It is shown that histidine-containing dipeptide carnosine (beta-alanyl-L-histidine), which is present in skeletal muscles in millimolar concentrations, decreases the rate of Ca2+ accumulation by the heavy fraction of sarcoplasmic reticulum from rabbit skeletal muscles. This effect results from the ability of carnosine to induce a rapid Ca2+ release from the heavy sarcoplasmic reticulum vesicles via activation of the ruthenium red-sensitive Ca-channels. The effect of carnosine is dose-dependent that indicates the presence of saturable site(s) for carnosine in the molecules of Ca-channels. The C0.5 value carnosine (the concentration that induces the half-maximal Ca2+ release) is 8.7 mM. The 1 N-methylated derivative of carnosine, i.e., anserine, also induces a rapid Ca2+ release with the half-maximal effect at 2.7 mM. Conversely, neither histidine nor beta-alanine (both separately and in the mixture) cause Ca2+ release. In addition, carnosine increases the sensitivity of Ca-channels to their well-known activators (caffeine, AMP, and Ca2+) and decreases inhibitory effect of low concentrations of Mg2+. It is concluded that carnosine as a component of skeletal muscles can be an endogenous regulator of the sarcoplasmic reticulum Ca-channel activity.

It is known that the light fraction of rabbit skeletal muscle sarcoplasmic reticulum vesicles can release Ca2+ from the intravesicular space, although the Ca2+-conductive channels are present only in the heavy fraction of sarcoplasmic reticulum vesicles. To study the possible pathways of the Ca2+ leakage from light vesicles we have used a short-term treatment for 4.5 min at 45°C which quickly decreases the efficiency of Ca2+-transporting ATPase operation without any visible effects on the hydrolytic activity of the Ca2+-ATPase in the membranes. The treatment of the vesicles decreased the negative membrane surface potential created by the Ca2+-ATPase. Comparative titration of control and heat-treated preparations of light sarcoplasmic reticulum vesicles by K+, Na+, Mg2+, and Ca2+ revealed clear differences in their surface properties. The short-term heating resulted in release of Ca2+ from the vesicles previously loaded with 45Ca2+, which indicates an increase in passive membrane permeability to Ca2+. Study of Ca2+-ATPase protein arrangement in the membrane indicated that the heat treatment induced protein oligomerization and some of the Ca2+-ATPase molecules acquired intermolecular and intramolecular covalent bonds. From these data, we have concluded that the changes in the surface and structure properties of the vesicle membranes after the short-term heat treatment were the result of clustering of the CaZ+-ATPase molecules. This protein rearrangement may create channels for calcium leakage from light sarcoplasmic reticulum vesicles.The sarcoplasmic reticulum (SR) plays a key role in excitation-contraction coupling between nerve impulses and muscle contraction. The skeletal muscle SR consists of two morphologically and functionally distinct regions, namely the lateral sacs (heavy fraction of vesicles after centrifugation in a sucrose density gradient) and the longitudinal tubules (light fraction of vesicles) which differ in protein composition [l]. The lateral sacs contain Ca2+ channels and provide fast release of stored Ca2+ [l, 21. The role of the longitudinal tubules, which are rich in CaZ+-ATPase protein, is calcium accumulation. The latter process involves active transport in which Ca" translocation is coupled with ATP hydrolysis [3]. It is known that short-term heating of SR vesicles at 42-45 "C leads to a quick decrease of Ca2+-transporting ATPase (Ca'+ pump) efficiency without an effect on the Ca2+-ATPase hydrolytic activity [4, 53. Studying the mechanism of the Ca2+ pump thermal uncoupling may be useful for understanding the possible pathways of Ca" release from SR. New pathways for Caz+ release from SR could be found which may be relevant to the regulation of the Caz+ exchange in muscle tissue under normal and pathological conditions. It has been shown that the decrease in the Ca2+ transport efficiency upon short-term heating of SR vesicles was not connected with true intramolecular uncoupling of Ca2+ pump [6]. To evaluate conformational changes of the Caz+-ATPase after heat treatment, kinetic paramete...

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It is known that short-term heating of sarcoplasmic reticulum (SR) vesicles at 42-45°C leads 'ю а sharp decrease of Ca-pump efficiency without any effect on the Ca-ATPase hydrolytic activity [!]. The decrease of the pump efficiency is connected with the release of accumulated Ca?+ from the vesicles as a result of a strong increase of the membrane permeability for this ion [%2]. Analysis of the phenomenon called thermouncoupling may be useful for understanding the possible pathways of Ca?t release from SR. The appearance of such new pathways for Са*+ release from SR can be relevant to the regulation of Ca?t exchange in muscle tissue under normal and pathological conditions.The formation of Ca-permeable channels between Ca-ATPase molecules at enzyme clusterization is one of the possibilities of the Ca?* release from SR [%]. It has been shown earlier that the Ca-pump thermouncoupling is connected with the decrease of the SR lipid bilayer microviscosity that can induce protein oligomerization [%%]. As shown elsewhere, thermotreatment of SR vesicles decreases the membrane surface potential that seems to reflect clusterization of the membrane proteins [?]. However, all approaches used previously have given only indirect information about the Ca-ATPase protein arrangement in the SR membranes.