Mitochondrial ryanodine-sensitive Ca2+ channels of rat liver.

To examine ryanodine-sensitive Ca2+ channels in mitochondria of rat hepatocytes and their role in energy state of the cells via investigation of the ryanodine effect on mitochondrial membrane potential. Oxygen consumption was measured by polarography using the Clark electrode. The substrates of oxidation such as pyruvate (5mM), α-ketoglutarate (5mM), or succinate (5mM) were used. Oxidative phosphorylation was stimulated by the addition of adenosine diphosphate (200nM). Mitochondrial membrane potential was measured using a voltage-sensitive fluorescent probe tetramethylrhodamine-methyl-ester (0.1µM) and was analyzed by a flow cytometer. To evaluate the intact mitochondria, we used carbonil cyanide m-chlorophenyl hydrazone (CCCP, 10µM). Changes in the ionized calcium concentration in rat liver mitochondria were measured using a fluorescent probe Fluo-4 AM. Effect of ryanodine on oxygen consumption of rat liver mitochondria depends on the oxidation substrate and the incubation time. Oxidation of pyruvate in the presence of ryanodine (0.05µM) decreased the membrane potential of rat liver mitochondria by 38.4%. At higher concentrations, ryanodine (0.1µM or 1µM) led to decrease of membrane potential by 51.7% and 42.8%, respectively. In contrast, oxidation of α-ketoglutarate in the presence of ryanodine (0.05µM) increased mitochondrial membrane potential by 16.8%. However, at higher concentrations, ryanodine (0.1µM or 1µM) triggered a decreasing of membrane potential by 42.5% and 31.0%, respectively. Therefore, ryanodine at various concentrations (0.05µM, 0.1µM, or 1µM) causes differential effects on Ca2+ concentration in the mitochondria matrix under oxidation of pyruvate or α-ketoglutarate. The data suggest the presence of ryanodine receptors in mitochondrial membrane of rat hepatocytes. Their inhibition with higher concentrations of ryanodine leads to decreasing of intra-mitochondrial Ca2+ concentration and affecting the energy state of mictochondria in hepatocytes.

Ca(2+)-dependent regulation of the Ca(2+) concentration in the myometrium mitochondria. I. Trifluoperazine effects on mitochondria membranes polarization and [Ca(2+)](m).

Са2+-dependent regulation of Ca2+ exchange in mitochondria is carried out with participation of calmodulin. We have shown previously that calmodulin antagonists reduced the level of mitochondrial membrane polarization and induced increase of the ionized Са concentration in both the mitochondrial matrix and cell cytoplasm. The concentration-dependent influence of trifluoperazine on the level of polarization of mitochondrial membranes has been shown in this work. The coordinates of the Hill graphs were used to calculate the constant K0.5 and the Hill coefficient. K0.5 was 24.4 ± 5 µM (n = 10). The Hill coefficient was 2.0 ± 0.2, indicating the presence of two centers of the trifluoperazine binding. We have also studied [Ca2+]m changes, when incubating mitochondria in mediums of different composition: without ATP and ions of Mg (0-medium), in the presence of 3 mM Mg (Mg-medium) and 3 mM Mg + 3 mM ATP (Mg,ATP-medium). It was shown that the composition of the incubation medium affected the [Ca2+]m values in the absence of exogenous Ca2+ and did not affect them in the presence of the latter. Preincubation of mitochondria in mediums of different composition with 25 µM trifluoperazine did not affect the [Ca2+]m values both before and after the addition of 100 µÐœ Са2+ to the incubation medium. It was concluded, that trifluoperazine depolarized myometrial mitochondria membranes in concentration-dependent manner. However, mitochondria preincubation with 25 µM trifluope­razine accompanied by 50% decrease in membrane polarization did not affect the [Ca2+]m values.

[Influence of Mg ions and spermine on ATP-dependent Ca2+ transport in myometrial intracellular structures. II. Comparative study of spermine, Mg ions and cyclosporin A effects on Ca2+ transport in mitochondria]. / Vliianie ionov Mg i spermina na ATR-zavisimyi transport Ca2+ vo vnutrikletochnykh strukturakh miometriia. II. Sravnitel'noe izuchenie deistviia spermina, ionov Mg i tsiklosporina A na transport Ca2+ v mitokhondriiakh.

In experiments carried out with the use of the radioactive label (45Ca2+) on suspension of the rat uterus myocytes processed by digitonin solution (0.1 mg/ml), influence of spermine and cyclosporin A on Mg2+, ATP-dependent Ca2+ transport in mitochondria at different Mg2+ concentration were investigated. Ca2+ accumulation in mitochondria was tested as such which was not sensitive to thapsigargin (100 nM) and was blocked by ruthenium red (10 microM). It has been shown, that spermine (1 mM) stimulates Mg2+, ATP-dependent Ca2+ accumulation in mitochondria irrespective of Mg2+ concentration (3 or 7 mM) in the incubation medium. At the same time cyclosporin A (5 microM) effects on Ca2+ accumulation in mitochondria depend on Mg2+ concentration in the incubation medium: at 3 mM Mg2+ the stimulating effect was observed, and at 7 mM Mg2+ - the inhibitory one. In conditions which led to the increase of nonspecific mitochondrial permeability and, accordingly, to dissipation of electrochemical potential (it was reached by 5 min. preincubation of myocytes suspension in the medium that contained 10 microM Ca2+, 2 mM phosphate and 3 or 7 mM Mg2+, but not ATP) significant inhibition of Mg2+, ATP-dependent Ca2+ accumulation in mitochondria was observed. The inhibition to the greater degree was observed when medium ATP and Mg2+ were absent simultaneously in the preincubation. Thus the quality of spermine effects on Ca2+ accumulation was kept: stimulation in the presence both of 3 mM and 7 mM Mg2+. Ca2+ accumulation did not reach the control level when 3 mM Mg2+ and 1 mM spermine was present and ATP absent in the preincubation medium. However, in the presence of 7 mM Mg2+ and 1 mM spermine practically full restoration (up to a control level) of Ca2+ accumulation was observed. At the same time with other things being equal such restoration was not observed at simultaneous absence of ATP and Mg2+ in the preincubation medium. The quality of cyclosporin A effects on Ca2+ accumulation in mitochondria was also kept: stimulation - in the presence of 3 mM Mg2+, inhibition - in the presence of 7 mM Mg2+ in the preincubation medium. And, at last, in the presence of cyclosporin A irrespective of the fact which preincubation medium was used, Ca2+ accumulation level practically did not depend on Mg2+ concentration.

[Effect of Mg ions and spermine on ATP-dependent Ca2+ transport in myometrial intracellular structures. I. Comparative study of Ca2+ accumulation in mitochondria and sarcoplasmic reticulum]. / Vliianie ionov Mg i spermina na ATP-zavisimyi transport Ca2+ vo vnutrikletochnykh strukturakh miometriia. 1. Sravnitel'noe izuchenie akkumuliatsii Ca2+ v mitokhondriiakh i sarkoplazmaticheskom retikulume.

In experiments, which were carried out with the use of a radioactive label (45Ca2+) on the suspension of rat uterus myocytes treated by digitonin solution (0.1 mg/ml), influence of Mg ions and spermine on Mg2+, ATP-dependent Ca2+ transport in mitochondria and sarcoplasmic reticulum was investigated. Ca2+ accumulation in mitochondria (1324 +/- 174 pmol Ca2+/10(6) cells for 1 min - the control) was tested as such which was not sensitive to thapsigargin (100 nM) and was blocked by ruthenium red (10 microM). Oxalate-stimulated Ca2+ accumulation in sarcoplasmic reticulum (136 +/- 17 pmol Ca2+/10(6) cells for 1 min - the control) was tested as such which was not sensitive to ruthenium red and was blocked by thapsigargin. It has been shown, that initial speed and level of energy-dependent Ca2+ accumulation in mitochondria considerably exceeded the values of these parameters for sarcoplasmic reticulum Ca2+-accumulation system. Ca2+ accumulation kinetic in mitochondria was characterized by a steady-state phase (for 5-10 min. of incubation) while accumulation kinetic of this cation in sarcoplasmic reticulum corresponded to zero order reaction. Increase of Mg2+ concentration up to 5 mM led to activation of Ca2+-accumulation systems in mitochondria and sarcoplasmic reticulum (values of activation constants K(Mg) for Mg2+ were 2.8 and 0.6 mM, accordingly). Concentration dependence of spermine action on Ca2+ accumulation in mitochondria was described by a dome-shaped curve with a maximum at 1 mM spermine. In case of sarcoplasmic reticulum Ca2+ pump only the inhibition phase was tested at spermine concentration above 1 mM. However values of inhibition constants for both transporting systems were practically identical--5.2 +/- 0.6 and 5.7 +/- 0.7 mM, accordingly. Hence, Mg ions carry out the important role in regulation of energy-dependent Ca2+ transporting systems both in uterus smooth muscle mitochondria and sarcoplasmic reticulum. Spermine acts first of all on mitochondrial calcium uniporter.

[Effect of ethanol on intracellular Ca2+ metabolism]. / Effect of ethanol on intracellular Ca2+ metabolism.

The aim of this review is to summarize current thinking on ethanol effects on the Ca2+ homeostasis in the excitable tissue cells. It has been shown that acute exposure to ethanol decreases cytoplasmic Ca2+ concentration due to Ca2+ channels inhibition and Ca2+ pumps activation. Whereas chronic exposure to ethanol increases the intracellular Ca2+ concentration in cells due to activation of passive Ca2+ transport systems and inhibition of energy-dependent Ca2+ transport systems. The emphasis is place on a possible role of pharmacologic agents that preserve Ca2+ homeostasis in protecting against ethanol-induced diseases.

Adenylate cyclase of myometrium and its regulation by cAMP-dependent protein kinase.

The purpose of this study was to examine activity of adenylate cyclase (AC) and its cAMP-dependent phosphorylation by cAMP-dependent protein kinase (PKA) in the membrane of rabbit myometrium. Isoproterenol (IP) significantly increased AC activity of nonpregnant rabbits myometrium plasma membranes. However, during pregnancy AC of myometrium plasma membranes did not respond to IP. Phosphorylation of the myometrium membrane by PKA was followed by significantly decreased response of AC to IP.

[Effect of ethanol on the activity of the energy-dependent Ca2+-transporting system of myometrial cells]. / Vliianie étanola na aktivnost' énergozavisimykh Ca2+-transportiruiushchikh sistem kletok miometriia.

In order of estimating some regularities of ethanol direct (effectory) effect to transmembrane calcium metabolism in the myometrium the action of this substance on the energy-dependent Ca(2+)-transporting systems of the uterine myocytes subcellular structures has been studied. The systems of Mg2+, ATP-dependent Ca2+ transport regarding their sensitivity to ethanol inhibitory effect were displayed as satisfying the following sequences: endoplasmic reticulum calcium pump > plasma membrane solubilized Ca2+, Mg2+, ATP-ase > mitochondrial Ca(2+)-accumulating system = plasma membrane calcium pump. Alongside with the latter, the oxytocin-insensitive component of Mg2+, ATP-dependent Ca2+ accumulation in the endoplasmic reticulum was defined to be less resistant to inhibitory effect of ethanol if compared with the oxytocin-sensitive one. On the base of the data received some mechanisms of ethanol effectory action on the intracellular calcium homeostasis in the myometrium cells are under the discussion.

[Properties of the smooth muscle cell endoplasmic reticulum calcium pump]. / Svoistva kal'tsievogo nasosa éndoplazmaticheskogo retikuluma gladkomyshechnky kletok.

In experiments with 45Ca2+ conducted on digitonin-treated (0.1 mg/ml) myometrium cells suspension, the properties of ruthenium red-insensitive, oxalate- or phosphate-stimulated and thapsigargin- or cyclopiasonic acid-suppressed Mg2+, ATP-dependent calcium pump of myometrium sarcoplasmic reticulum was studied. The Ca2+ accumulation increased linearly in time up to 10 min, the average initial rate was 80-130 pmol Ca2+/10(6) cells per min. In the presence of 10 mM oxalate the values of the activation constant KMg for Mg2+ and K(m) for ATP were 0.6 and 1.0 mM, respectively. The relative efficiency of the different cations in insuring of the ATP-dependent Ca2+ accumulation was Mg2+ > Mn2+ = Co2+ >> Ni2+; the Ca2+ accumulation was not observed in the presence of 3 mM Zn2+ or Cu2+. We observed the suppression of calcium pump activity by different inhibitors such as thapsigargin, cyclopiazonic acid, p-chloromercuribenzoic acid, eosin Y ad Na3 VO4: the values of K0.5 were 2.0 nM, 0.3 microM, 0.6 microM, 0.8 microM and 45 microM respectively. The conclusion was made that suspension of myometrial cells treated with digitonin represent a suitable experimental model for studying the properties of myometrium sarcoplasmic reticulum calcium pump.

Evidence for Mg2, ATP-dependent accumulation of Ca2+ by the intracellular non-mitochondrial calcium store in uterine smooth muscle cells.

Using carbachol contracture as the experimental model for testing the properties of the intracellular calcium store in intact tissue and 45Ca2+ accumulation in the chemically skinned by digitonin smooth muscle cells isolated from oestrogen-dominated rat uterus the evidence for the presence of Mg2+, ATP-dependent Ca2+ pump in the non-mitochondrial store has been found which is supposed to play a key role in the process of refilling' of the store on the cytoplasmic level. The experiments performed on intact muscle showed that the functional activity of the carbachol-releasable Ca2+ store is critically dependent on Ca2+ entry. It is found that Ca2+ entry via voltage operated Ca2+ channels or on the Na(+)-Ca2+ exchange was needed to refill the store in this tissue. However, when Ca2+ extrusion systems located in the plasma membrane were inhibited by La3+, the store retained its ability to discharge and reaccumulate Ca2+ released on the regular basis suggesting the presence of the energy-dependent Ca2+ accumulating system in the store. The process of the store refilling was totally inhibited by cyclopiazonic acid. Chemically skinned uterine smooth muscle cells demonstrated the presence of Mg2+, ATP-dependent accumulation of Ca2+ in the non-mitochondrial (ruthenium red insensitive) intracellular store(s) potentiated by Ca(2+)-precipitating anions (potassium oxalate and phosphate), in a time- and concentration dependent way which was inhibited by Ca(2+)-ionophore A 23187 (5 microM) and cyclopiazonic acid with Ki = 0.4 microM. It is suggested that in the uterine smooth muscle of the oestrogen-dominated rats, nonmitochondrial receptor-operated intracellular calcium store is represented by endoplasmic reticulum.

Suspension of smooth muscle cells treated with digitonin as a model for studying the myometrial endoplasmic reticulum calcium pump.

The effect of the nonionic detergent digitonin on ruthenium red-insensitive, oxalate-stimulated, and thapsigargin-suppressed accumulation of Ca2+ by suspension of myometrial cells from ATP- and Mg(2+)-containing incubation medium was studied using passive loading with 45Ca2+. The dependence of Ca2+ accumulation by the cells on the concentration of digitonin is bell-shaped, the maximal Ca2+ uptake being observed at 0.05-0.1 mg/ml digitonin. Myocytes chemically skinned with digitonin represent a suitable experimental model for studying kinetic properties of the uterine myocyte endoplasmic reticulum Mg2+, ATP-dependent Ca2+ pump and assessing sensitivity of this Ca(2+)-transporting system to various effectors including the uterotonic octapeptide oxytocin.

[Effect of inhibitors of energy-dependent Ca2+-transporting systems on calcium pumps of a smooth muscle cell]. / Vliianie ingibitorov énergozavisimykh Ca2+-transportiruiushchikh sistem na kal'tsievye nasosy gladkomyshechnoi kletki.

Comparative investigation of sensitivity of calcium pumps of sarcolemma, endoplasmic reticulum and mitochondria of the smooth muscle to some inhibitors of energy-dependent Ca(2+)-transporting systems has been carried out in experiments made using the isotope method (45Ca2+) on the fraction of plasmatic membranes and suspension of chemically scanned cells of myometrium as well as on the preparation of highly purified Ca2+, Mg(2+)-ATPase which was solubilized from plasmatic membrane of uterus myocytes. It is proved that the calcium pump of mitochondria is selectively inhibited by ruthenium red (imaginary inhibition constant K, is equal to 0.6 microM); it is nonselectively inhibited by o-vanadate (Ki = 0.6 mM), p-chloromercuribenzoate (Ki = 3.2 microM) and cosine (Ki = 2.1 microM), but is not sensitive to the effect of thapsigargin and cyclopiasonic acid. Mg2+, ATP-dependent calcium pump and transport Ca2+, Mg(2+)-ATPase of plasmatic membrane are nonspecifically inhibited by o-vanadate (the value Ki equals 45.0 and 5.0 microM, respectively) and by cosine (Ki = 0.8 microM in the both cases); this calcium pump is also nonspecifically inhibited by p-chlormercury benzoate (Ki = 3.2 microM), but it is not sensitive to the effect of ruthenium red. Mg2+, ATP-dependent calcium pump of endoplasmic reticulum is selectively inhibited by tapsigargin (Ki = 2.0 nM) and cyclopiasonium acid (Ki = 0.3 microM), but, like calcium pumps of mitochondria and plasma membrane, it is nonspecifically inhibited by o-vanadate (Ki = 45.0 microM); by p-chlormercurybenzoate (Ki = 0.6 microM) and eosin (Ki = 0.8 microM), but it is not sensitive to the effect of ruthenium red. Data that have been obtained can be of use for the further development of the ideas about biochemical regularities of Ca(2+)-dependent control of contraction-relax of the smooth muscles, identifications of energy-dependent calcium pumps of smooth cells and finding out of structure-functional organization of these pumps.

[Treatment of intact myometrial cells with oxytocin inhibits Mg2+, ATP-dependent accumulation of Ca2+ in endoplasmic reticulum]. / Obrabotka oksitotsinom intaktnykh kletok miometriia ingibiruet Mg2+,ATP-zavisimuiu akkumuliatsiiu Ca2+ v éndoplazmaticheskom retikulume.

Uterotonic peptide hormone oxytocin has been studied for its effect on ATP-dependent accumulation of Ca2+ nonsensitive to the effect of ruthenium red (10 microM) that is potentiated by Ca2+-precipitating anion - oxalate. That was studied in experiments made on the suspension of myometrium cells from estrogenized rats processed by digitonin (0.1 mg/ml) with the aim permeabilization of plasmatic membrane. The preliminary processing of intact myocytes by oxytocin solution (final concentration 100 mM) to permeabilization of plasmatic membrane causes partial inhibition (stimulated by oxalate by 25-30% (0-10 mM) of Mg2+, ATP-dependent accumulation of Ca2+ in nonmitochondrion intracellular calcium depot of perforates smooth-muscled cells. The value of the component of power-dependent accumulation of Ca2+, that is inhibited by oxytocin, increased with both the incubation time and oxalate concentration. Thapsigargin and cyclopiasonium acid, selective inhibitors of calcium pump of endo(sarco)plasmatic reticulum, when used in saturation concentration (50 mM and 10 mM, respectively), completely inhibit the component of Mg2+, ATP-dependent accumulation of Ca2+ in nonmitochondrial intracellular calcium depot that is stimulated with oxalate (< 10 mM) and inhibited with oxytocin (100 mM). It is concluded that oxytocin partially inhibits Mg2+, ATP-dependent calcium pump of myometrium cell endoplasmic reticulum.

[Mg2+,ATP-dependent transport of Ca2+ in the endoplasmic reticulum of myometrial cells]. / Mg2+,ATP-zavisimyi transport Ca2+ v éndoplazmatichesom retikulume kletok miometriia.

An in-depth study of the effects of Ca(2+)-precipitating anions (oxalate, phosphate), the Ca2+ ionophore A23187 and some highly effective selective inhibitors of the endo(sarco)plasmic reticulum Ca(2+)-pump (tapsigargin, cyclopiasonic acid) on ruthenium red (10 microM)-insensitive, Mg2+,ATP-dependent accumulation of Ca2+ in digitonin-permeabilized myometrium cells of nonpregnant estrogen-treated rats, has been carried out. Oxalate and phosphate stimulated Mg2+,ATP-dependent accumulation of Ca2+ in permeabilized smooth muscle cells. The Ca2+ ionophore A23187 (5 microM) added to the medium containing oxalate (10 mM) or phosphate (20 mM) prevented the energy-dependent accumulation of Ca2+. Tapsigargin (50 nM) and cyclopiasonic acid (10 microM) fully suppressed the Mg2+,ATP-dependent Ca2+ increase measured in the presence of the Ca(2+)-precipitating anions. The kinetic data indicate that the tapsigargin affinity for the energy-dependent Ca2+ transporting system is much higher than that of cyclopiasonic acid: Ki app values are 0.9 nM and 0.6 microM, respectively. Oxalate (10 mM) added to the incubation medium initially containing tapsigargin (50 nM) failed to induce the Mg2+,ATP-dependent accumulation of Ca2+ in permeabilized myocytes. The data obtained provide biochemical evidence for the existence in myometrium cells of a Ca(2+)-pump providing the Mg2+,ATP-dependent accumulation of Ca2+ in the nonmitochondrial Ca2+ depot--the endoplasmic reticulum. This reaction is stimulated by precipitating penetrating anions and suppressed by selective inhibitors.

[Energy-dependent Ca2+-transport in intracellular smooth muscle structures]. / Energozavisimyi transport Ca2+ vo vnutrikletochnykh strukturakh gladkoi myshtsy.

In experiments performed on digitonin-treated myometrium cells the existence of intracellular energy-dependent Ca(2+)-transporting system has been identified. The mitochondria-linked energy-dependent accumulation of Ca2+ was inhibited by ruthenium red and sodium azide with Ki of 0.7 +/- 0.3 microM and 1.1 +/- 0.4 mM, respectively. The Ca2+ ionophore A23187 added to the incubation medium following the termination of the accumulation process by ruthenium red caused a discharge of Ca2+ accumulated from the mitochondria. In the endoplasmic reticulum there exists a Mg2+,ATP-dependent Ca(2+)-pump which is potentiated by oxalate, immune to ruthenium red and sodium azide and inhibited by thapsigargin with Ki < 20 nM. In the absence of oxalate, the component of mitochondrial accumulation of Ca2+ was 3-4 times higher that of the endoplasmic reticulum. These results indicate that under the conditions used the calcium capacity of mitochondria was significantly higher than that of the endoplasmic reticulum. It is suggested that mitochondria serve as a high capacity intracellular Ca(2+)-accumulating store protecting smooth muscle cells from Ca2+ overload under some abnormal states. Under normal physiological conditions the endoplasmic reticulum serves as the major intracellular store involved in the pharmacomechanical coupling in the smooth muscle of the myometrium.

[Uterotonic action of sigetin and its effect on the Mg2+, ATP-dependent transport and stationary metabolism of Ca2+ through the myometrial sarcolemma]. / Uterotonicheskoe deistvie sigetina i ego vliianie na Mg2+, ATP-zavisimyi transport i statsionarnyi obmen Ca2+ cherez sarkolemmu miometriia.

The effect of sigetin (dipotassium salt, meso-3, 4-di-(n-sulfophenyl)- hexane) on the electrical and mechanical activity, Ca2+, Mg2+-ATPase activity and Mg2+, ATP-dependent Ca2+ transport in uterine smooth muscle have been investigated. Sigetin caused depolarization of the cell membrane, leading to Ca2+ dependent spike discharge and development of mechanical response. Sigetin also produced deceleration of the relaxation of carbachol contracture controlled mostly by the sarcolemmal ATP driven Ca2+ pump. The drug produced strong inhibition of Mg2+, ATP-dependent Ca2+ transport in sarcolemmal vesicles as well as the enzyme and transport activity of the solubilized Ca2+, Mg(2+)-ATPase reconstructed in liposomes. Under the experimental conditions close to the physiological (i.e. in the presence of an outwardly directed Ca2+ gradient) Ca2+ pump normally compensates passive release of this cation. Sigetin was found to shift this calcium equilibrium towards a non-compensated slow diffusion of the cation in the extravesicular space. Addition of sigetin to the incubation medium during the existence of the stationary transmembrane Ca2+ equilibrium stimulates transition of the stationary concentration of the cation in vesicles from high to lower level. The results suggest that the excitatory action of sigetin on myometrial smooth muscle may involve not only indirect stimulation of Ca2+ influx via the voltage operated Ca2+ channels but also through the inhibition of the ATP driven Ca2+ pump of the cell membrane.

[Uterotonic effect of oxytocin and transport of Ca2+ through the myometrial sarcolemma]. / Uterotonicheskoe deistvie oksitotsina i transport Ca2+ cherez sarkolemmu miometriia.

Effect of oxytocin on contractile activity, passive and Mg2+, ATP-dependent Ca2+ transport through myocyte plasmic membrane was studied in the experiments carried out on intact samples of the uterus smooth muscle and fraction of myometrium sarcolemma vesicules. It was shown that at short-term (30 s) application of oxytocin (2.10(-7) M) powerful prolonged contractile reaction of myometrium bands was observed. After the chelating of Ca2+ in the washing medium by means of EGTA (2mM) or while introducing into it blockers of electrically controlled calcium channels the bands responded to oxytocin application with a single physical contraction. High concentrations of peptide hormone (10(-6) M) introduced into the preincubation medium of the smooth muscle bands before the isolation of the membrane fraction or directly into the incubation medium produced no effect on passive liberation of Ca2+ vesicules from the sarcolemma vesicules. At the same time preincubation of the bands in the solution containing smaller concentration of octapeptide (10(-7) M) decreased the initial velocity of Mg2+, ATP-dependent Ca2+ transport in the vesicules by 30% on the average. Possible mechanisms of oxytocin effects on intracellular homeostasis of Ca2+ in the myometrium, on the contractile activity of the uterus are discussed.