Recognition and activation of ryanodine receptors by purines.

Ryanodine receptor (RyR) is a tetrameric, high molecular weight protein that functions as a calcium release channel. It plays a key role in phenomena such as signal transduction, excitation-contraction and excitation-secretion coupling. Hyperthermia maligna, central core disease and myocardial infarction have been related with RyR dysfunction. RyR is present as three isoforms in vertebrates: RyR 1 mainly localized in skeletal muscle, RyR 2 in cardiac muscle, and RyR 3 in nervous system. RyR is regulated by a number of physiological and pharmacological factors. Main physiological modulators: calcium, kinases and phosphatases, redox state and energy charge. Main pharmacological regulators: caffeine, dantrolene, ruthenium red, heavy metals and ryanodine. Purines have to do with both, physiological and pharmacological regulation of the RyR activity. So far, the mechanisms of RyR activation by ATP and caffeine have been described in detail using [3H]-ryanodine binding assays and unitary channel activity recorded in planar lipid bilayers. However, some questions remain to be addressed and are at present aim of active scrutiny: How many sites for purines are present in the RyR? Is the same site recognized by nucleotides and methylxanthines? What differences exist among the interaction between RyR and purine bases, nucleosides and nucleotides? Are the phosphate groups important for the recognition of nucleotides? Is the sugar moiety important for the recognition of nucleosides? The review article will examine the most recent specialized literature about the mechanism of activation of RyR by purines with emphasis on reports with approaches of structure-function and structure-activation.

Modulation of the skeletal muscle Ca2+ release channel/ryanodine receptor by adenosine and its metabolites: a structure-activity approach.

Activation of ryanodine receptor (RyR) from skeletal muscle sarcoplasmic reticulum by adenosine and adenosine's metabolites was studied. The purines tested increased the [3H]-ryanodine binding as follows: xanthine>adenosine>adenine >inosine>/=uric acid>hypoxanthine. The enhanced [3H]-ryanodine binding did not involve change in the RyR-Ca(2+) sensitivity and was due mainly to lower values in the affinity constant (K(d)) that corresponded with an increase in the association rate constant (K(+1)). [3H]-ryanodine maximum binding (B(max)) was much less affected. Adenosine and inosine effects were dependent on the presence beta-glycosidic bond within the ribose ring, since the combination of adenine or hypoxanthine with ribose was not able to emulate the nucleosides' original activation. Competition experiments with AMP-PCP, a non-hydrolyzable analogue of ATP, evidenced a nucleotide's inhibitory influence on the adenosine and xanthine activation of the RyR. As a result of a Quantitative Structure-Activity Relationship (QSAR) study, we found a significant correlation between the modulation by adenosine and its metabolites on RyR activity and the components of their calculated dipole moment vector. Our results show that the ribose moiety and the dipole moment vector could be factors that make possible the modulation of the RyR activity by adenosine and its metabolites.