Calcium-Induced Calcium Release in Skeletal Muscle

Frog alpha- and beta-ryanodine receptors provide distinct intracellular Ca2+ signals in a myogenic cell line

Background

In frog skeletal muscle, two ryanodine receptor (RyR) isoforms, α-RyR and β-RyR, are expressed in nearly equal amounts. However, the roles and significance of the two isoforms in excitation-contraction (E-C) coupling remains to be elucidated.

Methodology/Principal Findings

In this study, we expressed either or both α-RyR and β-RyR in 1B5 RyR-deficient myotubes using the herpes simplex virus 1 helper-free amplicon system. Immunological characterizations revealed that α-RyR and β-RyR are appropriately expressed and targeted at the junctions in 1B5 myotubes. In Ca²⁺ imaging studies, each isoform exhibited caffeine-induced Ca²⁺ transients, an indicative of Ca²⁺-induced Ca²⁺ release (CICR). However, the fashion of Ca²⁺ release events was fundamentally different: α-RyR mediated graded and sustained Ca²⁺ release observed uniformly throughout the cytoplasm, whereas β-RyR supported all-or-none type regenerative Ca²⁺ oscillations and waves. α-RyR but not β-RyR exhibited Ca²⁺ transients triggered by membrane depolarization with high [K⁺]_o that were nifedipine-sensitive, indicating that only α-RyR mediates depolarization-induced Ca²⁺ release. Myotubes co-expressing α-RyR and β-RyR demonstrated high [K⁺]_o-induced Ca²⁺ transients which were indistinguishable from those with myotubes expressing α-RyR alone. Furthermore, procaine did not affect the peak height of high [K⁺]_o-induced Ca²⁺ transients, suggesting minor amplification of Ca²⁺ release by β-RyR via CICR in 1B5 myotubes.

Conclusions/Significance

These findings suggest that α-RyR and β-RyR provide distinct intracellular Ca²⁺ signals in a myogenic cell line. These distinct properties may also occur in frog skeletal muscle and will be important for E-C coupling.

Energy drinks: a review of use and safety for athletes

Energy drinks have increased in popularity in adolescents and young adults; however, concerns have been raised regarding the ingredients in energy drinks and their potential negative effects on health. Caffeine, the most physiologically active ingredient in energy drinks, is generally considered safe by the US Food and Drug Administration (FDA), although adverse effects can occur at varying amounts. Guarana, which contains caffeine in addition to small amounts of theobromine, theophylline, and tannins, is also recognized as safe by the FDA, although it may lead to caffeine toxicity when combined with caffeine. The amount of ginseng in energy drinks is typically far below the amount used as a dietary supplement, and is generally considered safe. Taurine, an intracellular amino acid, has been reported to have positive inotropic effects; however, this claim is not supported by research. Most energy drinks also contain sugar in an amount that exceeds the maximum recommended daily amount. Young athletes are increasingly using energy drinks because of the ergogenic effects of caffeine and the other ingredients found in these beverages. Energy drinks combined with alcohol are also gaining popularity in young adults, which poses significant concerns about health risks. Other health concerns related to consumption of energy drinks include case reports of seizures and cardiac arrest following energy drink consumption and dental enamel erosion resulting from the acidity of energy drinks.

ORAI-mediated calcium entry: mechanism and roles, diseases and pharmacology

ORAI1 is a protein located on the plasma membrane that acts as a calcium channel. Calcium enters via ORAI1 as a mechanism to refill the sarcoplasmic/endoplasmic reticulum calcium stores, the depletion of which can be detected by the sensor protein STIM1. Isoforms of these proteins ORAI2, ORAI3 and STIM2 also have roles in cellular calcium homeostasis but are less well characterized. This pathway of filling the calcium stores is termed store-operated calcium entry and while the pathway itself was proposed in 1986, the identity of the key molecular components was only discovered in 2005 and 2006. The characterization of the ORAI and STIM proteins has provided clearer information on some calcium-regulated pathways that are important in processes from gene transcription to immune cell function. Recent studies have also suggested the importance of the components of ORAI-mediated calcium entry in some diseases or processes significant in disease including the migration of breast cancer cells and thrombus formation. This review will provide a brief overview of ORAI-mediated calcium entry, its role in physiological and pathophysiological processes, as well as current and potential pharmacological modulators of the components of this important cellular calcium entry pathway.

Synchronous intra-Golgi transport induces the release of Ca2+ from the Golgi apparatus

The mechanisms of secretory transport through the Golgi apparatus remain an issue of debate. The precise functional importance of calcium ions (Ca(2+)) for intra-Golgi transport has also been poorly studied. Here, using different approaches to measure free Ca(2+) concentrations in the cell cytosol ([Ca(2+)](cyt)) and inside the lumen of the Golgi apparatus ([Ca(2+)](GA)), we have revealed transient increases in [Ca(2+)](cyt) during the late phase of intra-Golgi transport that are concomitant with a decline in the maximal [Ca(2+)](GA) restoration ability. Thus, this redistribution of Ca(2+) from the Golgi apparatus into the cytosol during the movement of cargo through the Golgi apparatus appears to have a role in intra-Golgi transport, and mainly in the late Ca(2+)-dependent phase of SNARE-regulated fusion of Golgi compartments.

Spatiotemporal dynamics of Ca2+ signaling and its physiological roles

Changes in the intracellular Ca(2+) concentration regulate numerous cell functions and display diverse spatiotemporal dynamics, which underlie the versatility of Ca(2+) in cell signaling. In many cell types, an increase in the intracellular Ca(2+) concentration starts locally, propagates within the cell (Ca(2+) wave) and makes oscillatory changes (Ca(2+) oscillation). Studies of the intracellular Ca(2+) release mechanism from the endoplasmic reticulum (ER) showed that the Ca(2+) release mechanism has inherent regenerative properties, which is essential for the generation of Ca(2+) waves and oscillations. Ca(2+) may shuttle between the ER and mitochondria, and this appears to be important for pacemaking of Ca(2+) oscillations. Importantly, Ca(2+) oscillations are an efficient mechanism in regulating cell functions, having effects supra-proportional to the sum of duration of Ca(2+) increase. Furthermore, Ca(2+) signaling mechanism studies have led to the development of a method for specific inhibition of Ca(2+) signaling, which has been used to identify hitherto unrecognized functions of Ca(2+) signals.