Effect of natural phenol derivatives on skeletal type sarcoplasmic reticulum Ca2+-ATPase and ryanodine receptor

The effect of natural phenol derivatives was studied on skeletal type sarcoplasmic reticulum Ca²⁺-ATPase and ryanodine receptor. The majority of the tested derivatives exerted inhibitory effect on the Ca²⁺-ATPase with an ascending sequence in regard to their effectiveness (IC₅₀): cineole (3.33 mM) < ortho-vanillin (IC₅₀ =1.13 mM) < 4-methyl-2-nitrophenol (1104 μM) < vanillin (525 μM) < thymol (224 μM) < carvacrol (162 μM). In two cases biphasic characteristic was observed: trans-anethole and meta-anisaldehyde first caused activation followed by inhibition (with IC₅₀-s of 141 and 1903 μM respectively) as their concentration was increased. In some cases (cineole, ortho-vanillin, meta-anisaldehyde) total inhibition of Ca²⁺-ATPase could not be reached as the result of the limited solubility of these drugs. Para-anisaldehyde and 6-amino-meta-cresol did not show any effect up to 3 mM. In Ca²⁺ release experiments drugs were applied on heavy sarcoplasmic reticulum vesicles isolated from skeletal muscle and actively loaded with calcium. Only thymol and carvacrol were able to evoke Ca²⁺ release with EC₅₀ values of 158 ± 16 and 211 ± 55 μM respectively. Futhermore the effect of thymol and carvacrol was tested on the isolated ryanodine receptor incorporated into artificial lipid bilayer. Both drugs activated the RyR when applied in concentrations identical to their EC₅₀ values. These observations show that small differences in the structure of phenol derivatives sometimes have little impact on their effect on the sarcoplasmic reticulum Ca²⁺-ATPase or ryanodine receptor (thymol and carvacrol) whereas in certain cases they can completely abolish a particular effect (para- and meta-anysaldehide).