Summary
Ryanodine receptors (RyR for short) form a class of intracellular calcium channels in various forms of excitable animal tissue like muscles and neurons. There are three major isoforms of the ryanodine receptor, which are found in different tissues and participate in different signaling pathways involving calcium release from intracellular organelles. The RYR2 ryanodine receptor isoform is the major cellular mediator of calcium-induced calcium release (CICR) in animal cells. The ryanodine receptors are named after the plant alkaloid ryanodine which shows a high affinity to them. There are multiple isoforms of ryanodine receptors: RyR1 is primarily expressed in skeletal muscle RyR2 is primarily expressed in myocardium (heart muscle) RyR3 is expressed more widely, but especially in the brain. Non-mammalian vertebrates typically express two RyR isoforms, referred to as RyR-alpha and RyR-beta. Many invertebrates, including the model organisms Drosophila melanogaster (fruitfly) and Caenorhabditis elegans, only have a single isoform. In non-metazoan species, calcium-release channels with sequence homology to RyRs can be found, but they are shorter than the mammalian ones and may be closer to IP3 Receptors. Ryanodine receptors mediate the release of calcium ions from the sarcoplasmic reticulum and endoplasmic reticulum, an essential step in muscle contraction. In skeletal muscle, activation of ryanodine receptors occurs via a physical coupling to the dihydropyridine receptor (a voltage-dependent, L-type calcium channel), whereas, in cardiac muscle, the primary mechanism of activation is calcium-induced calcium release, which causes calcium outflow from the sarcoplasmic reticulum. It has been shown that calcium release from a number of ryanodine receptors in a ryanodine receptor cluster results in a spatiotemporally restricted rise in cytosolic calcium that can be visualised as a calcium spark. Ryanodine receptors are very close to mitochondria and calcium release from RyR has been shown to regulate ATP production in heart and pancreas cells.
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