Arterial tropomyosin and a relaxing protein fraction from vascular smooth muscle. Comparison with skeletal tropomyosin and troponin

The thin filaments of smooth muscles

Contraction in vertebrate smooth and striated muscles results from the interaction of the actin filaments with crossbridges arising from the myosin filaments. The functions of the actin based thin filaments are (1) interaction with myosin to produce force; (2) regulation of force generation in response to Ca2+ concentration; and (3) transmission of the force to the ends of the cell. The major protein components of smooth muscle thin filaments are actin, tropomyosin and caldesmon, present in molar ratios of 28:4:1 respectively. Other smooth muscle proteins which may be associated with the thin filaments in the cell are filamin, vinculin, alpha-actinin, myosin light chain kinase and calmodulin. We have reviewed the structural and functional properties of these proteins and where possible we have suggested what their function and mechanism of action may be. We propose that actin and tropomyosin are involved in the force producing interaction with myosin, and that this interaction is controlled by a Ca2+-dependent mechanism involving caldesmon, tropomyosin and calmodulin. Vinculin, alpha-actinin and filamin appear to be involved in the attachment of the thin filaments to the cell membrane and their spatial organization within the cell. We conclude that the filaments of smooth muscles share many common properties with those from skeletal muscle, but that they are also quite distinct in terms of both their caldesmon based regulatory mechanism and their mode of organization into a contractile apparatus.

Anti-troponin-T monoclonal antibody crossreacts with all muscle types

Monoclonal antibodies to troponin-T were produced by the hybridoma technique. Culture supernatants were initially screened using an enzyme-linked immunoabsorbent assay (ELISA). Positive clones were subcloned twice and further characterized. One of these, 7/H3:C9:D10, produced antibodies against troponin-T; immunoblotting experiments indicated its specificity for only troponin-T when challenged with a variety of striated muscle myofibrillar proteins. Indirect immunofluorescence staining with the antibody shows specific I-band staining in both adult and embryonic skeletal and cardiac muscle of various vertebrate species. In addition, intense but diffuse cytoplasmic staining was seen in chicken gizzard smooth muscle. Our results suggest that troponin-T contains an antigenic determinant that is common to both striated and smooth muscle.

A Ca2+-sensitive myosin light chain kinase, regulating pig carotid smooth muscle actomyosin ATPase

In this paper the correlation between phosphate incorporation into the regulatory light chain of myosin by a Ca2+-dependent myosin light chain kinase, and the Ca2+-sensitive ATPase activity and superprecipitation behaviour of arterial actomyosin, is described.

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.

Calcium-sensitivity of pig-carotid-actomyosin ATPase in relation to phosphorylation of the regulatory light chain

Ca2+-dependent phosphorylation of the 20000-Mr regulatory light chain was found to be a necessary condition for the Ca2+-sensitivity of the Mg2+-dependent ATPase activity and superprecipitation of pig carotid actomyosin. Actin-myosin interaction independent of phosphorylation and Ca2+ (ATPase activity and superprecipitation) were demonstrated in aged actomyosin preparations and in preparations from which the regulatory light chains were removed by papain digestion.

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.

Peripheral vascular dynamics studied by calcium ion inhibition

The study of peripheral vasomotility through inhibition of parietal smooth muscle cells is of great clinical interest for both diagnostic and therapeutic reasons. It permits the differentiation of the functional clinical forms from the organic. And in organic forms, once any spastic component is eliminated, it determines the site and severity. On the basis of these premises we have developed a test for the study of vascular dynamics based on the use of nifedipine as antagonist of calcium ions.

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.

Force-generating capacity and contractile protein content of arterial smooth muscle

After correction for extracellular space (40%) determined from electron micrographs, the maximum isometric force developed by strips prepared from the media of the hog carotid artery (2.2 x 10(6) dyn/cm(2)) can be extrapolated to give a value of 3.7 x 10(6) dyn/cm(2) for the smooth muscle component of the strip. Three independent estimates of the myosin content of the smooth muscle cells were made based on (a) exhaustive extraction and purification with estimates of preparative losses, (b) the myosin catalyzed ATPase activity of media homogenates, and (c) quantitative densitometry of the peaks containing myosin, actin, and tropomyosin after disk electrophoresis of sodium dodecyl sulfate-treated media homogenates. The results were consistent and gave a myosin content of 5-10 mg/g media, or 8-17 mg/g cell. Method (c) gave myosin:actin:tropomyosin weight ratios of 1:3.2:0.8. Although measured force developed by the smooth muscle cell exceeds that of mammalian striated muscle, the myosin content in smooth muscle is about five times lower. The actin content of smooth muscle is relatively high. The actin and myosin contents are consistent with thick and thin filament ratios observed in electron micrographs of vascular smooth muscle.