Ryanodine receptor isoforms in excitation-contraction coupling

Three genomically distinct isoforms of RyR are now known. RyR1 homologue is the primary isoform in skeletal muscles, whereas in cardiac muscles it is RyR2 homologue. RyR3 homologue occurs ubiquitously in many cells, but the biological function is little known, partly because of its minuscule amount in mammalian cells. The difference among RyR isoforms may not be so great in CICR activity, in other words, in the interaction of RyR isoforms with Ca²⁺, adenine nucleotides and caffeine. Species specificity among RyR1 homologues may be more important in the apparent difference between RyR1 and RyR3 homologues. CICR is likely to be the dominant underlying mechanism for E-C coupling in the cardiac muscle and probably in cells other than the skeletal muscle where the significance of CICR is controversial in physiological contraction. In E-C coupling of skeletal muscle (DICR), the reciprocal tight interactions between DHPR and RyR1 are critically required. The α₁ subunit of DHPR was only the main target of our current interests in the interaction with RyR1; the involvement of auxiliary subunits of $\text{α}{}_2\text{δ}$ and β subunits and their mutual interactions, however, are also important. DICR and CICR in RyR1 share common properties of stimulation by concentrated solutes and modulation by luminal calcium or Ca²⁺, suggesting that the main difference between the two Ca²⁺ release mechanisms may be in the gating mechanism of the channel. Further investigations are required to understand molecular interactions during E-C coupling.