Acting on an impulse (or two): Advantages of high-frequency tetanic onset in skeletal muscle

Ryanodine receptor activity and store-operated Ca2+ entry: Critical regulators of Ca2+ content and function in skeletal muscle

Store-operated Ca2+ entry (SOCE) is critical to cell function. In skeletal muscle, SOCE has evolved alongside excitation-contraction coupling (EC coupling); as a result, it displays unique properties compared to SOCE in other cells. The plasma membrane of skeletal muscle is mostly internalized as the tubular system, with the tubules meeting the sarcoplasmic reticulum (SR) terminal cisternae, forming junctions where the proteins that regulate EC coupling and SOCE are positioned. In this review, we describe the properties and roles of SOCE based on direct measurements of Ca2+ influx during SR Ca2+ release and leak. SOCE is activated immediately and locally as the [Ca2+ ] of the junctional SR terminal cisternae ([Ca2+ ]jSR ) depletes. [Ca2+ ]jSR changes rapidly and steeply with increasing activity of the SR ryanodine receptor isoform 1 (RyR1). The high fidelity of [Ca2+ ]jSR with RyR1 activity probably depends on the SR Ca2+ -buffer calsequestrin that is located immediately behind RyR1 inside the SR. This arrangement provides in-phase activation and deactivation of SOCE with a large dynamic range, allowing precise grading of SOCE flux. The in-phase activation of SOCE as the SR partially depletes traps Ca2+ in the cytoplasm, preventing net Ca2+ loss. Mild presentation of RyR1 leak can occur under physiological conditions, providing fibre Ca2+ redistribution without changing fibre Ca2+ content. This condition preserves normal contractile function at the same time as increasing basal metabolic rate. However, higher RyR1 leak drives excess cytoplasmic and mitochondrial Ca2+ load, setting a deleterious intracellular environment that compromises the function of the skeletal muscle.

Dietary nitrate supplementation increases diaphragm peak power in old mice

KEY POINTS

Respiratory muscle function declines with ageing, contributing to breathing complications in the elderly. Here we report greater in vitro respiratory muscle contractile function in old mice receiving supplemental NaNO₃ for 14 days compared with age-matched controls. Myofibrillar protein phosphorylation, which enhances contractile function, did not change in our study - a finding inconsistent with the hypothesis that this post-translational modification is a mechanism for dietary nitrate to improve muscle contractile function. Nitrate supplementation did not change the abundance of calcium-handling proteins in the diaphragm of old mice, in contrast with findings from the limb muscles of young mice in previous studies. Our findings suggest that nitrate supplementation enhances myofibrillar protein function without affecting the phosphorylation status of key myofibrillar proteins.

ABSTRACT

Inspiratory muscle (diaphragm) function declines with age, contributing to ventilatory dysfunction, impaired airway clearance, and overall decreased quality of life. Diaphragm isotonic and isometric contractile properties are reduced with ageing, including maximal specific force, shortening velocity and peak power. Contractile properties of limb muscle in both humans and rodents can be improved by dietary nitrate supplementation, but effects on the diaphragm and mechanisms behind these improvements remain poorly understood. One potential explanation underlying the nitrate effects on contractile properties is increased phosphorylation of myofibrillar proteins, a downstream outcome of nitrate reduction to nitrite and nitric oxide. We hypothesized that dietary nitrate supplementation would improve diaphragm contractile properties in aged mice. To test our hypothesis, we measured the diaphragm function of old (24 months) mice allocated to 1 mm NaNO₃ in drinking water for 14 days (n = 8) or untreated water (n = 6). The maximal rate of isometric force development (∼30%) and peak power (40%) increased with nitrate supplementation (P < 0.05). There were no differences in the phosphorylation status of key myofibrillar proteins and abundance of Ca²⁺-release proteins in nitrate vs. control animals. In general, our study demonstrates improved diaphragm contractile function with dietary nitrate supplementation and supports the use of this strategy to improve inspiratory function in ageing populations. Additionally, our findings suggest that dietary nitrate improves diaphragm contractile properties independent of changes in abundance of Ca²⁺-release proteins or phosphorylation of myofibrillar proteins.

Intramuscular Contributions to Low-Frequency Force Potentiation Induced by a High-Frequency Conditioning Stimulation

Electrically-evoked low-frequency (submaximal) force is increased immediately following high-frequency stimulation in human skeletal muscle. Although central mechanisms have been suggested to be the major cause of this low-frequency force potentiation, intramuscular factors might contribute. Thus, we hypothesized that two intramuscular Ca²⁺-dependent mechanisms can contribute to the low-frequency force potentiation: increased sarcoplasmic reticulum Ca²⁺ release and increased myofibrillar Ca²⁺ sensitivity. Experiments in humans were performed on the plantar flexor muscles at a shortened, intermediate, and long muscle length and electrically evoked contractile force and membrane excitability (i.e., M-wave amplitude) were recorded during a stimulation protocol. Low-frequency force potentiation was assessed by stimulating with a low-frequency tetanus (25 Hz, 2 s duration), followed by a high-frequency tetanus (100 Hz, 2 s duration), and finally followed by another low-frequency (25 Hz, 2 s duration) tetanus. Similar stimulation protocols were performed on intact mouse single fibers from flexor digitorum brevis muscle, whereby force and myoplasmic free [Ca²⁺] ([Ca²⁺]_i) were assessed. Our data show a low-frequency force potentiation that was not muscle length-dependent in human muscle and it was not accompanied by any increase in M-wave amplitude. A length-independent low-frequency force potentiation could be replicated in mouse single fibers, supporting an intramuscular mechanism. We show that at physiological temperature (31°C) this low-frequency force potentiation in mouse fibers corresponded with an increase in sarcoplasmic reticulum (SR) Ca²⁺ release. When mimicking the slower contractile properties of human muscle by cooling mouse single fibers to 18°C, the low-frequency force potentiation was accompanied by minimally increased SR Ca²⁺ release and hence it could be explained by increased myofibrillar Ca²⁺ sensitivity. Finally, introducing a brief 200 ms pause between the high- and low-frequency tetanus in human and mouse muscle revealed that the low-frequency force potentiation is abolished, arguing that increased myofibrillar Ca²⁺ sensitivity is the main intramuscular mechanism underlying the low-frequency force potentiation in humans.