Expression and regulation of ryanodine receptor/calcium release channels.

Intracellular calcium release channels on the endoplasmic or sarcoplasmic reticula (ryanodine receptors, RyR, and inositol 1,4,5-trisphosphate receptors, IP(3)R) comprise a unique family of molecules that are structurally and functionally distinct from all other known ion channels. These channels play crucial roles in many cellular signaling pathways including excitation-contraction coupling, oocyte fertilization, hormone secretion, neurotransmitter release, and T lymphocyte activation. Three forms of RyR have been identified: RyR1 expressed predominantly in skeletal muscle, RyR2 in cardiac muscle, and RyR3 in the brain. The tetrameric structures of RyR1 and RyR2 are stabilized by a channel-associated protein, FKBP12. The immunosuppressant drugs FK506 and rapamycin inhibit the prolyl isomerase activity of FKBP12 and could cause cardiac dysfunction by inducing a Ca(2+) leak from the sarcoplasmic reticulum. RyR2 is downregulated and IP(3)R is upregulated during severe end-stage heart failure secondary to dilated cardiomyopathies in humans, suggesting that these channels may contribute to abnormalities in Ca(2+) homeostasis.