The use of caged adenine nucleotides and caged phosphate in intact skeletal muscle fibres of the mouse

Mechanical isolation, and measurement of force and myoplasmic free [Ca2+] in fully intact single skeletal muscle fibers

Mechanical dissection of single intact mammalian skeletal muscle fibers permits real-time measurement of intracellular properties and contractile function of living fibers. A major advantage of mechanical over enzymatic fiber dissociation is that single fibers can be isolated with their tendons remaining attached, which allows contractile forces (in the normal expected range of 300-450 kN/m²) to be measured during electrical stimulation. Furthermore, the sarcolemma of single fibers remains fully intact after mechanical dissection, and hence the living fibers can be studied with intact intracellular milieu and normal function and metabolic properties, as well as ionic control. Given that Ca²⁺ is the principal regulator of the contractile force, measurements of myoplasmic free [Ca²⁺] ([Ca²⁺]_i) can be used to further delineate the intrinsic mechanisms underlying changes in skeletal muscle function. [Ca²⁺]_i measurements are most commonly performed in intact single fibers using ratiometric fluorescent indicators such as indo-1 or fura-2. These Ca²⁺ indicators are introduced into the fiber by pressure injection or by using the membrane-permeable indo-1 AM, and [Ca²⁺]_i is measured by calculating a ratio of the fluorescence at specific wavelengths emitted for the Ca²⁺-free and Ca²⁺-bound forms of the dye. We describe here the procedures for mechanical dissection, and for force and [Ca²⁺]_i measurement in intact single fibers from mouse flexor digitorum brevis (FDB) muscle, which is the most commonly used muscle in studies using intact single fibers. This technique can also be used to isolate intact single fibers from various muscles and from various species. As an alternative to Ca²⁺ indicators, single fibers can also be loaded with fluorescent indicators to measure, for instance, reactive oxygen species, pH, and [Mg²⁺], or they can be injected with proteins to change functional properties. The entire protocol, from dissection to the start of an experiment on a single fiber, takes ∼3 h.

Caged nucleotides/nucleosides and their photochemical biology

Nucleotides and nucleosides are not only key units of DNA/RNA that store genetic information, but are also the regulators of many biological events of our lives. By caging the key functional groups or key residues of nucleotides with photosensitive moieties, it will be possible to trigger biological events of target nucleotides with spatiotemporal resolution and amplitude upon light activation or photomodulate polymerase reactions with the caged nucleotide analogues for next-generation sequencing (NGS) and bioorthogonal labeling. This review highlights three different caging strategies for nucleotides and demonstrates the photochemical biology of these caged nucleotides.

Synthetic localized calcium transients directly probe signalling mechanisms in skeletal muscle

The contribution of Ca2+-induced Ca2+ release (CICR) to trigger muscle contraction is controversial. It was studied on isolated muscle fibres using synthetic localized increases in Ca2+ concentration, SLICs, generated by two-photon photorelease from nitrodibenzofuran (NDBF)-EGTA just outside the permeabilized plasma membrane. SLICs provided a way to increase cytosolic [Ca2+] rapidly and reversibly, up to 8 μM, levels similar to those reached during physiological activity. They improve over previous paradigms in rate of rise, locality and reproducibility. Use of NDBF-EGTA allowed for the separate modification of resting [Ca2+], trigger [Ca2+] and resting [Mg2+]. In frog muscle, SLICs elicited propagated responses that had the characteristics of CICR. The threshold [Ca2+] for triggering a response was 0.5 μM or less. As this value is much lower than concentrations prevailing near channels during normal activity, the result supports participation of CICR in the physiological control of contraction in amphibian muscle. As SLICs were applied outside cells, the primary stimulus was Ca2+, rather than the radiation or subproducts of photorelease. Therefore the responses qualify as ‘classic' CICR. By contrast, mouse muscle fibres did not respond unless channel-opening drugs were present at substantial concentrations, an observation contrary to the physiological involvement of CICR in mammalian excitation–contraction coupling. In mouse muscle, the propagating wave had a substantially lower release flux, which together with a much higher threshold justified the absence of response when drugs were not present. The differences in flux and threshold may be ascribed to the absence of ryanodine receptor 3 (RyR3) isoforms in adult mammalian muscle.

Carbohydrate administration and exercise performance: what are the potential mechanisms involved?

It is well established that carbohydrate (CHO) administration increases performance during prolonged exercise in humans and animals. The mechanism(s), which could mediate the improvement in exercise performance associated with CHO administration, however, remain(s) unclear. This review focuses on possible underlying mechanisms that could explain the increase in exercise performance observed with the administration of CHO during prolonged muscle contractions in humans and animals. The beneficial effect of CHO ingestion on performance during prolonged exercise could be due to several factors including (i) an attenuation in central fatigue; (ii) a better maintenance of CHO oxidation rates; (iii) muscle glycogen sparing; (iv) changes in muscle metabolite levels; (v) reduced exercise-induced strain; and (vi) a better maintenance of excitation-contraction coupling. In general, the literature indicates that CHO ingestion during exercise does not reduce the utilization of muscle glycogen. In addition, data from a meta-analysis suggest that a dose-dependent relationship was not shown between CHO ingestion during exercise and an increase in performance. This could support the idea that providing enough CHO to maintain CHO oxidation during exercise may not always be associated with an increase in performance. Emerging evidence from the literature shows that increasing neural drive and attenuating central fatigue may play an important role in increasing performance during exercise with CHO supplementation. In addition, CHO administration during exercise appears to provide protection from disrupted cell homeostasis/integrity, which could translate into better muscle function and an increase in performance. Finally, it appears that during prolonged exercise when the ability of metabolism to match energy demand is exceeded, adjustments seem to be made in the activity of the Na+/K+ pump. Therefore, muscle fatigue could be acting as a protective mechanism during prolonged contractions. This could be alleviated when CHO is administered resulting in the better maintenance of the electrical properties of the muscle fibre membrane. The mechanism(s) by which CHO administration increases performance during prolonged exercise is(are) complex, likely involving multiple factors acting at numerous cellular sites. In addition, due to the large variation in types of exercise, durations, intensities, feeding schedules and CHO types it is difficult to assess if the mechanism(s) that could explain the increase in performance with CHO administration during exercise is(are) similar in different situations. Experiments concerning the identification of potential mechanism(s) by which performance is increased with CHO administration during exercise will add to our understanding of the mechanism(s) of muscle/central fatigue. This knowledge could have significant implications for improving exercise performance.

The α,5-dicarboxy-2-nitrobenzyl caging group, a tool for biophysical applications with improved hydrophilicity: synthesis, photochemical properties and biological characterization

Earlier we reported on the synthesis of α,4-dicarboxy-2-nitrobenyzl caged compounds (Schaper, K. et al. [2002] Eur. J. Org. Chem., 1037-1046). These compounds have the advantage of an increased hydrophilicity compared with the well-established α-carboxy-2-nitrobenzyl caged compounds; however, the release of the active compound becomes slower due to the introduction of the additional carboxy group. Based upon theoretical calculations we predicted that the release would become faster when the additional carboxy group is moved to the 5-position. Here we describe the synthesis and the photochemical and biological characterization of an α,5-dicarboxy-2-nitrobenyzl caged compound. The high hydrophilicity of the new caging group is maintained due to the fact that the additional carboxy moiety is preserved, while the release of the active species from the new derivative is even faster than for the reference, an α-CNB caged compound.

Caged protein prenyltransferase substrates: tools for understanding protein prenylation

Originally designed to block the prenylation of oncogenic Ras, inhibitors of protein farnesyltransferase currently in preclinical and clinical trials are showing efficacy in cancers with normal Ras. Blocking protein prenylation has also shown promise in the treatment of malaria, Chagas disease and progeria syndrome. A better understanding of the mechanism, targets and in vivo consequences of protein prenylation are needed to elucidate the mode of action of current PFTase (Protein Farnesyltransferase) inhibitors and to create more potent and selective compounds. Caged enzyme substrates are useful tools for understanding enzyme mechanism and biological function. Reported here is the synthesis and characterization of caged substrates of PFTase. The caged isoprenoid diphosphates are poor substrates prior to photolysis. The caged CAAX peptide is a true catalytically caged substrate of PFTase in that it is to not a substrate, yet is able to bind to the enzyme as established by inhibition studies and X-ray crystallography. Irradiation of the caged molecules with 350 nm light readily releases their cognate substrate and their photolysis products are benign. These properties highlight the utility of those analogs towards a variety of in vitro and in vivo applications.

Effect of low cytoplasmic [ATP] on excitation-contraction coupling in fast-twitch muscle fibres of the rat

In this study we investigated the roles of cytoplasmic ATP as both an energy source and a regulatory molecule in various steps of the excitation-contraction (E-C) coupling process in fast-twitch skeletal muscle fibres of the rat. Using mechanically skinned fibres with functional E-C coupling, it was possible to independently alter cytoplasmic [ATP] and free [Mg2+]. Electrical field stimulation was used to elicit action potentials (APs) within the sealed transverse tubular (T-) system, producing either twitch or tetanic (50 Hz) force responses. Measurements were also made of the amount of Ca2+ released by an AP in different cytoplasmic conditions. The rate of force development and relaxation of the contractile apparatus was measured using rapid step changes in [Ca2+]. Twitch force decreased substantially (approximately 30%) at 2 mm ATP compared to the level at 8 mm ATP, whereas peak tetanic force only declined by approximately 10% at 0.5 mm ATP. The rate of force development of the twitch and tetanus was slowed only slightly at [ATP] > or = 0.5 mm, but was slowed greatly (> 6-fold) at 0.1 mm ATP, the latter being due primarily to slowing of force development by the contractile apparatus. AP-induced Ca2+ release was decreased by approximately 10 and 20% at 1 and 0.5 mm ATP, respectively, and by approximately 40% by raising the [Mg2+] to 3 mm. Adenosine inhibited Ca2+ release and twitch responses in a manner consistent with its action as a competitive weak agonist for the ATP regulatory site on the ryanodine receptor (RyR). These findings show that (a) ATP is a limiting factor for normal voltage-sensor activation of the RyRs, and (b) large reductions in cytoplasmic [ATP], and concomitant elevation of [Mg2+], substantially inhibit E-C coupling and possibly contribute to muscle fatigue in fast-twitch fibres in some circumstances.

Insulin does not mediate the attenuation of fatigue associated with glucose infusion in rat plantaris muscle

Glucose infusion attenuates fatigue in rat plantaris muscle stimulated in situ, and this is associated with a better maintenance of electrical properties of the fiber membrane (Karelis AD, Péronnet F, and Gardiner PF. Exp Physiol 87: 585-592, 2002). The purpose of the present study was to test the hypothesis that elevated plasma insulin concentration due to glucose infusion ( approximately 900 pmol/l), rather than high plasma glucose concentration ( approximately 10-11 mmol/l), could be responsible for this phenomenon, because insulin has been shown to stimulate the Na+-K+ pump. The plantaris muscle was indirectly stimulated (50 Hz, for 200 ms, 5 V, every 2.7 s) via the sciatic nerve to perform concentric contractions for 60 min, while insulin (8 mU x kg-1x min-1: plasma insulin approximately 900 pmol/l) and glucose were infused to maintain plasma glucose concentration between 4 and 6 [6.2 +/- 0.4 mg x kg-1x min-1: hyperinsulinemic-euglycemic (HE)] or 10 and 12 mmol/l [21.7 +/- 1.1 mg. kg-1. min-1: hyperinsulinemic-hyperglycemic clamps (HH)] (6 rats/group). The reduction in submaximal dynamic force was significantly (P < 0.05) less with HH (-53%) than with HE and saline only (-66 and -70%, respectively). M-wave characteristics were also better maintained in the HH than in HE and control groups. These results demonstrate that the increase in insulin concentration is not responsible for the increase in muscle performance observed after the elevation of circulating glucose.

Rapid photolytic release of cytidine 5'-diphosphate from a coumarin derivative: a new tool for the investigation of ribonucleotide reductases

In order to study the long-range radical transfer in the Escherichia coli ribonucleotide reductase (RNR), caged cytidine 5'-diphosphate (CDP) 1 was synthesized, which contains the photolabile (7-diethylaminocoumarin-4-yl)methyl moiety. The caged CDP 1 triggers the release of CDP when irradiated at wavelengths between 365 and 436 nm. The rate constant of the formation of alcohol 2 and cytidine 5'-diphosphate 3 is 2x10(8) s(-1) and the quantum efficiency for the disappearance of caged CDP 1 is 2.9%.

Mechanisms of reduced SR Ca(2+) release induced by inorganic phosphate in rat skeletal muscle fibers

The effects of inorganic phosphate (P(i)) on Ca(2+) release from the sarcoplasmic reticulum (SR) were studied in mechanically skinned rat skeletal muscle fibers. Application of caffeine or T-tubule depolarization was used to induce Ca(2+) release from the SR, which was detected using fura 2 fluorescence. Addition of P(i) (1-40 mM) caused a reversible and concentration-dependent reduction in the caffeine-induced Ca(2+) transient. This effect was apparent at low P(i) concentration (<5 mM), which did not result in detectable precipitation of calcium phosphate within the SR. The inhibitory effect of P(i) exhibited a marked dependence on free Mg(2+) concentration. At 0.5 mM free Mg(2+), 5 mM P(i) reduced the caffeine-induced transient by 25.1 +/- 4.1% (n = 13). However, at 1.5 mM free Mg(2+), 5 mM P(i) reduced the amplitude of caffeine-induced Ca(2+) transients by 68.9 +/- 3.1% (n = 10). Depolarization-induced SR Ca(2+) release was similarly affected. These effects of P(i) may be important in skeletal muscle fatigue, if an inhibitory action of P(i) on SR Ca(2+) release is augmented by the rise in cytosolic Mg(2+) concentration, which accompanies ATP breakdown.