"Current" advances in mechanically skinned skeletal muscle fibres

Store-Operated Calcium Entry in Skeletal Muscle: What Makes It Different?

Current knowledge on store-operated Ca²⁺ entry (SOCE) regarding its localization, kinetics, and regulation is mostly derived from studies performed in non-excitable cells. After a long time of relative disinterest in skeletal muscle SOCE, this mechanism is now recognized as an essential contributor to muscle physiology, as highlighted by the muscle pathologies that are associated with mutations in the SOCE molecules STIM1 and Orai1. This review mainly focuses on the peculiar aspects of skeletal muscle SOCE that differentiate it from its counterpart found in non-excitable cells. This includes questions about SOCE localization and the movement of respective proteins in the highly organized skeletal muscle fibers, as well as the diversity of expressed STIM isoforms and their differential expression between muscle fiber types. The emerging evidence of a phasic SOCE, which is activated during EC coupling, and its physiological implication is described as well. The specific issues related to the use of SOCE modulators in skeletal muscles are discussed. This review highlights the complexity of SOCE activation and its regulation in skeletal muscle, with an emphasis on the most recent findings and the aim to reach a current picture of this mesmerizing phenomenon.

Distinct effects of subcellular glycogen localization on tetanic relaxation time and endurance in mechanically skinned rat skeletal muscle fibres

In vitro experiments indicate a non-metabolic role of muscle glycogen in contracting skeletal muscles. Since the sequence of events in excitation\#8211;contraction (E\#8211;C) coupling is known to be located close to glycogen granules, at specific sites on the fibre, we hypothesized that the distinct compartments of glycogen have specific effects on muscle fibre contractility and fatigability. Single skeletal muscle fibres (n = 19) from fed and fasted rats were mechanically skinned and divided into two segments. In one segment glycogen localization and volume fraction were estimated by transmission electron microscopy. The other segment was mechanically skinned and, in the presence of high and constant myoplasmic ATP and PCr, electrically stimulated (10 Hz, 0.8 s every 3 s) eliciting repeated tetanic contractions until the force response was decreased by 50% (mean +/- S.E.M., 81 +/- 16, range 22-252 contractions). Initially the total myofibrillar glycogen volume percentage was 0.46 +/- 0.07%, with 72 +/- 3% in the intermyofibrillar space and 28 +/- 3% in the intramyofibrillar space. The intramyofibrillar glycogen content was positively correlated with the fatigue resistance capacity (r(2) = 0.32, P = 0.02). Intermyofibrillar glycogen was inversely correlated with the half-relaxation time in the unfatigued tetanus (r(2) = 0.25, P = 0.03). These results demonstrate for the first time that two distinct subcellular populations of glycogen have different roles in contracting single muscle fibres under conditions of high myoplasmic ATP.

The Sarcoplasmic Reticulum in Muscle Fatigue and Disease: Role of the Sarco(endo)plasmic Reticulum Ca2+-ATPase

Skeletal muscles induced to contract repeatedly respond with a progressive loss in their ability to generate a target force or power. This condition is known simply as fatigue. Commonly, fatigue may persist for prolonged periods of time, particularly at low activation frequencies, which is called low-frequency fatigue. Failure to activate the contractile apparatus with the appropriate intracellular free calcium ([Ca2+]f) signal contributes to fatigue but the precise mechanisms involved are unknown. The sarcoplasmic reticulum (SR) is the major organelle in muscle that is responsible for the regulation of [Ca2+]f, and numerous studies have shown that SR function, both Ca2+ release and Ca2+ uptake, is impaired following fatiguing contractile activity. The major aim of this review is to provide insight into the various cellular mechanisms underlying the alterations in SR Ca2+ cycling and cytosolic [Ca2+]f that are associated both with the development of fatigue during repeated muscle contraction and with low-frequency or long-lasting fatigue. The primary focus will be on the role of the sarco(endo)plasmic reticulum Ca2+-ATPase (SERCA) in normal muscle function, fatigue, and disease. Key words: calcium release, calcium uptake, muscle relaxation, low-frequency fatigue, Brody disease

MHC-based fiber type and E-C coupling characteristics in mechanically skinned muscle fibers of the rat

In this study, we investigated whether the previously established differences between fast- and slow-twitch single skeletal muscle fibers of the rat, in terms of myosin heavy chain (MHC) isoform composition and contractile function, are also detectable in excitation-contraction (E-C) coupling. We compared the contractile responsiveness of electrophoretically typed, mechanically skinned single fibers from the soleus (Sol), the extensor digitorum longus (EDL), and the white region of the sternomastoid (SM) muscle to t-system depolarization-induced activation. The quantitative parameters assessed were the amplitude of the maximum depolarization-induced force response (DIFR(max); normalized to the maximum Ca(2+)-activated force in that fiber) and the number of responses elicited until the force declined by 75% of DIFR(max) (R-D(75%)). The mean DIFR(max) values for type IIB EDL and type IIB SM fibers were not statistically different, and both were greater than the mean DIFR(max) for type I Sol fibers. The mean R-D(75%) for type IIB EDL fibers was greater than that for type I Sol fibers as well as type IIB SM fibers. These data suggest that E-C coupling characteristics of mechanically skinned rat single muscle fibers are related to MHC-based fiber type and the muscle of origin.