During the contraction of a skeletal muscle fiber an action potential running along the fiber surface membrane results in the conformational change of the dihydropyridine receptors which in turn causes the opening of the sarcoplasmic reticulum (SR) calcium channels (RyR1). Calcium ions - released from the SR through the channels – increase the myoplasmic calcium concentration that finally evokes the contraction of the fibre. The decrease in the myoplasmic calcium concentration causing relaxation of the fiber is achieved by the action of the SR Ca²⁺-ATPase (SERCA).

It's known from the literature and our earlier results that thymol and its structural analogues – which are widely used in the food, cosmetic and pharmaceutical industry as preservatives – have influence on the activity of the RyR1 and SERCA. Continuing our previous work our aim was to study the effect of further phenol derivatives on the SERCA.

Light SR (LSR) vesicles were prepared from rabbit skeletal muscle (m. longissimus dorsi) containing the SR Ca²⁺-pump in their membrane. ATP dependent hydrolytic activity of LSR vesicles was measured using "coupled enzyme assay" at 37^oC. Specific activity of SERCA was calculated after determination of the non-specific activity in the presence of 20 mM cyclopiazonic acid which specifically blocks SERCA.

Pump activities were plotted against the concentration values of different drugs, dots were fitted by Hill-equation revealing the following parametes:

4-Chloro-meta-crezol IC₅₀=167 ± 8 mM, n_Hill ~3;

5-Chloro-orho-crezol IC₅₀=554 ± 45 mM, n_Hill ~2,

4-Chloro-ortho-crezol IC₅₀=1370 ± 88 mM, n_Hill ~8.

Almost all the compounds investigated here inhibited the pump except cresol which didn't exert any effect in concentration range 0–3 mM. Other compounds inhibited SERCA activity, but affinity and the number of ligands differed from each other.

Our results prove that phenol derivative structural analogues have an inhibitory effect on SERCA activity but this effect is significantly modified by the relative position of the different substituent groups and the presence of cloride is also required for inhibition. The alterations in the shape of the pi electron cloud caused by the different substituents can be also involved in the effect of these compounds.

Supported by: OTKA 81923