Preparation and properties of a calcium ion-sensitive actomyosin from arteries

HTS driven by fluorescence lifetime detection of FRET identifies activators and inhibitors of cardiac myosin

Small molecules that bind to allosteric sites on target proteins to alter protein function are highly sought in drug discovery. High-throughput screening (HTS) assays are needed to facilitate the direct discovery of allosterically active compounds. We have developed technology for high-throughput time-resolved fluorescence lifetime detection of fluorescence resonance energy transfer (FRET), which enables the detection of allosteric modulators by monitoring changes in protein structure. We tested this approach at the industrial scale by adapting an allosteric FRET sensor of cardiac myosin to high-throughput screening (HTS), based on technology provided by Photonic Pharma and the University of Minnesota, and then used the sensor to screen 1.6 million compounds in the HTS facility at Bristol Myers Squibb. The results identified allosteric activators and inhibitors of cardiac myosin that do not compete with ATP binding, demonstrating high potential for FLT-based drug discovery.

Purification of Myosin from Bovine Tracheal Smooth Muscle, Filament Formation and Endogenous Association of Its Regulatory Complex

Dynamic regulation of myosin filaments is a crucial factor in the ability of airway smooth muscle (ASM) to adapt to a wide length range. Increased stability or robustness of myosin filaments may play a role in the pathophysiology of asthmatic airways. Biochemical techniques for the purification of myosin and associated regulatory proteins could help elucidate potential alterations in myosin filament properties of asthmatic ASM. An effective myosin purification approach was originally developed for chicken gizzard smooth muscle myosin. More recently, we successfully adapted the procedure to bovine tracheal smooth muscle. This method yields purified myosin with or without the endogenous regulatory complex of myosin light chain kinase and myosin light chain phosphatase. The tight association of the regulatory complex with the assembled myosin filaments can be valuable in functional experiments. The purification protocol discussed here allows for enzymatic comparisons of myosin regulatory proteins. Furthermore, we detail the methodology for quantification and removal of the co-purified regulatory enzymes as a tool for exploring potentially altered phenotypes of the contractile apparatus in diseases such as asthma.

Cyclooxygenase, p38 Mitogen-Activated Protein Kinase (MAPK), Extracellular Signal-Regulated Kinase MAPK, Rho Kinase, and Src Mediate Hydrogen Peroxide-Induced Contraction of Rat Thoracic Aorta and Vena Cava

In hypertension, blood vessels exhibit increased reactive oxygen species production that may alter vascular tone. We previously observed that H2O2 contracted rat thoracic vena cava under resting tone and aorta contracted with KCl. In arteries but not veins, H2O2-induced contraction required extracellular Ca2+ influx. Because of this difference in Ca2+ utilization, we hypothesized that signaling pathways mediating H2O2-induced contraction in vena cava under resting tone differed from those mediating H2O2-induced contraction in aorta contracted with KCl. Inhibitors of cyclooxygenase (COX) 1 and 2 (indomethacin, 10 microM), thromboxane A2 (TXA2) receptors [ICI185282 (2RS,4RS,5SR-4-o-hydroxyphenyl-2-trifluoromethyl-1,3-dioxan-5-yl heptenoic acid), 10 microM], p38 mitogen-activated protein kinase (MAPK) [SB203580 (4-[5-(4-fluorophenyl)-2-[4-(methylsulfonyl)phenyl]-1H-imidazol-4-yl]pyridine), 10 microM], extracellular signal-regulated kinase (Erk) [PD98059 (2'-amino-3'-methoxyflavone), 10 microM], src [PP1 (4-amino-5-(4-methylphenyl)-7-(t-butyl)pyrazolo[3,4-d]pyrimidine, 10 microM], and rho kinase [Y27632 (trans-4-[(1R)-1-aminoethyl]-N-4-pyridinylcyclohexanecarboxamide dihydrochloride), 10 microM], significantly reduced H2O2-induced contraction in vena cava under resting tone and aorta after KCl (30 mM) contraction. In contrast, the phosphatidylinositol 3-kinase (PI3-K) inhibitor LY294002 [2-(4-morpholinyl)-8-phenyl-4H-1-benzopyran-4-one, 20 microM] did not reduce aortic or venous H2O2-induced contraction. p38 MAPK, Erk MAPK, and src inhibition did not reduce aortic or venous contraction to the TXA2 receptor agonist U46619 (9,11-dideoxy-9alpha,11alpha-methanoepoxy PGF(2alpha), 1 microM), whereas rho kinase inhibition significantly reduced aortic and venous contraction to U46619, and PI3-K inhibition reduced venous contraction to U46619. Our data suggest that, in rat thoracic aorta and vena cava, a COX-derived metabolite is one important mediator of H2O2 contraction, possibly via rho kinase activation, and that H2O2-induced contraction via p38 and Erk MAPK probably occurs independently of TXA2 receptor activation.

MAPK and PKC activity are not required for H(2)O(2)-induced arterial muscle contraction

H(2)O(2)-induced pulmonary arterial smooth muscle (PASM) contractions are independent of Ca(2+) and myosin light chain phosphorylation. The purpose of this study was to determine whether mitogen-activated protein kinase (MAPK), extracellular signal-regulated kinase (ERK) 1 and ERK2, or protein kinase C (PKC) activation is required for H(2)O(2)-induced contraction. Porcine PASM strips were stimulated with 1 mM H(2)O(2), 120 mM KCl, or 10 microM phorbol myristic acetate and freeze clamped at various times during the contractions. Changes in relative amounts of tyrosine/threonine phosphorylated MAPK compared with total MAPK were measured. MAPK tyrosine phosphorylation levels increased in correlation with tension development. However, 50 microM PD-98059, a MAPK/ERK kinase-MAPK kinase blocker, reduced MAPK phosphorylation below resting levels, even though the magnitude of the isometric tension development was unaltered. Freeze-clamped PASM strips were placed in a PKC activity assay buffer containing (32)P and CaCl(2) to measure the total myelin basic protein phosphorylation. The data show that: 1) the time courses of PKC activity and force produced in response to H(2)O(2) do not correlate, and 2) MAPK activation may be a concurrent event with, or a consequence of, tension development in response to a variety of agonists but is not responsible for contractions to H(2)O(2), high K(+), or phorbol esters.

Caldesmon: a common actin-linked regulatory protein in the smooth muscle and nonmuscle contractile system

Caldesmon was originally purified from gizzard smooth muscle as a major calmodulin-binding protein which also interacts with actin filaments. It has an alternative binding ability to either calmodulin or actin filaments depending upon the concentration of Ca2+ ("flip-flop binding"). Two forms of caldesmon (Mr's in the range of 120-150 kDa and 70-80 kDa) have been demonstrated in a wide variety of smooth muscles and nonmuscle cells. Immunohistochemical studies suggest that caldesmon is colocalized with actin filaments in vivo. Considering its abundance, the Ca2+-dependent flip-flop binding ability to either calmodulin or actin filaments, and its intracellular localization, caldesmon is expected to be involved in contractile events. Recent results from our laboratory have led to the conclusion that caldesmon regulates the smooth muscle and nonmuscle actin-myosin interaction and the smooth muscle actin-high Mr actin-binding protein (ABP or filamin) interactin in a flip-flop manner. It might function in cell motility by regulating the contractile system.

Vasodilating effects of carbon monoxide

The vasodilator effects of carbon monoxide (CO) were studied in an isolated perfused rat thoracic aorta preparation. Thoracic aortas from male Sprague-Dawley laboratory rats were dissected free of surrounding tissue, cannulated proximally, and tethered to in situ length. The vessels were perfused with oxygenated Krebs-Henseleit (KH) solution at 37 degrees C in a constant flow system with a circumferentially-applied, pulsatile (300/min), basal "systolic" pressure of 100 mm Hg. Aortas were precontracted with high-potassium (K+) or norepinephrine (NE). Changes in perfusion pressure were indicative of changes in vascular resistance induced by the test gas mixtures. Oxygen (O2) content of the perfusate was kept constant, while CO and nitrogen (N2) were altered. CO (2.5, 5 and 10%) dilated both K+-contracted and NE-contracted aortas in a dose-dependent manner. A significant vasodilation in response to 5% CO (24.5% of maximal), but not to 5% N2, was obtained in the K+-contracted aortas. After the endothelium was removed chemically, the aortas continued to dilate in response to CO. These results suggest that CO has a direct vasorelaxant effect on vascular smooth muscle which is nonspecific and is not endothelium-dependent.

Purification of smooth-muscle myosin free of calmodulin and myosin light-chain kinase. Susceptibility to oxidation

Smooth-muscle myosin purified as described by Persechini & Hartshorne [(1983) Biochemistry 22, 470-476] contains trace amounts of calmodulin and myosin light-chain kinase, which can be removed by Ca2+-dependent hydrophobic-interaction chromatography followed by calmodulin-Sepharose affinity chromatography. The resultant column-purified myosin exhibits properties similar to those of the non-purified myosin, e.g. actin activation of the Mg2+-ATPase requires Ca2+/calmodulin-dependent phosphorylation of the two 20 kDa light chains. However, unlike the non-purified myosin, the column-purified myosin undergoes a time-dependent transition to a form which no longer requires phosphorylation for actin activation of the myosin Mg2+-ATPase. This transition is identified as a time-dependent change in conformation of the column-purified myosin from a 10 S to 6 S form and is caused by slow oxidation of the column-purified myosin, since it could be prevented by storage under N2 and reversed by 5 mM-dithiothreitol.

Purification and properties of Ca2+-regulated thin filaments and F-actin from sheep aorta smooth muscle

We have investigated the conditions for isolation of Ca2+-regulated thin filaments from sheep aorta. Inhibition of proteolysis by 2 micrograms ml-1 leupeptin and chymostatin and of oxidation with 5 mM dithiothreitol were essential. Washed homogenates were extracted in 10 mM ATP of low ionic strength at pH 6.1 to minimize coextraction of myosin with thin filaments. Thin filaments were separated from myosin by high speed sedimentation; 20% glycol was added to prevent loss of regulatory factors and tropomyosin. The resulting thin filaments (yield 2.5 mg protein g-1 artery wet weight) were made up of actin, tropomyosin and a 120 000 Mr protein (molar ratio 1:1/5:1/29) and were up to 4 micron long. They activated skeletal muscle myosin at least 50 times in presence of Ca2+. Up to 80% inhibition was observed in the absence of Ca2+. We also prepared pure arterial F-actin, which activated skeletal myosin more than the thin filaments, but was similar to skeletal F-actin. We conclude that Ca2+ regulation is negative, involves cooperative interactions between actin, myosin and tropomyosin and suggest that it is mediated by the 120 000 Mr protein.

Phosphagen and metabolite content during contraction in porcine carotid artery

Previous studies (Paul, R. J. Chemical energetics of vascular smooth muscle. In: Handbook of Physiology: The Cardiovascular System. Bethesda, MD: Am. Physiol. Soc., 1980, p. 201-235) have shown that vascular oxygen consumption reaches a steady state at approximately twice the basal rate during maintenance of isometric contraction. The time course of the attainment of a metabolic steady state, the metabolic signal for the observed increase in respiration, and the contribution of endogenous phosphagens to the energetics of isometric contraction are not known with certainty. To this end, the time course of the tissue content of ATP, ADP, AMP, phosphocreatine (PCr), inorganic phosphate (Pi), and lactate were measured during a KCl-induced isometric contraction in porcine carotid artery and compared with values in the basal state. Oxygenated unpoisoned strips were frozen at 0, 0.5, 1, and 15 min of contraction, and tissue extracts were analyzed using analytical isotachophoresis. No statistically significant changes from the basal levels of ATP and PCr were measured. A small but significant increase in ADP was seen at all times. An increase in Pi of 1.25 mumol/g was observed at 0.5 min, which decreased in time. Tissue lactate content increased by 1.79 mumol/g after 1 min of contraction. The calculated range of cellular free ADP (ADPfree), 44-123 microM, may be sufficient to saturate oxidative phosphorylation. This and the apparent lack of change of ADPfree from basal during contraction suggest that it does not play a role in the coordination of metabolism and contractility. From as early as 0.5 min, when less than 40% of peak isometric force is attained, intermediary metabolism provides the total ATP required for contraction.

Vascular smooth muscle. Calmodulin and cyclic AMP-dependent protein kinase after calcium sensitivity in porcine carotid skinned fibers

Recent work on vascular smooth muscle actomyosin has indicated that the Ca2+ sensitivity of both ATPase and superprecipitation are affected by calmodulin (CaM) and cyclic AMP-dependent protein kinase (cPK). Using a "chemically skinned" arterial preparation, we have extended these observations to the intact structured contractile system. Media from hog carotid artery were skinned with 1% Triton X-100 followed by a 50% glycerol-ATP salt solution, in which the strips were stored at -25 degrees C. Small strips (thickness between 0.1 and 0.2 mm) were mounted isometrically and relaxed in a Mg-ATP salt solution, pH 6.7, Ca2+ 10(-8) M, 30 degrees C. Ca2+ elicited a contraction with an ED50 of 10(-6) M. Isometric force was between 1 and 4 mN, consistent with the force observed before skinning. With time, the preparation became less sensitive with an increase in ED50 to 10(-5.7) M. CaM (4 micro M) reverses this loss, stabilizes the preparation, and sharply accelerates the rate of tension development. The ED50 in the presence of 4 micro M CaM shifts to about 10(-7) M. This effect is dose-dependent, with the half maximal effect at about 0.4 micro M CaM. Submaximal Ca2+ contractions can be reversibly depressed by preincubation of relaxed fibers with cPK catalytic subunit (300 U/ml), even in the presence of 4 micro M CaM. An inhibition of about 50% of the contraction at 0.2 micro M Ca2+ was obtained, whereas only 20% inhibition was found at 6 micro M Ca2+. Our findings suggest that changes in vascular contractility cannot be described solely in terms of changes in cytoplasmic Ca2+, and that changes in the sensitivity of the contractile protein to a given Ca2+ concentration are also potential mechanisms for vasodilation.

Studies on high affinity binding of mineralo- and glucocorticoids in rabbit aorta cytosol

High affinity, specific binding-sites to mineralocorticoids and glucocorticoids, with characteristics of steroid receptors, have been found in rabbit aorta cytosol. Binding parameters (dissociation constants and number of binding sites per mg of cytosol protein) were determined from Scatchard plots, after statistical treatment of the data with the aid of a computer program, for the following tritiated steroids: 11-desoxycorticosterone (DOC), aldosterone (Aldo), progesterone (Prog), corticosterone (BK), cortisol (FK) and dexamethasone (Dex). The specificity of binding was then examined by means of steroid competition studies. The results of these experiments indicate that three different types of high-affinity binding sites to adrenal steroids are present in aorta cytosol: Type A, with the highest affinity for DOC; Type B, with the highest affinity for FK; Type C, with the highest affinity for Dex. In accordance with the relative competitive potencies of various steroids for these binding sites, Type A is designated as the "arterial mineralocorticoid binder", clearly differing in its binding characteristics from the cytoplasmic mineralocorticoid binders in known target tissues to these steroids (e.g. the renal receptor), while Type C is designated as the "arterial glucocorticoid binder", closely resembling the classical glucocorticoid receptor in known target tissues to glucocorticoids. Type B exhibited some of the binding characteristics of transcortin and may represent a modified, intracellular transcortin. While Types B and C are present also in the cytosol of inferior vena cava. Type A was only in the aorta. The role of these binders is not known at present. Arguments are presented in favor of a hypothesis that the Type A (mineralocorticoid) binder represents an arterial wall; and that, under certain conditions, this action leads to an increased contractility of arterial and arteriolar smooth muscles, increased peripheral resistance and hypertension.

Evidence for the presence in arterial walls of intracellular-molecular mechanism for action of mineralocorticoids

Data from clinical and experimental studies indicate that mechanism(s) for action of mineralocorticoids, other than renal, must be involved in the overall effect of mineralocorticoids on circulation--increased peripheral resistance and hypertension. We have postulated existence of such a mechanism in the arterial wall and have looked for the evidence for its presence. We have found high affinity, specific binders for mineralocorticoids, and glucocorticoids, with characteristics of steroid receptors, in the cytosol of rabbit aorta and femoral and carotid arteries. These binders possess physicochemical properties of steroid receptors and, moreover, they translocate to cell nuclei (as steroid-receptor complexes) and bind to relatively specific "acceptor-sites" on nuclear chromatin. This provides evidence for the existence of an intracellular-molecular mechanism for a direct in situ action of mineralocorticoids, and also glucocorticoids, in the arterial wall. We postulate that the demonstrated previously effect of mineralocorticoids on arterial smooth muscle cell-membrane permeability to electrolytes, leading presumably to increased peripheral resistance and hypertension, is elicited through the receptor-mediated mechanism for action of mineralocorticoids in the arterial wall.

Phosphorylation of the calcium ion-regulated thin filaments from vascular smooth muscle. A new regulatory mechanism?

The thin filaments of vascular smooth muscle (pig aorta) contain a Ca2+-sensitive regulatory system that resembles troponin-tropomyosin [Marston, Trevett & Walters (1980) Biochem. J. 185, 355-365]. Our thin-filament preparations also contain enzymes that phosphorylate and dephosphorylate a specific protein. Initial rate of phosphorylation was 0.42 +/- 0.10 (95% confidence limits) mumol of Pi/min per g of thin filaments; half-maximal incorporation was obtained in 4 1/2 min, and a maximum of 1.8 +/- 0.1 mumol of Pi/g of thin filaments was incorporated after 40 min (conditions: 1 mM-MgATP, 60 mM-MgATP, 60 mM-KCl, 10 mM-imidazole, pH 7.0, 5 mM-MgCl2, 10 mM-NaN3, 0.5 mM-dithiothreitol, 0.1 mM-CaCl2, 25 degrees C). On gel electrophoresis in polyacrylamide (4-30% gradient)/0.25% sodium dodecyl sulphate gel over 75% of protein-bound phosphate was in a single protein of mol.wt. 21000. On electrophoresis in polyacrylamide (8%)/6 M-urea (pH 8.6) gel the phosphoprotein remained at the origin. Phosphorylation was associated with an increase in the concentration of high-affinity (K congruent to 10(6) M-1) Ca2+-binding sites from 0.8-1.5 to 6.3 mumol of Ca2+/g of thin filaments. Phosphorylation also changed the regulatory properties of the skeletal-muscle myosin-aorta thin-filament MgATPase; maximum activity was unaltered, but the phosphorylated thin filaments required only 0.36 microM-Ca2+ for half-activation compared with 2.7 microM-Ca2+ for unphosphorylated thin filaments. The possible regulatory role of thin-filament phosphorylation is discussed.

Tension responses of chemically skinned fibre bundles of the guinea-pig taenia caeci under varied ionic environments

1. Thin fibre bundles (about 100 mum in width and 1.5 mm in length) were dissected from taenia caeci of the guinea-pig, and were mounted in an apparatus to record isometric tension mainly at 25 degrees C.2. Fibre bundles were chemically skinned by treating with 50 mug saponin/ml. for 20 min in a relaxing solution. Upon application of an activating solution containing 80 mum-Ca(2+) the fibre bundles rapidly developed a tension which was comparable in magnitude to that elicited with 10 mum-carbachol before the saponin treatment.3. It was confirmed that skinned fibre bundles develop active tension slowly in the virtual absence of Ca(2+) when Mg(2+) concentration was raised above 2 mm in the presence of MgATP. However, this tension was not developed in the presence of an ATP regenerating system.4. Maximum Ca(2+) activated tension and Ca sensitivity of skinned smooth muscle fibre bundles were examined using solutions in which pH, ionic strength, and the concentrations of Mg(2+) and MgATP(2-) were varied around supposedly physiological values. An ATP regenerating system was used when necessary.5. Maximum Ca(2+) activated tension changed only slightly with alteration in ionic strength (0.15-0.3 m). It decreased by 40% with increase in pH from 6.4 to 7.2, and decreased by 10-30% when Mg(2+) concentration was either lowered to 0.2 mm or raised to 2 and 10 mm from 1 mm. The tension increased by about 10-20% as MgATP(2-) concentration was raised from 0.4 to 4 or 8 mm.6. Changes in pH, ionic strength, and MgATP(2-) concentration in the above range had no important effects on the relative isometric tension-pCa relation. Decrease of Mg(2+) concentration from 1 to 0.2 mm had little effect, but increase to 2 and 10 mm shifted the curve some 0.2 and 0.6 pCa units, respectively, toward a lower pCa. There was not large difference between the relative isometric tension-pCa relation at 25 degrees C and that at 38 degrees C.7. The Ca(2+) concentration in the smooth muscle cell of taenia during physiological contraction can be estimated from the results obtained and the values for the relevant in vivo ionic constituents given in the literature; the lowest Ca(2+) concentration for detectable tension development is between 0.2 and 0.5 mum, and tension tends to level off above 10 mum-Ca(2+).

Contraction in intact pig aortic strips is not always associated with phosphorylation of myosin light chains

Intact pig aortic strips were incubated in medium containing [32P]P1 and various Ca2+ concentrations. The 32P content of the myosin P-light chain was determined by radioautography after electrophoresis in the presence of sodium dodecyl sulphate. Although treatment of the strips with noradrenaline always caused a rise in tension, this was not necessarily accompanied by increased phosphorylation of the P-light chain. These results indicate that, in aortic smooth muscle, phosphorylation of the P-light chain is not obligatory for contraction.

Calcium ion-regulated thin filaments from vascular smooth muscle

Myosin and actin competition tests indicated the presence of both thin-filament and myosin-linked Ca2+-regulatory systems in pig aorta and turkey gizzard smooth-muscle actomyosin. A thin-filament preparation was obtained from pig aortas. The thin filaments had no significant ATPase activity [1.1 +/- 2.6 nmol/mg per min (mean +/- S.D.)], but they activated skeletal-muscle myosin ATPase up to 25-fold [500 nmol/mg of myosin per min (mean +/- S.D.)] in the presence of 10(-4) M free Ca2+. At 10(-8) M-Ca2+ the thin filaments activated myosin ATPase activity only one-third as much. Thin-filament activation of myosin ATPase activity increased markedly in the range 10(-6)-10(-5) M-Ca2+ and was half maximal at 2.7 x 10(-6) M (pCa2+ 5.6). The skeletal myosin-aorta-thin-filament mixture gave a biphasic ATPase-rate-versus-ATP-concentration curve at 10(-8) M-Ca2+ similar to the curve obtained with skeletal-muscle thin filaments. Thin filaments bound up to 9.5 mumol of Ca2+/g in the presence of MgATP2-. In the range 0.06-27 microM-Ca2+ binding was hyperbolic with an estimated binding constant of (0.56 +/- 0.07) x 10(6) M-1 (mean +/- S.D.) and maximum binding of 8.0 +/- 0.8 mumol/g (mean +/- S.D.). Significantly less Ca2+ bound in the absence of ATP. The thin filaments contained actin, tropomyosin and several other unidentified proteins. 6 M-Urea/polyacrylamide-gel electrophoresis at pH 8.3 showed proteins that behaved like troponin I and troponin C. This was confirmed by forming interspecific complexes between radioactive skeletal-muscle troponin I and troponin C and the aorta thin-filament proteins. The thin filaments contained at least 1.4 mumol of a troponin C-like protein/g and at least 1.1 mumol of a troponin I-like protein/g.

Temperature- and Mg-ATP-dependent regulation of Ca2+ sensitivity of smooth muscle actomyosin ATPase

Many smooth muscles on metabolic depletion undergo a contraction that is insensitive to EGTA [ethylene glycol-bis (beta-aminoethylether)N,N-tetraacetic acid]. Chicken gizzard actomyosin shows a progressive loss of Ca sensitivity accompanied by activation of EGTA-Mg-ATPase at temperatures near 37 degrees C with decreasing ATP concentrations. Ca2+-dependent phosphorylation still occurs under these conditions when the ATPase is Ca insensitive. Activation of EGTA-Mg-ATPase at low ATP concentration is not due to a pseudo-ATPase, or due to denautration of the actomyosin at 37 degrees C. Magnesium concentrations above 1 mM are required for observing the enhanced EGTA-Mg-ATPase activity and the Ca sensitivity is very markedly influenced by the magnesium concentrations of medium at low ATP. When the Mg-to-ATP ratio (5:1) was kept constant for varying ATP concentrations, activation of EGTA-ATPase was not observed. This activation was not due to the characteristics of the ATP regenerating system (phosphoenolpyruvate and pyruvate kinase) because with phosphocreatine and creatine phosphokinase similar results were obtained. Thus the EGTA-insensitive rise in tension during metabolic depletion is due to activation of Mg-ATPase and loss of Ca sensitivity at 37 degrees C, a temperature at which mammalian smooth muscles normally function.

Drug interactions in cat isolated tracheal smooth muscle

1. The interactions between some drugs that contract and relax airways smooth muscle have been investigated in the cat isolated trachea. 2. Isoprenaline and theophylline inhibited serotonin-elicited contractions more than acetylcholine-mediated responses. This was observed both in terms of the degree of inhibition and the concentration of the relaxant drug producing this inhibition. 3. The acetylcholine- and serotonin-induced contractions were inhibited more by theophylline than by isoprenaline displaced acetylcholine and serotonin response curves to the right whereas theophylline caused a flattening of the curves. 4. Isoprenaline was more effective in hibiting serotonin contractions than acetylcholine contractions when the tracheas were bathed in K+ depolarizing solution, suggesting that the difference in the suscepbibility of serotonin and acetylcholine contractions to isoprenaline was not dependent on the electrical membrane potential. 5. Isoprenaline inhibited the tonic component of acetylcholine contractions more than the phasic component. 6. The differences in the pharmacological responses to the contractile and relaxant drugs in cat tracheal preparations provide further examples in smooth muscle of different mechanisms by which acetylcholine and serotonin induce contraction and isoprenaline and theophylline relaxation.

The calcium accumulation in a microsomal fraction from porcine coronary artery smooth muscle. A study of the heterogeneity of the fraction

1. Microsomes prepared from the combined media and intima of pig coronary artery, take up Ca in an ATP-dependent way. This uptake is stimulated by oxalate. 2. Conditions have been determined to optimize the preparation of the microsomes in terms of their Ca accumulation activity. Careful homogenization of the tissue mince in 0.25 M sucrose by means of a Potter-Elvehjem homogenizer gives microsomal preparations with the highest specific activity for Ca accumulation. 3. Arguments are presented to support the hypothesis that, even in the absence of oxalate, Ca accumulation occurs into the lumen of the vesicles, and that these vesicles have a low Ca permeability. 4. Density gradient analysis shows that the microsomal fraction prepared from pig coronary artery media and intima is composed of vesicles that are heterogeneous in enzymatic composition. 5. Adenylate cyclase appears to be a predominantly plasma membrane-bound enzyme. Rotenone-insensitive NADH-cytochrome c reductase and choline phosphotransferase, two putative markers for internal membranes, give distinct banding patterns on on isopycnic centrifugation, indicating different intracellular localization. 6. There is a difference between the density gradient distribution pattern of Ca uptake measured in the presence or absence of oxalate. The latter coincides more closely with plasma membrane markers. The former resembles more the distribution of rotenone-insensitive NADH-cytochrome c reductase.

Inhibitory action of prostaglandin E1 on smooth muscle contraction and calcium responses

The relation between the inhibitory action of prostaglandin (E1 (PGE1) and external Ca concentration was investigated using the guinea-pig isolated ureter and the perfused central artery of the rabbit isolated ear. PGE1 20 ng/ml reduced the ureteral contraction evoked by a single electrical stimulation. This inhibitory action of PGE1 was enhanced with a decreased external Ca concentration. PGE1 100 ng/ml also reduced Ca-induced contracture of the ureter depolarized in Ca-free K(80 mM)-Krebs' solution. Furthermore, PGE1 50 ng/ml inhibited the responses of peripheral vascular resistance to noradrenaline, and this effect increased with a reduced external Ca concentration.

Energy-dependent calcium uptake activity of microsomes from the aorta of normal and hypertensive rats

Energy-dependent calcium uptake activity of microsomes isolated from the rat aorta has been characterized. The microsomes consist of smooth membrane vesicles which in the presence of MG-ATP as an energy source continuously sequester calcium over a 60-min period. This calcium uptake is greatly stimulated by oxalate anion which serves as a calcium trapping agent. Unlike the calcium uptake of mitochondria this uptake is not inhibited by sodium azide. Sucrose density gradient analysis of the microsomal calcium uptake suggests that the system is associated with the sarcoplasmic reticulum. In presence of 5 mM Mg-ATP and 20 muM calcium approximately 38 nmol of calcium per mg of microsomal protein are taken up in 20 min. In the absence of ATP, less than 2 nmol of calcium per mg of protein are taken up in the first 2 min with no further uptake of calcium in subsequent time periods. When calcium uptake activity is plotted against calcium or ATP concentration of the medium, half maximal activity is calculated for 24.3 muM calcium and for 1.6 mM ATP. The calcium uptake characteristics of the rat aorta microsomes are compatible with a postulated role in the relaxation of the vascular smooth muscle and the provision of an intracellular calcium store for muscle contraction. Aorta microsomes from SHR rats (a genetic strain that is spontaneously hypertensive) have a significantly reduced uptake when compared with the corresponding nonhypertensive control strain. The level of calcium and ATP for half maximal activity of the rat aorta microsomal calcium uptake system is approximately the same in the SHR and the control strain. The rate of release of calcium from rat aorta microsomes is apparently identical in SHR strain and control. The calcium uptake activity of kidney and liver microsomes isolated from the SHR strain and control. The calcium uptake activity of kidney and liver microsomes isolated from the SHR rat appears to be identical to that found in the control strain.

Preparation and properties of vertebrate smooth-muscle myofibrils and actomyosin

A new technique for obtaining a myofibril-like preparation from vertebrate smooth muscle has been developed. An actomyosin can be readily extracted from these myofibrils at low ionic strength and in yields 20 times as high as previously reported. The protein composition of all preparations has been monitored using dodecylsulfate-gel electrophoresis. By this method smooth muscle actomyosin showed primarily only the major proteins, myosin, actin and tropomyosin, while the myofibrils contained, additionally, three new proteins not previously described with polypeptide chain weights of 60000, 110000 and 130000. The ATPase activities of both the myofibrils and actomyosin preparations are considerably higher than previously described for vertebrate smooth muscle. They are sensitive to micromolar Ca2+ ion concentrations to the same degree as comparable skeletal and cardiac muscle preparations, even though troponin-like proteins could not be identified in these smooth muscle preparations. From the latter observation and the presence of Ca2+-sensitivity in tropomyosin-free actomyosin it is suggested that this calcium sensitivity is, as in some invertebrate muscles, a property of the myosin molecule.

Effects of hypoxia on the distribution of calcium in arterial smooth muscle cells of rats and swine

Exposure to hypoxia caused an increase in the hematocrit and right heart weight of experimental rats, but did not affect calcium-45 uptake by pulmonary arterial smooth muscle cells. However, autoradiographic studies showed that hypoxia apparently caused a shift of 45-Ca from primarily extracellular sites in arteries of control rats to intracellular sites in tissues of hypertensive rats. Cytochemical studies of calcium distributions in pulmonary arterial smooth muscle cells support the autoradiographic data and show that in both rats and swine the majority of pyroantimonate granules occur extracellulary in control tissues. In contrast, hypoxic tissues displayed a greatly reduced number of granules in extracellular sites and an increase in the amount of precipitate in intracellular sites. In pulmonary arterial smooth muscle cells from hypoxic rats most of the precipitate was associated with the caveolae intracellulares, while in corresponding cells from hypoxic swine in the majority of the pyroantimonate granules were localized to the sarcoplasmic reticulum. Hypoxia may produce pulmonary hypertension by interfering with the ability of the arterial smooth muscle cells to maintain transmembrane ionic gradients, thus producing an effective increase in cytoplasmic calcium levels. The increased calcium may then activate the contractile apparatus to produce a sustained vasoconstriction