Inorganic phosphate promotes relaxation of chemically skinned smooth muscle of guinea-pig Taenia coli

Mechanism of catch force: tethering of thick and thin filaments by twitchin

Catch is a mechanical state occurring in some invertebrate smooth muscles characterized by high force maintenance and resistance to stretch during extremely slow relaxation. During catch, intracellular calcium is near basal concentration and myosin crossbridge cyctng rate is extremely slow. Catch force is relaxed by a protein kinase A-mediated phosphorylation of sites near the N- and C- temini of the minititin twitchin (~526 kDa). Some catch force maintenance car also occur together with cycling myosin crossbridges at submaximal calcium concentrations, but not when the muscle is maximally activated. Additionally, the link responsible for catch can adjust during shortening of submaximally activated muscles and maintain catch force at the new shorter length. Twitchin binds to both thick and thin filaments, and the thin filament binding shown by both the N- and Cterminal portions of twitchin is decreased by phosphorylation of the sites that regulate catch. The data suggest that the twitchin molecule itself is the catch force beanng tether between thick and thin filaments. We present a model for the regulation of catch in which the twitchin tether can be displaced from thin filaments by both (a) the phosphorylation of twitchin and (b) the attachment of high force myosin crossbridges.

Molecular basis of the catch state in molluscan smooth muscles: a catchy challenge

The catch state (or 'catch') of molluscan smooth muscles is a passive holding state that occurs after cessation of stimulation. During catch, force and, in particular, resistance to stretch are maintained for long time periods with low (or no) energy consumption at basal intracellular free [Ca2+]. The catch state is initiated by Ca2+-stimulated dephosphorylation of the titin-like protein twitchin and is inhibited by cAMP-dependent phosphorylation of twitchin. In addition, catch is pH sensitive, but the reason for this is unknown. According to a traditional model, catch is due to slower cross-bridge cycles where myosin heads remain longer attached to the actin filaments after force generation, possibly caused by a hindered release of ADP from the myosin heads. However, this model was disproved by recent findings which showed that (i) inhibitors of myosin function, such as vanadate, do not affect catch force; (ii) factors which terminate the catch state do not accelerate myosin head detachment kinetics and (iii) a catch-like high resistance to stretch is still inducible when force development is prevented. Thus, catch probably involves passive linkage structures interconnecting the myofilaments (catch linkages). For example twitchin could (i) tie myosin heads to the thin filaments, (ii) mechanically lock them in a stretch resistant state or (iii) interconnect thick and thin filaments directly. However, it is questionable if these mechanisms are sufficient since twitchin seems to be about 15-times less abundant than myosin. Therefore, in addition, interconnections between thick filaments could exist, which could involve e.g. paramyosin or twitchin. Catch could even involve changes in the compliance of thick filaments. The function of myorod, found specifically in catch muscles in equal abundance with myosin, is not known. The suggestion is made here that catch linkages are present already during active contraction either as ratchet-like elements resisting stretch and not opposing shortening or in some kind of 'standby' mode ready to transform suddenly into the working mode by stretches or after Ca2+ removal following cessation of stimulation.

Cross-bridge cycling in smooth muscle: a short review

This review is focused on the cross-bridge interaction of the organized contractile system of smooth muscle fibres. By using chemically skinned preparations the different enzymatic reactions of actin-myosin interaction have been associated with mechanical events. A rigor state has been identified in smooth muscle and the binding of ATP causes dissociation of rigor cross-bridges at rates slightly slower than those in skeletal muscle, but fast enough not to be rate-limiting for cross-bridge turn over in the muscle fibre. The release of inorganic phosphate (Pi) is associated with force generation, and this process is not rate-limiting for maximal shortening velocity (Vmax) in the fully activated muscle. The binding of ADP to myosin is strong in the smooth muscle contractile system, a property that might be associated with the generally slow cross-bridge turn over. Both force and Vmax are modulated by the extent of myosin light chain phosphorylation. Low levels of activation are considered to be associated with the recruitment of slowly cycling dephosphorylated cross-bridges which reduces shortening velocity. The attachment of these cross-bridge states in skinned smooth muscles can be regulated by cooperative mechanisms and thin filament associated systems. Smooth muscles exhibit a large diversity in their Vmax and the individual smooth muscle tissue can alter its Vmax under physiological conditions. The diversity and the long-term modulation of phenotype are associated with changes in myosin heavy and light chain isoform expression.

Effects of hypoxia on [Ca2+]i, pHi and myosin light chain phosphorylation in guinea-pig taenia caeci

1. Hypoxia (achieved by bubbling with N2 instead of O2) reduces the force of a KCl (40 mM)-induced contracture to approximately 10% of the control value in guinea-pig taenia caeci. The underlying mechanism of this relaxation in response to hypoxia was investigated by measuring the major cell signalling parameters, intracellular Ca2+ concentration ([Ca2+]i) and myosin regulatory light chain (LC20) phosphorylation (MLC-P1), as well as intracellular pH (pHi), a factor often suggested to mediate hypoxic relaxation of muscle. 2. [Ca2+]i, measured using the ratiometric fluorescent dye fura-2, increased when 40 mM KCl was added to physiological saline solution (PSS) (peak value assigned 100%), and the steady state after 15 min was 92.8%. There were no detectable decreases in [Ca2+]i during hypoxia. 3. MLC-Pi, measured using isoelectric focusing-polyacrylamide gel electrophoresis and identified using Western blotting, increased from 9% of the total LC20 in Ca(2+)-free PSS to a peak value of 51% in 40 mM KCl-PSS. The steady-state value in hypoxia of 43% was not significantly different from that in control oxygenated conditions at the same point in time. 4. pHi, measured using the ratiometric fluorescent dye 2',7'-bis(carboxyethyl)-5(6)-carboxy-fluorescein (BCECF), under quiescent conditions (Ca(2+)-free PSS) was 7.23 and increased to 7.36 with 40 mM KCl. After imposition of hypoxia pHi remained unchanged despite the known increase in both lactate content and production. 5. As [Ca2+]i and MLC-Pi, key factors in activation, were not decreased by hypoxia and changes in pHi were minor, hypoxic relaxation in guinea-pig taenia caeci appears to be directly related to energy limitation rather than any oxygen-sensing mechanism.

Effects of inorganic phosphate on cross-bridge kinetics at different activation levels in skinned guinea-pig smooth muscle

1. The effects of inorganic phosphate (P(i)) on force, Ca(2+)-force relationship, ATPase activity, maximal shortening velocity (Vmax) and rate of tension development were investigated in chemically skinned preparations of smooth muscle from the guinea-pig taenia coli. 2. In maximally thiophosphorylated fibres, P(i) in the range 1-40 mM inhibited isometric force, with a reduction of 20% at 20 mM P(i). 3. The relative force was similar at all [Ca2+], i.e. the Ca(2+)-force relationship was not affected, when 20 mM P(i) was present. 4. After photolytic release of ATP from caged ATP in maximally thiophosphorylated fibres in the presence of 20 mM P(i), tension rose to a lower level but with a higher rate constant than in the absence of P(i). 5. Inorganic phosphate (20 mM) did not affect the ATP hydrolysis in fibres activated at intermediate [Ca2+] or by maximal thiophosphorylation. 6. Inorganic phosphate (20 mM) decreased force but did not influence Vmax in maximally activated fibres. At lower levels of activation by Ca2+, P(i) increased the Vmax and decreased force slightly without affecting the degree of myosin light chain phosphorylation. 7. We conclude that P(i) influences cross-bridge reactions associated with force generation in smooth muscle. These reactions are not rate limiting for cross-bridge turnover under isotonic or isometric conditions in maximally activated smooth muscle fibres, since P(i) did not influence Vmax or the rate of ATP turnover. 8. Since P(i) increased Vmax in submaximally activated muscles, we propose that, under these conditions, shortening velocity is rate limited by cross-bridge states, reached early after attachment, which impose a mechanical resistance to shortening.

Flash photolysis studies of relaxation and cross-bridge detachment: higher sensitivity of tonic than phasic smooth muscle to MgADP

The effects of MgADP and inorganic phosphate (Pi) on cross-bridge detachment were determined in tonic (rabbit femoral artery) and phasic (rabbit bladder and guinea pig portal vein) smooth muscles permeabilized with staphylococcal alpha-toxin. Relaxation from rigor was induced by photolysis of ATP (1.2-1.5 mM) from caged ATP. The initial one second of relaxation from rigor was resolved into two exponential components: a rapid component with normalized amplitudes, Af, of 8, 15 and 26% and rate constants, kf (in s-1) of 26, 36 and 30 in rabbit femoral artery, guinea pig portal vein, and rabbit bladder; the respective rate constants of the second, slower component, ks, were 0.07, 0.2 and 0.1. Removal of residual endogenous ADP with apyrase treatment increased the amplitude Af and accelerated ks; addition of MgADP reduced Af. The combination of these effects (increases in Af and ks) decreased the t1/2 of relaxation from control values by factors of 2.6 (femoral artery), 6.7 (portal vein) and 10 (bladder). Pi (30 mM) further increased the amplitudes Af. The affinity of MgADP for myosin cross-bridges, estimated as the reduction of the relative amplitude of the rapid component, Af, was significantly higher in tonic than in phasic smooth muscle: the KD of MgADP was 1.1 +/- 0.3 microM in rabbit femoral artery and 4.9 +/- 1.0 microM in rabbit bladder. The higher affinity of tonic smooth muscle myosin for MgADP correlated with its relatively high LC17b isoform content (58 +/- 4.2%) in contrast to the lower affinity of the phasic, bladder detrusor smooth muscle that contained only the LC17a isoform.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of pH and inorganic phosphate on force production in alpha-toxin-permeabilized isolated rat uterine smooth muscle

1. Strips of longitudinal smooth muscle isolated from rat uterus were permeabilized using crude alpha-toxin from the bacterium Staphylococcus aureus. This treatment rendered the surface membrane permeable to small molecular weight substances. Simultaneous measurements of tension and calcium concentration ([Ca2+]) (using indo-1 fluorescence) were used to investigate the effects of pH and inorganic phosphate concentration ([Pi]) on Ca(2+)-activated force generated by the contractile proteins. 2. Raising the [Pi] from 1 to 11 mM at a pH of 7.2 depressed both maximal and submaximal Ca(2+)-activated force. This effect of Pi was concentration dependent having the majority of its effect by 6 mM. 3. Further experiments at a submaximal [Ca2+] showed that Ca(2+)-activated force was enhanced by raising [Pi] from 6 to 11 mM suggesting that Pi increased the Ca2+ sensitivity of tension production. Based on these results, calculations indicate that the apparent affinity constant of Ca2+ for the contractile proteins increased from 4 x 10(6) M-1 to 6 x 10(6) M-1 on raising [Pi] from 1 to 11 mM. 4. Lowering pH from 7.2 to 6.7 at a [Pi] of 1 mM potentiated Ca(2+)-activated force with a small depression in the apparent Ca2+ sensitivity of tension production. This effect of pH on maximum (100 microM Ca2+) and submaximum (0.3 microM Ca2+) Ca(2+)-activated force was observed over a range of acidic pHs (7.0-6.7). 5. Increasing pH from 7.2 to 7.7 at a [Pi] of 1 mM depressed Ca(2+)-activated force with no effect on Ca2+ sensitivity of tension production. 6. Spontaneous contractions in intact rat myometrium are abolished under hypoxic conditions. Under these same conditions intracellular [Pi] rises and pH falls. The results of this study suggest that taken individually neither the effect of a rise in [Pi] nor a fall in pH on Ca(2+)-activated force generated by the contractile proteins can account for the effect of hypoxia on spontaneous contractions.

Tension responses of sheep aorta to simultaneous decreases in phosphocreatine, inorganic phosphate and ATP

1. Tension responses of sheep aortae were investigated when different substrates were included in the superfusing medium. The magnitude of tension development was similar whether or not 5 mM glucose was present in the medium. However, the rate of tension development was greater in the absence of glucose. 2. When 5 mM 2-deoxyglucose (2DG) was present in the medium, the magnitude of tension generation was 1.6 times that in the absence of exogenous substrate. A second sequential contraction with 2DG generated tension 1.25 times that in the absence of exogenous substrate. The rate of tension development during the first contraction in the presence of 2DG was similar to that in the absence of substrate. However, the second contraction in the presence of 2DG had a substantially increased rate of tension development. 3. 31P nuclear magnetic resonance (NMR) spectroscopy revealed that, at resting tone, in the presence of 2DG, inorganic phosphate (P(i)) and phosphocreatine (PCr) simultaneously decrease while 2-deoxyglucose-6-phosphate accumulates. During contraction-relaxation cycles, in the presence of 2DG, P(i) and PCr become undetectable while ATP declines to approximately 50% of control values as determined by NMR. During the second contraction in the presence of 2DG, the area of the ADP resonance was similar to that of the alpha-ATP resonance. 4. The increase in the magnitude of tension generation, during 2DG administration, correlated with a decrease in P(i) levels. The rate of relaxation from a contraction, in the presence of 2DG, was slower than in the presence of glucose or in the absence of exogenous substrate. These results are consistent with the role of P(i) in the release of the proposed 'latch-bridge' state of maintained contraction at low energy demand. 5. The increase in isometric tension generation during contraction in the presence of 2DG appears to be related to the decreased levels of P(i). In the presence of 2DG, the reduction of PCr and of ATP occur to a similar extent to that during hypoxia, yet no inhibition of force takes place. The low levels of ATP and PCr reported with 2DG administration in these studies do not energetically limit the contractile apparatus.

Myosin-product complex in the resting state and during relaxation of smooth muscle

Previous findings suggested that in resting smooth muscle ADP is bound to myosin and that phosphorylation of the myosin, and its subsequent interaction with actin, increases the rate of ADP release. We have now extended these studies to include measurements of bound Pi as well as bound ADP in permeabilized rabbit portal vein. We report that in resting smooth muscle that has been exposed to [3H]ATP and [gamma-32P]ATP, followed by a chase in an unlabeled relaxing solution, the ratio of bound [3H]ADP to bound [32P]Pi is close to unity, and both are released at approximately the same rate. This suggests that myosin exists predominantly with both ADP and Pi bound under resting conditions and that the release of one is quickly followed by the release of the other. In contrast, there is a significant 30% excess of bound Pi over ADP in a muscle during relaxation from an isometric contraction. Under these conditions, while force output is slowly decreasing, both light chain phosphorylation and adenosinetriphosphatase (ATPase) activity have decreased to near-resting values. The time course of relaxation is similar to the time course of Pi release from both the resting and relaxing muscle. We propose that during relaxation the dephosphorylated cross bridges which are bearing force have Pi but not ADP bound and that detachment of the cross bridge (and thus force decay) is limited by Pi release from myosin which occurs at the same rate as in the resting muscle.

Effects of hypoxia on high-energy phosphagen content, energy metabolism and isometric force in guinea-pig taenia caeci

1. Previously, Ishida, Takagi & Urakawa (1984) reported that, in the presence of high K+ (45.4 mM) under hypoxia (95% N2-5% CO2 bubbling), tension and ATP content of the smooth muscle of the guinea-pig taenia caeci increased concomitantly when the glucose concentration was raised. Tension and energy metabolism of the taenia seemed to be closely correlated. In the present experiments, we investigated the metabolic changes during the relaxation phase after the taenia was exposed to hypoxia in the presence of high K+ by measuring the content of phosphagen, inorganic phosphate (Pi) and lactate in the tissue. Oxygen consumption and lactate release from the tissue were also determined to estimate the rate of ATP synthesis. 2. Under hypoxic conditions, high-K(+)-induced tension decreased to one-tenth of maximum in aerobic conditions (95% O2-5% CO2 bubbling); increasing the calcium concentration from 2.5 to 10 mM had no effect. To test receptor-associated stimuli, carbachol or histamine (both 5 microM) had little effect on tension in hypoxia. The calcium ionophore A23187 (10 microM) also did not produce any significant contraction in the presence of high K+ under hypoxia. 3. Hypoxia in the presence of high K+ elicited a rapid decrease in phosphocreatine, approximately in parallel with the decrease in tension. The ATP content decreased gradually while Pi content increased. Lactate content increased rapidly and then partially decreased. 4. The rate of ATP synthesis estimated from the oxygen consumption and lactate release of the taenia was linearly correlated with tension development under aerobic and hypoxic conditions. 5. These results suggest that the decrease in tension of the taenia observed under hypoxic conditions is due to an inhibition of energy metabolism, and not due to an oxygen-sensing step in excitation-contraction coupling.

Metabolism and force in hypertrophic smooth muscle from rat urinary bladder

Ten days of urinary outlet obstruction in the rat induced a threefold increase in bladder weight. Active force of control and hypertrophic bladder muscle strips was measured at varying PO2 levels after high-K+, carbachol, or electrical field stimulation. Highest force output was obtained with carbachol. Force per muscle area was lower in the hypertrophic muscles. The basal rates of oxygen consumption and lactate formation were similar in the two groups. The metabolic tension cost (ATP turnover/active force) was similar in the two groups for activation with high K+ and carbachol. In anoxia the active force decreased, but this was less pronounced in the hypertrophied muscle. Hypertrophied muscle could, in contrast to the controls, maintain a sustained K+ contracture in anoxia. Basal metabolic rates and tension cost were markedly reduced in anoxia for both groups. The lower force per area with unaltered tension cost, in hypertrophic muscles under all experimental conditions, may reflect unaltered intrinsic properties of the contractile system, although the amount of contractile material has decreased relative to cell volume. The increased resistance to anoxia may reflect a metabolic adaptation to impaired oxygen supply to the hypertrophied tissue.

Influence of pH on isometric force development and relaxation in skinned vascular smooth muscle

The effects of pH (from pH values 6.50-7.10) on isometric tension development and relaxation were investigated in Triton X-100 "skinned" rat caudal artery. Helically cut skinned strips contracted in 21 microM Ca2+ were studied with respect to maximal isometric tension (Po) and rate of contraction (T0.5 C), and following relaxation in 18 nm Ca2+, the rate of relaxation (T0.5 R). Acidic pH (pH 6.50) decreased Po to 87% of isometric force obtained at pH 6.90, and increased the rate of contraction as shown by a decrease of T0.5 C to 80%. In contrast, T0.5 R increased 4.5-fold, indicating that with a change of only 0.40 pH units, relaxation rates were dramatically decreased. pCa-tension curves at pH values 6.50, 6.70, 6.90 and 7.10 indicated no significant shift in half maximal activation (pCa50) between pH 6.50 and 6.70, but a significant (P less than 0.01) shift in pCa50 between pH 6.70 [( Ca2+] = 0.46 microM) and pH 7.10 [( Ca2+] = 0.87 microM). Compared to contractions at pH 6.90, myosin light chain (LC20) phosphorylation at pH 6.50 was significantly greater at 30 and 60 s into contraction but not significantly different at 3-10 min. At both pH 6.50 and 6.90, dephosphorylation was rapid and substantially preceded relaxation; LC20 dephosphorylation and relaxation occurred more rapidly at pH 6.90 than at 6.50. At pH 6.50 and 6.90, relax solutions made with increased Ca2+ buffering capacity showed no effect in enhancing T0.5 R, suggesting the difference between relaxation rates was not due to Ca2+ diffusion limitations from the skinned strip.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of okadaic acid on isometric tension and myosin phosphorylation of chemically skinned guinea-pig taenia coli

1. In guinea-pig taenia coli skinned with Triton X-100, the marine sponge toxin okadaic acid (OA; 0.1-10 microM) produced a dose-dependent enhancement of isometric tension in the presence of low concentrations (0.1-1 microM) of Ca2+. 2. The Ca2+-tension relation of the skinned taenia showed a high co-operativity (Hill coefficient, h = 5) in the presence of 0.2 microM-calmodulin. The concentration of Ca2+ required to obtain half-maximal tension (ED50) was 1.8 microM. OA (5 microM) reduced the co-operativity (h = 2.3) and increased the Ca2+ sensitivity (ED50 = 0.92 microM-Ca2+). OA further increased the tension produced with 30 microM-Ca2+, while it failed to produce any mechanical effect in Ca2+-free solution. When the calmodulin concentration was increased the Ca2+ sensitivity increased as well, but the co-operativity was not affected both in the absence and in the presence of OA. 3. The level of myosin phosphorylation was analysed by two-dimensional gel electrophoresis. OA produced an increase in phosphorylated light chains and a concomitant decrease in unphosphorylated light chains. The effect was completely reversed when OA was washed out. 4. In solutions containing more than 1 microM-Ca2+, a third protein band appeared on the gels next to the bands of light chains. OA markedly increased the third band which disappeared when OA and Ca2+ were simultaneously removed. 5. OA reversibly slowed down both relaxation and dephosphorylation induced by Ca2+ removal following activation with 30 microM-Ca2+. Complete relaxation did not occur in the presence of more than 1 microM-OA. The concentration of OA required to produce a 50% reduction (ID50) of the relaxation rate was 78 nM. 6. The phosphatase activity in the taenia extract was inhibited by OA (1-10 microM) in a dose-dependent manner. The inhibition was well described as a mixed noncompetitive inhibition, and the dose-inhibition relation was shifted to the right when the concentration of substrate (phosphorylated light chains) was increased. The lower and upper limits of the change of ID50 produced by changing the substrate concentration were estimated to be 10 and 165 nM-OA, respectively. 7. These results strongly suggest that the tension enhancement and the slow-down of relaxation are both causally related to inhibition of myosin phosphatase activity by OA.

Cross-bridge kinetics, cooperativity, and negatively strained cross-bridges in vertebrate smooth muscle. A laser-flash photolysis study

The effects of laser-flash photolytic release of ATP from caged ATP [P3-1(2-nitrophenyl)ethyladenosine-5'-triphosphate] on stiffness and tension transients were studied in permeabilized guinea pig protal vein smooth muscle. During rigor, induced by removing ATP from the relaxed or contracting muscles, stiffness was greater than in relaxed muscle, and electron microscopy showed cross-bridges attached to actin filaments at an approximately 45 degree angle. In the absence of Ca2+, liberation of ATP (0.1-1 mM) into muscles in rigor caused relaxation, with kinetics indicating cooperative reattachment of some cross-bridges. Inorganic phosphate (Pi; 20 mM) accelerated relaxation. A rapid phase of force development, accompanied by a decline in stiffness and unaffected by 20 mM Pi, was observed upon liberation of ATP in muscles that were released by 0.5-1.0% just before the laser pulse. This force increment observed upon detachment suggests that the cross-bridges can bear a negative tension. The second-order rate constant for detachment of rigor cross-bridges by ATP, in the absence of Ca2+, was estimated to be 0.1-2.5 X 10(5) M-1s-1, which indicates that this reaction is too fast to limit the rate of ATP hydrolysis during physiological contractions. In the presence of Ca2+, force development occurred at a rate (0.4 s-1) similar to that of intact, electrically stimulated tissue. The rate of force development was an order of magnitude faster in muscles that had been thiophosphorylated with ATP gamma S before the photochemical liberation of ATP, which indicates that under physiological conditions, in non-thiophosphorylated muscles, light-chain phosphorylation, rather than intrinsic properties of the actomyosin cross-bridges, limits the rate of force development. The release of micromolar ATP or CTP from caged ATP or caged CTP caused force development of up to 40% of maximal active tension in the absence of Ca2+, consistent with cooperative attachment of cross-bridges. Cooperative reattachment of dephosphorylated cross-bridges may contribute to force maintenance at low energy cost and low cross-bridge cycling rates in smooth muscle.

Smooth muscle intracellular pH: measurement, regulation, and function

Smooth muscle performs many functions that are essential for the normal working of the human body. Changes in pH are thought to affect many aspects of smooth muscle. Despite this, until recently little was known about either intracellular pH (pHi) values or pHi regulation in smooth muscle. Recent work measuring pHi with either microelectrodes or nuclear magnetic resonance spectroscopy is now providing some of this much needed information for smooth muscles. From these studies, it can be concluded tentatively that pHi is the same in different smooth muscles, approximately 7.06 (37 degrees C). This value is very close to those obtained in cardiac and skeletal muscle. It is clear that H+ is not in equilibrium across the smooth muscle membrane; i.e., pHi is regulated. Preliminary results in smooth muscle suggest that certain aspects of this regulation are different from that described for other muscle types. Changes in pHi have been found to produce marked effects on contraction in smooth muscle. Of particular interest is the fact that, unlike striated muscles, some smooth muscles can product more force during an intracellular acidification.

P1- and P2-purine receptors in brain circulation

As there is increasing evidence that purines are involved in cerebral vasodilatation, purine receptors were characterized pharmacologically in pial arteries from rabbit, cat and man, and compared with purine receptors in various non-cerebral vascular beds. Use of agonists and antagonists allows purine receptors to be divided into P1 and P2 receptors. The location of these receptors on the surface of the various cells in the cerebrovascular wall was determined. P1 receptors, stimulated primarily by adenosine, are located on the smooth muscle cells, causing relaxation. P2 receptors, stimulated primarily by ADP and ATP, are located on endothelial cells and cause relaxation, and are also found on smooth muscle cells where they cause contraction. Slowly degradable ATP analogues were considerably more potent constrictors than ATP but not equally potent as dilators, which might indicate minor differences between the P2 receptors on smooth muscle and on endothelium. The P1 receptor was further subclassified into the A2 type. Adenosine may also reach an intracellular purine receptor, via an uptake process, to cause additional relaxation, as was shown with an intracellular purine receptor agonist in cat pial arteries. Brain vessels were found to be considerably more sensitive to purines than most non-cerebral vessels studied.

Influence of some variables in the Triton X-100 method of skinning the plasmalemmal membrane from guinea pig trachealis muscle

Electron microscopy showed that the process of skinning guinea pig trachaelis muscle using Triton X-100 destroys the plasma membranes and causes some retraction of the myofibrils toward the center of the cells. Seven days represents an optimal period for the low temperature storage of the skinned fibers. Increasing the imposed tension from 0.25 to 1.0 g reduces the size of the maximal response to Ca2+. Some sensitization of the skinned fibers to Ca2+ follows the construction of the initial Ca2+ log concentration-response curve. Exogenous calmodulin is not essential for Ca2+-induced tension development and, at 20 degrees C, does not potentiate Ca2+. Useful economy can be achieved without compromising Ca2+ sensitivity or responsiveness by using ATP (a component of the relaxing solution) of purity 99% rather than of 99-100%.

Actin polymerization and ATP hydrolysis

F-actin is the major component of muscle thin filaments and, more generally, of the microfilaments of the dynamic, multifunctional cytoskeletal systems of nonmuscle eukaryotic cells. Polymeric F-actin is formed by reversible noncovalent self-association of monomeric G-actin. To understand the dynamics of microfilament systems in cells, the dynamics of polymerization of pure actin must be understood. The following model has emerged from recent work. During the polymerization process, adenosine 5'-triphosphate (ATP) that is bound to G-actin is hydrolyzed to adenosine 5'-diphosphate (ADP) that is bound to F-actin. The hydrolysis reaction occurs on the F-actin subsequent to the polymerization reaction in two steps: cleavage of ATP followed by the slower release of inorganic phosphate (Pi). As a result, at high rates of filament growth a transient cap of ATP-actin subunits exists at the ends of elongating filaments, and at steady state a stabilizing cap of ADP.Pi-actin subunits exists at the barbed ends of filaments. Cleavage of ATP results in a highly stable filament with bound ADP.Pi, and release of Pi destabilizes the filament. Thus these two steps of the hydrolytic reaction provide potential mechanisms for regulating the monomer-polymer transition.

Relaxation of chemically skinned guinea pig taenia coli smooth muscle from rigor by photolytic release of adenosine-5'-triphosphate

The mechanical events following release of ATP from P3-1-(2-nitro)phenylethyladenosine-5'-triphosphate (caged-ATP) in skinned guinea pig taenia coli smooth muscle in rigor were investigated. A rigor force of about 25-35% of the maximal active force was obtained by removing ATP at the plateau of a maximal active contraction. In the rigor solution free-Mg2+ was 2 mM, ionic strength 90 mM and pH 7.0. When caged-ATP (12.5 mM) was diffused into the preparation there was no change in the rigor force. Photolytic production of about 2 mM ATP was achieved with a xenon flash lamp. Following illumination, force decreased with an approximate initial rate constant of 0.7 s-1. The rate of relaxation was increased in the presence of inorganic phosphate (at 3 mM: 1.3 s-1; 10 mM: 2.2 s-1). At higher Mg2+ concentrations the rate of relaxation was slower (5 mM: 0.2 s-1) and at lower concentrations the rate was faster (0.5 mM: 1.2 s-1). An increased rate of relaxation was observed when ionic strength was increased to 150 mM (2.2 s-1). Phosphate increased the rate of relaxation at the different levels of Mg2+ (0.5-10 mM) and ionic strength (90, 150 mM). In preparations shortened (by 1-3%) to give reduced rigor force, a small transient increase in tension was recorded after ATP release. In comparison to the rates of ATP-induced dissociation of actomyosin in solution, reported in the literature, the rate of relaxation from rigor is slower. This may reflect a slow rigor cross-bridge dissociation or mechanical interactions not associated with cross-bridges in the muscle fibre.(ABSTRACT TRUNCATED AT 250 WORDS)

Structural and biochemical analysis of skinned smooth muscle preparations

This paper describes a biochemical and immunocytochemical analysis of smooth muscle strips that were chemically skinned and subjected to contraction and relaxation cycles according to procedures commonly employed in current skinned smooth muscle work. The fate of four major proteins, myosin, filamin, caldesmon and actin, was followed with respect to the proportionate loss of these proteins to the bathing medium as well as to their structural redistribution within the cells in the muscle strips. Large losses (of the order of 50%) of both myosin and filamin occurred at the skinning step, using either Triton X-100 or Saponin as the detergent; losses of actin were up to 30% with Triton X-100 and around 15% with Saponin. Losses of caldesmon were difficult to assess due to the rapid degradation of this protein in the bathing medium. Subsequent cycles of contraction and relaxation resulted in accumulated loss, notably of myosin and filamin, so that after the third contraction as little as 20% and 40% respectively of the original complement of these proteins remained in the muscle strips. These changes in protein composition were accompanied by a drastic redistribution of the proteins in the muscle cells. Most marked were the changes seen with myosin, significant amounts of this protein being already found in the connective tissue space after the first relaxation. These findings point to the need for a careful reappraisal of the conditions currently used in skinned smooth muscle research.

Effect of inorganic phosphate on the Ca2+ sensitivity in skinned Taenia coli smooth muscle fibers. Comparison of tension, ATPase activity, and phosphorylation of the regulatory myosin light chains

Inorganic phosphate (Pi) decreases maximal tension in contracted skeletal and heart muscle fibers. We investigated the effects of 10 mM Pi on the force-calcium relationship in Triton X-100-skinned Taenia coli smooth muscle fibers. Isometric force measurements show that the calcium sensitivity of the force depends on the phosphate concentration. Furthermore 10 mM Pi relaxes the fibers more at intermediate than at high calcium ion concentrations: At pCa 4.5 tension decreases in the presence of 10 mM Pi by approximately 12% but it decreases 70% at pCa 6.17. Removal of phosphate partially reverses the relaxation. Simultaneous determination of actomyosin ATPase activity and force (Güth, K., and J. Junge, 1982, Nature (Lond.), 300:775-776) shows that the ATPase activity does not correlate with the changes in force. In the presence of Pi, tension decreases more than the ATPase activity. The level of phosphorylation of the 20,000-D regulatory myosin light chain is not changed in the presence or absence of 10 mM Pi. The results are discussed in terms of slowly or noncycling myosin crossbridges formed at lower calcium concentrations, which contribute to the force development but not to the ATPase activity. These crossbridges are considered to be dissociated in the presence of phosphate.

Inorganic phosphate regulates the contraction-relaxation cycle in skinned muscles of the rabbit mesenteric artery

The effects were investigated of inorganic phosphate (Pi) on the Ca2+-dependent and Ca2+-independent contractions evoked in chemically skinned smooth muscles of the rabbit mesenteric artery. The relation between the concentration of Ca2+ and tension showed a sigmoidal curve in the range of pCa 7-5.5. Pi (over 1 mM) inhibited the Ca2+-induced contraction, shifted the pCa-tension curve to the right and increased the Hill number from 2 to 3. Calmodulin did not change the Hill number and attenuated the inhibitory action of Pi as estimated from the shift of the curve, but this agent did not modify the increased Hill number in the presence of Pi. Pi consistently inhibited the Ca2+-independent contractions provoked by application of trypsin-treated myosin light chain kinase, of MgATP following adenosine-5'-o-(3-thiotriphosphate) (ATP gamma S) and Ca2+, and of a solution containing high Mg2+. These inhibitory actions of Pi were inversely proportional to the amplitude of the contraction. When Pi was applied simultaneously with ATP gamma S and Ca2+, there was no change in the amplitude of Ca2+-independent contractions provoked by the application of MgATP. The amplitude of the rigor contraction evoked by ATP-free solution was less than 7% of that of the 10 microM-Ca2+-induced contraction. When ATP was removed from the solution during the Ca2+ contraction, the rigor contraction was also generated. Pi did not inhibit either type of contraction. With a decrease in the concentration of Ca2+ from 10 microM to below 1 nM, the tissue relaxed at a slower rate than the rate of rise of the Ca2+-induced contraction. The slow relaxation was not modified by a change in the concentration of EGTA or addition of 1 microM-calmodulin. Pi reduced, and high Mg2+ prolonged the time required for the relaxation. This action of Pi was not prevented in the presence of calmodulin or of high Mg2+. The rates of rise and fall of the Ca2+-induced contraction depended on the concentration of MgATP, and Pi consistently inhibited the Ca2+-induced contraction in the presence of any given concentration of MgATP. We conclude that Pi may accelerate the detachment of cross-bridges between the contractile proteins. Thus, the amplitude of Ca2+-induced contraction is slightly inhibited and the relaxation is markedly facilitated. However, the site of action of Pi may differ from that of MgATP.(ABSTRACT TRUNCATED AT 400 WORDS)

Inhibition of cycling and noncycling cross bridges in skinned smooth muscle by vanadate

Vanadate (Vi, 3-300 microM) reversibly inhibited force development elicited by micromolar Ca2+ in membrane-skinned fibers of smooth muscle from taenia coli and trachea of guinea pig. When relaxed fibers were preincubated with Vi, the contraction to Ca2+ was characterized by a peak response followed by a lower steady-state phase. The peak phase depended on the rate of contraction and the [Vi]and was absent after Vi incubation during a previous contraction. These observations were consistent with Vi binding to a site that was exposed during the cross-bridge cycle but absent in the relaxed state. The actin X myosin X ADP intermediate formed at the active site during the cross-bridge cycle is suggested as the site of action of Vi. A weak antagonism between Pi and Vi was demonstrated during contractions activated by myosin thiophosphorylation. High concentrations of Pi (6-12 mM) were needed to produce a small inhibition (10%) of maximal Ca2+-activated tension. Skinned fibers relaxed slowly after Ca2+ removal, and the absence of an active state suggested that tension was maintained by noncycling cross bridges. Both Vi and Pi increased the rate of tension loss by 10-fold, but Vi was 5-10 times more potent than Pi. It is suggested that Vi and Pi both act on the active site but that Pi has a more efficacious action on slowly cycling than rapidly cycling cross bridges.

cGMP and cAMP inhibit tension development in skinned coronary arteries

The effects of physiological concentrations of cGMP and cAMP on tension development in skinned coronary arteries (Triton X-100) were studied. cGMP inhibited tension elicited at intermediate Ca2+ concentrations at pH 7.0 but not at more acidic or alkaline pH values. cAMP, on the other hand, decreased submaximal tension development independent of pH (from pH 6.5 to pH 7.2). Neither nucleotide affected tension development at maximally activating Ca2+ concentrations.

Effect of calmodulin, Ca2+, and cAMP protein kinase on skinned tracheal smooth muscle

The characteristics of contraction and relaxation of membrane skinned smooth muscle from guinea pig trachealis muscle are described. Micromolar Ca2+ elicited reproducible contractions in Mg-ATP salt solution at 20 degrees C. The speed of contraction was much faster at 30 and 37 degrees C, enabling cumulative concentration-response curves to be obtained. At these temperatures, a progressive increase in basal tension occurred in the absence of Ca2+. This tension was active and developed more rapidly at pH 6.7 than at pH 7.0. Calmodulin (0.1-10 microM) greatly increased the speed of contraction and lowered the threshold Ca2+ concentration ([Ca2+]) required to initiate contraction from 0.13 to 0.02 microM Ca2+. Trifluoperazine antagonized responses to Ca2+. Thiophosphorylation with adenosine 5'-O-(3-thiotriphosphate) produced maximum tension development, which was Ca2+-independent. This effect was reversible. The results are compatible with myosin-linked regulation of contraction in which a Ca2+ X calmodulin complex activates myosin light chain kinase to phosphorylate myosin. The catalytic subunit of cAMP-dependent protein kinase strongly inhibited tension development and slowly relaxed fibers contracted with threshold [Ca2+] consistent with an action via phosphorylation of myosin light chain kinase. This effect was extremely slow compared with the rate of relaxation by Ca2+ withdrawal or with relaxation of intact smooth muscle by beta-adrenergic agonists.

Skinned smooth muscle: time course of force and ATPase activity during contraction cycle

The time course of ATPase activity and force has been determined during contraction and relaxation in skinned (hyperpermeable) anterior byssus retractor muscle, ABRM, of Mytilus edulis and compared with corresponding measurements on skinned taenia coli of guinea-pigs. Following a calcium-induced contraction, lowering the [Ca++] to 10(-8) M rapidly reduces ATPase activity within 2 min to resting levels while force declines only to about 30-50% of maximal tension within the same time. Thus slow relaxation is due to a 'catch-like-state' which is common to different kinds of smooth muscles and can be reduced with cAMP in ABRM and by Pi in taenia coli.

Tension transients in skinned muscle fibres of insect flight muscle and mammalian cardiac muscle: effect of substrate concentration and treatment with myosin light chain kinase

Glycerinated single fibres from the dorsal longitudinal muscle of Lethocerus maximus were isometrically contracted in MgATP-salines (10 microM Ca2+; 1.5 mM Mg2+; pH 6.7; 22 degrees C and 20 mM PEP; 100 U/ml pyruvate kinase). The ratio of ATPase activity to tension decreased by a factor of 2 after reducing the ATP-concentration from 15 to 0.5 mM. At all ATP-concentrations (0.5-15 mM), the fibres showed tension adjustments in response to small step changes in length characteristic to an actively contracting muscle: i) an elastic phase which did not depend on ATP-concentration ii) a quick phase of stress relaxation with at least two exponential components; iii) a phase of delayed tension generation. An increase in size of the length step and/or a decrease of ATP-concentration slowed the quick phase and the delayed phase. Similar results have been obtained with skinned cardiac muscle (pig right ventricle). To see, how the isolated contractile system is affected by an increase in the light chain phosphorylation, tension transients were studied in skinned right ventricular muscle fibres before and after incubation with ATP gamma S (2 mM), pure myosin light chain kinase (9 micrograms/ml), Calmodulin (1 microM) and Ca2+ (0.8 microM). While isometric tension development elicited by 20 microM Ca2+ in the ATP salt solution was barely affected in presence of the enzyme, the ATPase activity was decreased by about 25% of the control. There was also a marked decrease (about 50%) in the contraction velocity as determined by the recovery of tension following a quick release. Quick stretches cause an immediate increase in tension followed by a rapid fall and a subsequent rise in tension. The velocity of this tension rise decreased by approximately 30% after incubation with myosin light chain kinase.

Dependence of cyclic-AMP induced relaxation on Ca2+ and calmodulin in skinned smooth muscle of guinea pig Taenia coli

cAMP (10(-6) - 10(-4) M) produced a dose-dependent relaxation of Ca2+-induced contraction in the guinea-pig taenia coli skinned with 1% Triton X-100. At 0.53 microM Ca2+ and 0.05 microM calmodulin (CaM), cAMP (10(-4) M) produced a maximal relaxation of 75% (pH 6.7; 25 degrees C). Increasing Ca2+ (0.8 microM) or CaM (0.37 microM) reduced cAMP-induced relaxation to 25 and 5% respectively. At high CaM (5 microM), cAMP-induced relaxation could be completely inhibited by as low as 0.25 microM Ca2+. Furthermore, small increases in Ca2+ or CaM could effectively reverse the cAMP-induced relaxation in the continuous presence of cAMP. These results demonstrate that small modulations in the Ca2+-calmodulin activity have a strong effect on the ability of cAMP to produce a direct relaxing effect on the contractile proteins in skinned fiber. It is suggested that the effects of cAMP on the cellular mechanisms that lower cytoplasmic free Ca2+ concentration may act as the important determinants of the extent of the direct inhibitory effect of cAMP on the contractile elements. These two mechanisms may act in concert in this fashion to effect cAMP-induced relaxation in smooth muscle during beta-adrenergic stimulation.

Mechanism of vanadate-induced contraction of airways smooth muscle of the guinea-pig

The characteristics of vanadate-induced contraction of airways smooth muscle are described in isolated preparations of guinea-pig central and peripheral airways. Vanadate (1-1000 microM) induced sustained contractions of trachea and lung parenchymal strips within 1 min of challenge. It was more potent (P less than 0.001) on the lung strip (EC50 = 63 microM) than on the trachea (EC50 = 123 microM). The lung strip also developed greater maximum isometric tension (P less than 0.001) than the trachea. The efficacy on the lung strip was 2 and the trachea 0.6, relative to the response to acetylcholine (efficacy = 1). Vanadate-induced contractions of the trachea were not inhibited by atropine, mepyramine, phentolamine or indomethacin, nor after mast cell depletion by compound 48/80, showing that contractions were not mediated via specific receptors or by release of endogenous mediators of tone. Inorganic phosphate specifically inhibited vanadate responses in a dose-dependent and reversible manner, suggesting a common site of action. Contractions could be elicited in depolarized muscle and after treatment with ouabain plus propranolol, showing that membrane depolarization and inhibition of the Na, K-ATPase system were not involved in the contractile action of vanadate. Pretreatment of tracheal smooth muscle with verapamil had no influence on contractions elicited by vanadate. After removal of extracellular calcium, vanadate-induced contractions declined slowly with time, indicating that influx of extracellular calcium was not giving rise to contractions elicited by vanadate. Vanadate markedly increased the rate of calcium efflux from trachealis muscle loaded with 45Ca into both Ca2+-free and normal Krebs solutions; this is compatible with vanadate mobilizing an intracellular store of Ca2+. Such a store involving sites with Ca-ATPase activity would be consistent with the action of vanadate in isolated membrane preparations. Membrane-skinned tracheal fibres contracted by micromolar Ca2+ were relaxed by vanadate in a reversible dose-related manner, indicating that the contractile action of vanadate was not related to its interaction with proteins at the cross-bridge level.

Low Ca2+ impedes cross-bridge detachment in chemically skinned Taenia coli

Muscle force is generated by cycling cross-bridges between actin and myosin filaments. In smooth muscle, cyclic attachment and detachment of cross-bridges is thought to be induced by a Ca2+- and calmodulin-dependent myosin light chain kinase which phosphorylates myosin. The relaxation that occurs after Ca2+ removal is usually ascribed to dephosphorylation of myosin by a phosphatase as non-phosphorylated myosin is unable to form force-generating criss-bridges. Recently, Dillon et al. claimed, however, that dephosphorylation of attached cross-bridges may impede cross-bridge detachment, thus forming so-called 'latch bridges'. Here we present evidence that after a Ca2+- and calmodulin-induced contraction of chemically skinned guinea pig Taenia coli, the rapid removal of Ca2+ impedes the detachment of the myosin cross-bridges from the actin filament; force can then be maintained without energy consumption. The extremely slowly detaching cross-bridges which maintain the force after Ca2+ removal may indeed correspond to the 'latch bridges' mentioned above.

Mechanical characteristics of chemically skinned guinea-pig taenia coli

Strips of intact and chemically skinned (Triton X-100) taenia coli were mounted for isometric and quick-release experiments at 23 degrees C. Active force increased in repeated high-K+ induced contractures in the intact muscle. Stable maximal force was 313 +/- 24 mN/mm2 (n = 6). The skinned preparations activated by Ca2+, at 2 mM Mg2+, 3.2 mM MgATP and ionic strength 0.085 M, gave half maximal force at pCa = 5.62 +/- 0.4 and a maximal force (63 +/- 8 mN/mm2) at pCa = 4.5 (20-25% of the control K+-responses prior to skinning but about 60% of the first K+-response). Force-velocity relations were obtained from intact muscles and from the same muscles chemically skinned and activated at optimal Ca2+. Maximal shortening velocity (Vmax) was unaltered in the skinned preparation compared to the intact muscle (0.138 +/- 0.011 vs 0.140 +/- 0.006 L/s) indicating similar kinetics of actomyosin interaction. In the intact muscle a decrease in Vmax was found when the Ca2+ concentration was reduced. Calmodulin (1 microM) increased Ca2+ sensitivity (by about 0.6 log units) of the skinned preparation but at optimal Ca2+ caused no alteration in isometric force or Vmax.