Cellular mechanisms of thromboxane A2-mediated contraction in pulmonary veins

Our objectives were to identify the relative contributions of [Ca2+]i and myofilament Ca2+ sensitivity in the pulmonary venous smooth muscle (PVSM) contractile response to the thromboxane A2 mimetic U-46619 and to assess the roles of PKC, tyrosine kinases (TK), and Rho-kinase (ROK) in that response. We tested the hypothesis that U-46619-induced contraction in PVSM is mediated by both increases in [Ca2+]i and myofilament Ca2+ sensitivity and that the PKC, TK, and ROK signaling pathways are involved. Isometric tension was measured in isolated endothelium-denuded (E-) canine pulmonary venous (PV) rings. In addition, [Ca2+]i and tension were simultaneously measured in fura-2-loaded E- PVSM strips. U-46619 (0.1 nM-1 microM) caused dose-dependent (P < 0.001) contraction in PV rings. U-46619 contraction was attenuated by inhibitors of L-type voltage-operated Ca2+ channels (nifedipine, P < 0.001), inositol 1,4,5-trisphosphate-mediated Ca2+ release (2-aminoethoxydiphenylborate, P < 0.001), PKC (bisindolylmaleimide I, P < 0.001), TK (tyrphostin A-47, P = 0.014), and ROK (Y-27632, P = 0.008). In PV strips, U-46619 contraction was associated with increases in [Ca2+]i and myofilament Ca2+ sensitivity. Both Ca2+ influx and release mediated the early transient increase in [Ca2+]i, whereas the late sustained increase in [Ca2+]i only involved Ca2+ influx. Inhibition of both PKC and ROK (P = 0.006 and P = 0.002, respectively), but not TK, attenuated the U-46619-induced increase in myofilament Ca2+ sensitivity. These results suggest that U-46619 contraction is mediated by Ca2+ influx, Ca2+ release, and increased myofilament Ca2+ sensitivity. The PKC, TK, and ROK signaling pathways are involved in U-46619 contraction.     

Effects of anoxia on cellular damage in the incubated mouse diaphragm

Satisfactory ultrastructural integrity of the mouse diaphragm was maintained in vitro in modified Krebs-henseleit saline for 3h when the rate of oxygenation was 2.8 ml sec-1 (95% O2 + 5% CO2). Hypoxic (O2 = 1.6-2.0 ml sec-1) or anoxic (95% N2 + 5% CO2) conditions triggered typical Ca-triggered myofilament damage, believed to be induced by a rise in [Ca]i. It was unaffected by omission of Ca from the saline, but the muscle was protected at 7.8 degrees C. 'High-O2' gassing (10 ml sec-1) also caused a characteristic, but different, damage with swollen sarcoplasmic reticulum and spacing of the myofibrils.     

Role of PKC,tyrosine kinases,and Rho kinase in alpha-adrenoreceptor-mediated PASM contraction

Our objectives were to identify the relative contributions of intracellular free Ca2+ concentration ([Ca2+]i) and myofilament Ca2+ sensitivity in the pulmonary artery smooth muscle (PASM) contractile response to the alpha-adrenoreceptor agonist phenylephrine (PE) and to assess the role of PKC, tyrosine kinases (TK), and Rho kinase (ROK) in that response. Our hypothesis was that multiple signaling pathways are involved in the regulation of [Ca2+]i, myofilament Ca2+ sensitization, and vasomotor tone in response to alpha-adrenoreceptor stimulation of PASM. Simultaneous measurement of [Ca2+]i and isometric tension was performed in isolated canine pulmonary arterial strips loaded with fura 2-AM. PE-induced tension development was due to sarcolemmal Ca2+ influx, Ca2+ release from inositol 1,4,5-trisphosphate-dependent sarcoplasmic reticulum Ca2+ stores, and myofilament Ca2+ sensitization. Inhibition of either PKC or TK partially attenuated the sarcolemmal Ca2+ influx component and the myofilament Ca2+ sensitizing effect of PE. Combined inhibition of PKC and TK did not have an additive attenuating effect on PE-induced Ca2+ sensitization. ROK inhibition slightly decreased [Ca2+]i but completely inhibited myofilament Ca2+ sensitization. These results indicate that PKC and TK activation positively regulate sarcolemmal Ca2+ influx in response to alpha-adrenoreceptor stimulation in PASM but have relatively minor effects on myofilament Ca2+ sensitivity. ROK is the predominant pathway mediating PE-induced myofilament Ca2+ sensitization.     

Are lysosomal enzymes involved in rapid damage in vertebrate muscle cells?

Summary Experiments with lysosomotropic agents suggest that the sarcotubular system subserves some of the functions of the lysosomal apparatus in frog skeletal muscle. Dinitrophenol or A23187 trigger lysosome labilization and myofilament damage in mammalian cardiac muscle. Lysolecithin labilizes isolated liver lysosomes, but has no action following phospholipase A₂ activation in vivo. Zinc ions or a pH_i of 7.5 do not protect against myofilament damage. In fractions from mammalian cardiac muscle, calcium and calmodulin do not cause lysosomal labilization whereas cGMP does but only at high concentration (10^-4 M). It is concluded that lysosomal hydrolases play no significant part in rapid muscle damage. It is suggested that rises in [Ca]_i activate two separate pathways causing (i) myofilament damage; (ii) sarcolemmal (and possibly lysosomal) membrane damage via phospholipase A₂ and lipoxygenase activity. Dinitrophenol triggers both pathways independently and thus may cause lysosome labilization. The possibility that the sarcoplasmic reticulum is the site generating myofilament damage is discussed.     

The C terminus (amino acids 75-94) and the linker region (amino acids 42-54) of the Ca2+-binding protein S100A1 differentially enhance sarcoplasmic Ca2+ release in murine skinned skeletal muscle fibers

S100A1, a Ca2+-binding protein of the EF-hand type, is most highly expressed in striated muscle and has previously been shown to interact with the skeletal muscle sarcoplasmic reticulum (SR) Ca2+ release channel/ryanodine receptor (RyR1) isoform. However, it was unclear whether S100A1/RyR1 interaction could modulate SR Ca2+ handling and contractile properties in skeletal muscle fibers. Since S100A1 protein is differentially expressed in fast- and slow-twitch skeletal muscle, we used saponin-skinned murine Musculus extensor digitorum longus (EDL) and Musculus soleus (Soleus) fibers to assess the impact of S100A1 protein on SR Ca2+ release and isometric twitch force in functionally intact permeabilized muscle fibers. S100A1 equally enhanced caffeine-induced SR Ca2+ release and Ca2+-induced isometric force transients in both muscle preparations in a dose-dependent manner. Introducing a synthetic S100A1 peptide model (devoid of EF-hand Ca2+-binding sites) allowed identification of the S100A1 C terminus (amino acids 75-94) and hinge region (amino acids 42-54) to differentially enhance SR Ca2+ release with a nearly 3-fold higher activity of the C terminus. These effects were exclusively based on enhanced SR Ca2+ release as S100A1 influenced neither SR Ca2+ uptake nor myofilament Ca2+ sensitivity/cooperativity in our experimental setting. In conclusion, our study shows for the first time that S100A1 augments contractile performance both of fast- and slow-twitch skeletal muscle fibers based on enhanced SR Ca2+ efflux at least mediated by the C terminus of S100A1 protein. Thus, our data suggest that S100A1 may serve as an endogenous enhancer of SR Ca2+ release and might therefore be of physiological relevance in the process of excitation-contraction coupling in skeletal muscle.     

Norepinephrine and GTP-gamma-S increase myofilament Ca2+ sensitivity in alpha-toxin permeabilized arterial smooth muscle

A new method for preparing permeabilized smooth muscle fibers from rabbit mesenteric artery has been developed using alpha-toxin, a transmembrane pore-making exo-protein produced by Staphylococcus aureus. After alpha-toxin treatment the fibers developed tension as a function of Ca2+ concentration (EC50 = 890 nM). But they could not contract without added ATP, indicating ATP is permeable. When the sarcoplasmic reticulum was loaded with 5 X 10(-7) M Ca2+ solution, NE induced a transient contraction in 2 mM EGTA 0 M Ca2+ solution and a transient and maintained contraction in 5 X 10(-7) M Ca2+ solution. GTP-gamma-S, a non-hydrolyzable analogue of GTP, substituted for NE in producing these contractile effects. The analysis of the relationship between Ca2+ and maintained tension revealed that NE and GTP-gamma-S cause increases in Ca2+ sensitivity of myofilament shifting the EC50 to 280 nM and 160 nM, respectively. We conclude that NE or GTP-gamma-S causes an increase in myofilament Ca2+ sensitivity and that G protein may be involved in receptor signal transduction system. alpha-Toxin is a useful tool to permeabilize the smooth muscle tissue to ions and small molecules without any damage of receptor and signal transduction system.     

Challenging Current Paradigms Related to Cardiomyopathies: ARE CHANGES IN THE Ca2+ SENSITIVITY OF MYOFILAMENTS CONTAINING CARDIAC TROPONIN C MUTATIONS (G159D AND L29Q) GOOD PREDICTORS OF THE PHENOTYPIC OUTCOMES?*

Two novel mutations (G159D and L29Q) in cardiac troponin C (CTnC) associate their phenotypic outcomes with dilated (DCM) and hypertrophic cardiomyopathy (HCM), respectively. Current paradigms propose that sarcomeric mutations associated with DCM decrease the myofilament Ca2+ sensitivity, whereas those associated with HCM increase it. Therefore, we incorporated the mutant CTnCs into skinned cardiac muscle in order to determine if their effects on the Ca2+ sensitivities of tension and ATPase activity coincide with the current paradigms and phenotypic outcomes. The G159D-CTnC decreases the Ca2+ sensitivity of tension and ATPase activation and reduces the maximal ATPase activity when incorporated into regulated actomyosin filaments. Under the same conditions, the L29Q-CTnC has no effect. Surprisingly, changes in the apparent G159D-CTnC Ca2+ affinity measured by tension in fibers do not occur in the isolated CTnC, and large changes measured in the isolated L29Q-CTnC do not manifest in the fiber. These counterintuitive findings are justified through a transition in Ca2+ affinity occurring at the level of cardiac troponin and higher, implying that the true effects of these mutations become apparent as the hierarchical level of the myofilament increases. Therefore, the contractile apparatus, representing a large cooperative machine, can provide the potential for a change (G159D) or no change (L29Q) in the Ca2+ regulation of contraction. In accordance with the clinical outcomes and current paradigms, the desensitization of myofilaments from G159D-CTnC is expected to weaken the contractile force of the myocardium, whereas the lack of myofilament changes from L29Q-CTnC may preserve diastolic and systolic function.     

Failure to detect changes in sarcolemma permeability in amphibian muscle following severe cellular damage

1. Isolated amphibian hearts and pectoris cutaneous muscles were exposed either to DNP or to caffeine, thereby producing severe myofilament damage. 2. No accompanying change in sarcolemma permeability was detected by monitoring either CK or LDH release or Procion yellow entry in the heart, or by Procion entry in amphibian skeletal muscle. 3. The findings are in contrast with mammalian cardiac and skeletal muscles, and confirm that the pathways leading to myofilament degradation and to the breakdown in sarcolemma organization are separate.     

Pharmacological activities of a toxic phospholipase A isolated from the venom of the snake Bothrops asper

A toxic phospholipase A was isolated from the venom of Bothrops asper. It induced skeletal muscle damage, anticoagulant effects and edema in the foot pad. The toxin had an intravenous LD50 of 95 micrograms/16-18 g mouse body wt and an intraventricular LD50 of 0.42 micrograms/16-18 g mouse body wt. Upon intramuscular and intravenous injections, the toxin induced a prominent increase in serum creatine kinase (CK) levels; only the CK-MM isozyme increased markedly. The toxin induced CK and creatine release from skeletal muscle incubated in vitro. The rate of efflux of creatine was higher than that of CK, although both markers were partially released as early as 15 min after incubation. The toxin also induced elevation of serum levels of lactic dehydrogenase isozymes. However, histological examination of skeletal muscle, kidneys, heart and lungs revealed cell damage only in skeletal muscle. The toxin was not cytotoxic to erythrocytes, lymphocytes or macrophages. In addition, it did not induce a mitogenic response on lymphocytes. In the absence of albumin in the medium, there was no significant difference between myotoxic activities in Ca2+-free and Ca2+-containing bathing solutions. However, when albumin was added, there was a significantly higher myotoxic effect in the presence of Ca2+. Thus, although phospholipolytic activity of the toxin plays a role in muscle damage when albumin is present, the toxin induces muscle damage even when phospholipase A activity is inhibited.     

Cytotoxic effect of phenazine methosulphate on the isolated frog heart

1. Perfusion of isolated frog hearts with phenazine methosulphate (PMS) at 0.3-1.0 mM caused a fall in amplitude and frequency of beat, and finally a cessation of contractile activity, together with widespread ultrastructural damage. 2. Sarcolemma blebs were a characteristic feature of the damage. 3. No protection was provided by mannitol (10-100 mM), superoxide dismutase, catalase or a pHo of 6.6. 4. Potassium ferricyanide (1-6 mM), an artificial electron acceptor, also caused ultrastructural damage. 5. Comparisons are made with the oxygen paradox of mammalian heart, and the possible role of Ca2+ fluxes and oxygen radicals in muscle damage are discussed.     

Ionizing radiation alters myofilament calcium sensitivity in vascular smooth muscle: potential role of protein kinase C

Radiation exposure increases vascular responsiveness, and this change involves endothelial damage, as well as direct effects on vascular smooth muscle. In this study, we tested the hypothesis that myofilament Ca(2+) sensitivity in vascular smooth muscle is increased from single whole body gamma irradiation (6 Gy). We measured contractile responses from intact and permeabilized rat thoracic aortic rings combined with cytosolic Ca(2+) ([Ca(2+)](i)) measurements. The sensitivity to KCl and phenylephrine increased significantly in tissues from animals on the 9th and 30th days postirradiation compared with control. Irradiation also significantly increased Ca(2+) sensitivity in beta-escin permeabilized smooth muscle on the 9th and 30th days postirradiation. Inhibitors of protein kinase C, chelerythrine, and staurosporine, had no effect on the pCa-tension curves in control permeabilized tissues but significantly decreased Ca(2+) sensitivity in permeabilized tissues on the 9th and 30th days postirradiation. Phorbol dibutyrate (PDBu, 10(-7) M) increased Ca(2+) sensitivity in control skinned smooth muscle but was without effect in irradiated vascular rings. Simultaneous measurement of contractile force and [Ca(2+)](i) showed that myofilament Ca(2+) sensitivity defined as the ratio of force change to [Ca(2+)](i) significantly increased following gamma-irradiation. PDBu (10(-6) M) stimulation of intact aorta produced a sustained contraction, while the increase in [Ca(2+)](i) was transient. In irradiated tissues, PDBu-induced contractions were greater than those seen in control tissues but there was no elevation in [Ca(2+)](i). Taken together, these data strongly support the hypothesis that irradiation increases the sensitivity of vascular smooth muscle myofilaments to Ca(2+) and this effect is dependent on activation of protein kinase C.     

Amphidinolide B, a powerful activator of actomyosin ATPase enhances skeletal muscle contraction

Amphidinolide B caused a concentration-dependent increase in the contractile force of skeletal muscle skinned fibers. The concentration-contractile response curve for external Ca2+ was shifted to the left in a parallel manner, suggesting an increase in Ca2+ sensitivity. Amphidinolide B stimulated the superprecipitation of natural actomyosin. The maximum response of natural actomyosin to Ca2+ in superprecipitation was enhanced by it. Amphidinolide B increased the ATPase activity of myofibrils and natural actomyosin. The ATPase activity of actomyosin reconstituted from actin and myosin was enhanced in a concentration-dependent manner in the presence or absence of troponin-tropomyosin complex. Ca2+-, K+-EDTA- or Mg2+-ATPase of myosin was not affected by amphidinolide B. These results suggest that amphidinolide B enhances an interaction of actin and myosin directly and increases Ca2+ sensitivity of the contractile apparatus mediated through troponin-tropomyosin system, resulting in an increase in the ATPase activity of actomyosin and thus enhances the contractile response of myofilament.     

State of the contractile apparatus during the development of a pathological process in the muscles. II. The effect of Ca2+ on the morphology of spreading necrosis caused by UV light]

Using phase-contrast technique and electron microscopy, a study was made of morphological changes of contractile system of striated muscle fibre during the spreading necrosis caused by ultraviolet light damage. It has been shown that the degree of manifestation of destructive changes in the contractile system depends upon Ca2+-ion concentration. The ultrastructural study of the damage region, under condition of muscle fibre stretching, made it possible to reveal the initial stages of formation of this pathological process. A possible contribution of intracellular membranous structures in spreading the destructive process along the muscle fibre is discussed.     

Cytosolic heparin inhibits muscarinic and alpha-adrenergic Ca2+ release in smooth muscle. Physiological role of inositol 1,4,5-trisphosphate in pharmacomechanical coupling

In order to test the physiological significance of inositol 1,4,5-trisphosphate (InsP3) in pharmacomechanical coupling, we have utilized two near-physiological systems, in which relatively high molecular weight solutes can be applied intracellularly and receptor coupling is retained: beta-escin permeabilization and reversible permeabilization. We showed that in smooth muscle permeabilized with beta-escin, one of the saponin esters, alpha 1-adrenergic (phenylephrine) and muscarinic (carbachol) agonists, as well as caffeine and InsP3, cause contractions mediated by Ca2+ release. These contractions were calmodulin-dependent and blocked by depletion of Ca2+ stored in the sarcoplasmic reticulum. Intracellular heparin (Mr = about 5000), a blocker of InsP3 binding to its receptor and a specific inhibitor of InsP3-induced Ca2+ release in smooth muscles, inhibited the responses to the agonists and to InsP3, but not those to caffeine, nor did it block the enhanced contractile response to cytoplasmic Ca2+ induced by agonists and by GTP gamma S. Neomycin blocked Ca2+ release induced by carbachol, but not by caffeine. In reversibly permeabilized ileum smooth muscle cells, loaded with Fura-2 acid and heparin, the intracellular heparin inhibited Ca2+ release and contractions induced by carbachol in Ca2+-free, high K+ solution. Heparin did not inhibit the high K+ contractions (with 1.2 mM Ca2+) and had no significant inhibitory effects on carbachol-induced responses in the presence of extracellular Ca2+. These results, obtained under near-physiological conditions, support the conclusion that InsP3 is the major physiological messenger of the Ca2+ release component of pharmacomechanical coupling, but not of the components mediated by Ca2+ influx or by potentiation of the contractile response to Ca2+.     

Morphology, ultrastructure and contractile properties of muscles responsible for superior tentacle movements of the snail

Bending, twitching and quivering are different types of tentacle movements observed during olfactory orientation of the snail. Three recently discovered special muscles, spanning along the length of superior tentacles from the tip to the base, seem to be responsible for the execution of these movements. In this study we have investigated the ultrastructure, contractile properties and protein composition of these muscles. Our ultrastructural studies show that smooth muscle fibers are loosely embedded in a collagen matrix and they are coupled with long sarcolemma protrusions. The muscle fibers apparently lack organized SR and transverse tubular system. Instead subsarcolemmal vesicles and mitochondria have been shown to be possible Ca2+ pools for contraction. It was shown that external Ca2+ is required for contraction elicited by high (40 mM) K+ or 10-4 M ACh. Caffeine (5 mM) induced contraction in Ca2+-free solution suggesting the presence of a substantial intracellular Ca2+ pool. High-resolution electrophoretic analysis of columellar and tentacular muscles did not reveal differences in major contractile proteins, such as actin, myosin and paramyosin. Differences were observed however in several bands representing presumably regulatory enzymes. It is concluded that, the ultrastructural, biochemical and contractile properties of the string muscles support their special physiological function.     

Comparative studies on the role of calcium in triggering subcellular damage in cardiac muscle

Isolated cardiac muscle strips from amphibians and mammals, together with isolated frog hearts, have been used as model systems for studying the action of elevated [Ca2+]i in promoting severe damage. A23187 and caffeine are believed to cause a rise in [Ca2+]i. Elevated [Ca2+]i causes characteristic damage which has been categorized and includes hypercontraction, Z-line damage and myofilament dissolution. The damage closely resembles that described in the isolated mammalian heart and in skeletal muscle preparations when [Ca2+]i is raised dramatically. Damage can therefore be triggered by releasing Ca2+ from intracellular sites, as distinct from increasing Ca2+ entry (as in the Ca2+-paradox). DNP and ruthenium red also cause identical damage and the results suggest that whilst the fall in pHi associated with ischaemia is probably the consequence of Ca2+/2H+ exchange at the mitochondria, coupled with ATP hydrolysis, lowered pHi by mitochondrial action is probably not the only cause of myofilament dissolution. Damage is not prevented by pretreatment with leupeptin, an inhibitor of Ca2+-activated neutral proteases, and it is concluded that the latter are probably not implicated in rapid and dramatic damage. The possible involvement of lysosomal enzymes in damage triggered by high [Ca2+]i is discussed.     

Calcium release in smooth muscle

In smooth muscle, maintenance of the contractile response is due to Ca2+ influx through two types of Ca2+ channel, a voltage-dependent Ca2+ channel and a receptor-linked Ca2+ channel. However, a more transient contraction can be obtained by release of Ca2+ from a cellular store, possibly the sarcoplasmic reticulum. In spike generating smooth muscle (e.g., guinea-pig taenia caeci), spike discharges may trigger the release of cellular Ca2+ by activating a Ca2+-induced Ca2+ release mechanism. Caffeine directly activates this mechanism in the absence of a triggered Ca2+ influx. In contrast to this, maintained depolarization may not only release but also refill the Ca2+ store. Drug-receptor interactions also release Ca2+ from a cellular store. This release may be elicited with inositol trisphosphate produced by receptor-linked phosphoinositide turnover. In non-spike generating smooth muscle (e.g., rabbit thoracic aorta), maintained membrane depolarization does not release but, instead, fills the Ca2+ store. However, caffeine and receptor-agonists release the Ca2+ store - possibly by activating the Ca2+-induced Ca2+ release mechanism and phosphoinositide turnover, respectively. The Ca2+ store in smooth muscle is filled by Ca2+ entry through voltage dependent Ca2+ channels and also by resting Ca2+ influx in the absence of receptor-agonists. The Ca2+ entering the cells through these pathways may be accumulated by the Ca2+ store and may activate the contractile filaments.     

A comparison of no-flow and low-flow ischemia in the rat heart: an energetic study

Heat production under no-flow ischemia (ISCH) and under hypoperfusion (HYP) conditions was measured in single isovolumetric contractions of perfused rat ventricles at 25 degrees C. Resting heat production (Hr) and resting pressure decreased when the perfusion rate was reduced from 6 to 1.5 mL min(-1) or lower flows (HYP) and by ISCH. Maximal developed pressure (P) decreased to 29% and 20% of control by HYP at 0.8 mL min(-1) and ISCH, respectively. The tension-independent heat (TIH) fraction attributed to Ca2+-binding, measured during single contractions, decreased under HYP with an increase in the ratio between the maximum relaxation rate and P (-P/P ratio). The TIH fractions (attributed to Ca2+ binding and Ca2+ removal processes) decreased under ISCH. The long duration TIH fraction associated with Ca2+-dependent mitochondrial activity disappeared at flow rates of 1.5 mL min(-1) or lower. The ratio between the tension-dependent energy release and P was decreased by ISCH but not by HYP, indicating that under ISCH there was an improvement in contractile economy, but this was not modified by HYP. Overall, the results indicate that no-flow and low-flow ischemias are energetically different models. While the contractile failure under HYP seems to be related to a decrease in myofilament Ca2+ sensitivity, under ISCH it appears to be related to decreased cytosolic Ca2+ availability combined with a more noticeable effect on a fraction of energy that has been attributed to mitochondrial activity. Furthermore, mechanical and energetic responses of both models (i.e., ISCH and HYP) found in the present work were not the same as those previously observed in severe hypoxia so that all these models should not be used indistinctly.     

Cytotoxicity of phenazine methosulphate on skeletal muscle. The role of the sarcoplasmic reticulum in initiating myofilament damage

Phenazine methosulphate (PMS) or ferricyanide caused ultrastructural damage, including sarcolemma folds and swelling of the sarcoplasmic reticulum (SR), in amphibian skeletal muscle which corresponds with that triggered by a rise in [Ca]i and which, it is suggested, is caused by the activation of NAD(P)H oxidases at the sarcolemma (where it causes sarcolemma folding) and SR (where it causes myofilament damage). PMS also caused SR swelling and more limited damage in chemically-skinned muscle at zero [Ca]. In contrast with the oxygen paradox of cardiac muscle, there is no evidence for the production of oxygen radicals since no protection was provided by N2, mannitol, desferrioxamine or alpha-tocopherol, nor was the cell damage produced by an influx of Ca across the sarcolemma.     

Nitric oxide as the regulator of intracellular homeostasis in the uterus myocytes

The published data on the mechanisms and regulation of active and passive Ca2+ transport in the myometrium have been analyzed. Particular attention is paid to the cGMP-dependent and independent pathways of action of nitric oxide or its derivatives on intracellular Ca2+ homeostasis of uterine smooth muscle and its contractile activity. Information on the effect of nitric oxide on Ca2+ -transport systems of other types of smooth muscles is provided in a comparative aspect. Based on own experimental results and literature data a scheme of NO action in the myometrium is suggested in which nitric oxide or its derivatives cause Ca2+ -dependent polarization of the sarcolemma. In accordance with our results, this effect may be based on the increase of sarcolemma Ca2+ permeability under the influence of NO or its derivatives and the stimulation of at least the initial passive transport of the cation in the myocytes mediated by dihydropyridine-sensitive channels. Additional factors that contribute to the polarization of the membrane are the increase of protons transport from the muscle cells and stimulation of Na+, K+ -ATPase. Acting on the sarcoplasmic reticulum, nitrosactive compounds activate the inclusion of calcium in this compartment and inhibit Ca2+ -induced release of the cation. The latter effects are able to provide compensation for NO-induced Ca2+ increase in myocytes and supress the electromechanical coupling at Ca2+ release from the reticulum. NO-derivates also inhibit a key link in the smooth muscle contractile act--the formation of the Ca2+ -calmodulin complex.