The ryanodine receptor (RyR) is a Ca2+ release channel in the sarcoplasmic reticulum of skeletal and cardiac muscles and plays a key role in excitation-contraction coupling. The activity of the RyR is regulated by many intracellular factors such as divalent cations (Ca2+ and Mg2+), nucleotides, associated proteins, and reactive oxygen species. Since these intracellular factors change depending on the condition of the muscle, e.g., exercise, fatigue, or disease states, the RyR channel activity will be altered accordingly. In this review, we describe how the RyR channel is regulated under various conditions and discuss the possibility that the RyR acts as a sensor for change in the cellular environment of muscles.

The color of a thin copper oxide layer formed on a copper plate was transformed from reddish-brown into blue-purple by irradiation with 5 keV Ar+ ions to a fluence as low as 1 1015 Ar+ cm–2. In the unirradiated copper oxide layer, the copper valence state of Cu2+ as well as Cu+ and/or Cu0 was included as indicated by the presence of a shake-up satellite line in a photoemission spectrum. While for the irradiated one, the satellite line decreased in intensity, indicating that irradiation resulted in the reduction from Cu2+ to Cu+ and/or Cu0. Furthermore, nuclear reaction analysis using a 16O(d, p)17O reaction with 0.85 MeV deuterons revealed a significant loss of oxygen (51015 O atoms cm–2) in the irradiated layer. Thus, the chromatic change observed in the present work originated in the irradiation-induced reduction of a copper oxide.