV to -50 mV (12), a range that is substantially depolarized from the value of -80 mV to -90 mV measured from intact whole muscle using sharp microelectrodes (13). Therefore, we applied a holding current to set 𝑉 𝑚 = -80 mV after fiber impalement with two microelectrodes.…”