INHIBITION OF CONTRACTION IN A MOLLUSCAN SMOOTH MUSCLE BY THIOUREA, AN INHIBITOR OF THE ACTOMYOSIN CONTRACTILE MECHANISM

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.

Invertebrate muscles: thin and thick filament structure; molecular basis of contraction and its regulation, catch and asynchronous muscle

This is the second in a series of canonical reviews on invertebrate muscle. We cover here thin and thick filament structure, the molecular basis of force generation and its regulation, and two special properties of some invertebrate muscle, catch and asynchronous muscle. Invertebrate thin filaments resemble vertebrate thin filaments, although helix structure and tropomyosin arrangement show small differences. Invertebrate thick filaments, alternatively, are very different from vertebrate striated thick filaments and show great variation within invertebrates. Part of this diversity stems from variation in paramyosin content, which is greatly increased in very large diameter invertebrate thick filaments. Other of it arises from relatively small changes in filament backbone structure, which results in filaments with grossly similar myosin head placements (rotating crowns of heads every 14.5 nm) but large changes in detail (distances between heads in azimuthal registration varying from three to thousands of crowns). The lever arm basis of force generation is common to both vertebrates and invertebrates, and in some invertebrates this process is understood on the near atomic level. Invertebrate actomyosin is both thin (tropomyosin:troponin) and thick (primarily via direct Ca(++) binding to myosin) filament regulated, and most invertebrate muscles are dually regulated. These mechanisms are well understood on the molecular level, but the behavioral utility of dual regulation is less so. The phosphorylation state of the thick filament associated giant protein, twitchin, has been recently shown to be the molecular basis of catch. The molecular basis of the stretch activation underlying asynchronous muscle activity, however, remains unresolved.

Effects of vanadate, phosphate and 2,3-butanedione monoxime (BDM) on skinned molluscan catch muscle

The effects of orthovanadate (V(i)), inorganic phosphate (P(i)) and 2,3-butanedione monoxime (BDM) on tension, force transients and the catch state (passive tension maintenance) were investigated in saponin-skinned fibre bundles of the anterior byssus retractor muscle (ABRM) of the bivalve mollusc Mytilus edulis at pH 6.7. During maximal Ca(2+) activation isometric force was depressed by V(i) (0.03-10 mM), P(i) (10 mM) and BDM (50 mM). Force transients following quick stretches (0.1-0.3% of fibre length) were accelerated substantially by 1 mM V(i), 10 mM P(i) or 50 mM BDM. These compounds also accelerated force responses in experiments in which ATP was released rapidly from caged ATP by flash photolysis at both pCa 4.7 (force rise) and at pCa>8 (force decline). The effects on the catch state were investigated in two types of experiments: (1) Ca(2+) removal after maximal Ca(2+) activation and (2) rapid ATP release during high-force rigor at pCa>8. In both cases rapid relaxation was followed by slow relaxation (slower than 2% of initial force per min). This later slow relaxation (catch) was insensitive to V(i) (1-10 mM), P(i) (10 mM) and BDM (50 mM) but was accelerated by 0.12 mM cAMP. Complete relaxation to almost zero force was attained by changing pH from 6.7 to 7.7 (pCa>8). We conclude that catch depends on cAMP- and pH-sensitive structures linking the myofilaments and not on the force-generating actomyosin cross-bridges that are sensitive to V(i), P(i) and BDM.

Activation of the contractile mechanism in the anterior byssal retractor muscle of Mytilus edulis

1. The electrical and mechanical responses of the anterior byssal retractor muscle (ABRM) of Mytilus edulis to acetylcholine (ACh), high [K]O or the removal of external Ca were examined under a variety of conditions. 2. ACh (10(-6)--10(-3)M) produced contracture tensions larger than those produced by high [K]O (30-300 mM) for a given amount of depolarization. In Ca-free solution, the rate of decline of ACh-contractures was much smaller than that of K contractures, though both ACh- and K-contractures eventually disappeared. 3. 5-HT (10(-4)M) of procaine (1 mM) markedly reduced the height of ACh-contractures, but had little or no effect on K-contractures. The height of K contractures was markedly decreased by Mn ions (20 mM) or low pH (4-5), while ACh-contractures remained unaffected. 4. Partial replacement of [Na]o by choline (30-100 mM) reduced both ACh-induced depolarization and contracture tension, whereas K-contractures remained unchanged even after total replacement of [Na]o by choline. 5. ACh could produce little or no tension when applied during the relaxation phase of K-contractures, while high [K]o produced the maximal contracture tension when applied during the relaxation phase of ACh-contractures. 6. Following the removal of external Ca from solutions containing less than 10 mM-Mg, the ABRM showed a marked tension development associated with repetitive electrical activity superimposed on a gradual decline of membrane potential. 7. These results suggest that ACh-contractures are mainly due to the release of intracellularly stored Ca, while K-contractures are mainly associated with the inward movement of external Ca.

Localization of calcium-accumulating structures in the anterior byssal retractor muscle of Mytilus edulis and their role in the regulation of active and catch contractions

1. The localization of Ca-accumulating structures in the anterior byssal retractor muscle (ABRM) of Mytilus edulis and their role in the contraction-relaxation cycle were studied by fixing the ABRM at rest or during various phases of mechanical activity with a 1% osmium tetroxide solution containing 2% potassium pyroantimonate. 2. In the resting ABRM, electron-opaque pyroantimonate precipitate was observed at the inner surface of the plasma membrane, the vesicles and the mitochondria. 3. Electron X-ray microanalysis showed the presence of Ca in the precipitate, indicating that the precipitate provides a valid measure for Ca localization. 4. In the ABRM fixed at the peak of mechanical response to the Ca-removal or to acetylcholine, the precipitate was found to be diffusely distributed in the myoplasm in the form of a number of particles. At the completion of spontaneous relaxation, the precipitate was again seen at the inner surface of the plasma membrane. 5. During the catch state, the precipitate was found to be re-accumulated in the peripheral structures with a corresponding decrease of the precipitate in the myoplasm. 6. These results not only provide evidence for the involvement of the Ca-accumulating structures in the contraction-relaxation cycle in the ABRM, but also suggest that the transition from active to catch contractions is related to a decrease in myoplasmic free Ca ion concentration.

Radiation damage to bull sperm motility. III. Further x-ray studies

The results of previous radiation experiments, which indicated that the centriole serves as a control center for bull sperm motility, appear to be in conflict with experiments showing that the bull sperm flagellum is an autonomous oscillator. To resolve this conflict experiments were conducted to calibrate absolutely the dose-response curves for the radiation damage, and to measure the force production and the mechanochemical energy conversion after irradiation in bull sperm. The results indicate that the centriole acts as a mechanical anchor for the contractile fibers.

The structure of Mytilus smooth muscle and the electrical constants of the resting muscle

The individual muscle fibers of the anterior byssus retractor muscle (ABRM) of Mytilus edulis L. are uninucleate, 1.2-1.8 mm in length, 5 microm in diameter, and organized into bundles 100-200 microm in diameter, surrounded by connective tissue. Some bundles run the length of the whole muscle. Adjacent muscle cell membranes are interconnected by nexuses at frequent intervals. Specialized attachments exist between muscle fibers and connective tissue. Electrical constants of the resting muscle membrane were measured with intracellular recording electrodes and both extracellular and intracellular current-passing electrodes. With an intracellular current-passing electrode, the time constant tau, was 4.3 +/- 1.5 ms. With current delivered via an extracellular electrode tau was 68.3 +/- 15 ms. The space constant, lambda, was 1.8 mm +/- 0.4. The membrane input resistance, R(eff), ranged from 23 to 51 MOmega. The observations that values of tau depend on the method of passing current, and that the value of lambda is large relative to fiber length and diameter are considered evidence that the individual muscle fibers are electrically interconnected within bundles in a three-dimensional network. Estimations are made of the membrane resistance, R(m), to compare the values to fast and slow striated muscle fibers and mammalian smooth muscles. The implications of this study in reinterpreting previous mechanical and electrical studies are discussed.

Ca-coupling in the anterior byssal retractor muscle of Mytilus edulis L

1. Experiments designed to elucidate the role of Ca in the excitation-contraction coupling of the anterior byssus retractor muscle (ABRM) were carried out. Ca influences membrane depolarization and provides for coupling of the contraction in response to repetitive electrical stimulation as well as of ACh and KCl contracture. Depriving ABRM of Ca results in two closely correlated events: disappearance of action potential and of the contraction in response to repetitive electrical stimulation.2. A sigmoid increase in tension with the log-Ca concentration in artificial medium was observed whereas, over the same range of concentrations, the tension remnant decreased.3. Induction of relaxation by 5-HT is Ca dependent. Either thiourea inactivation or Ca deprivation results in failure to relax. Low concentrations of 5-HT (10(-7) g/ml.) bring about increase in peak tension of the contraction in response to repetitive electrical stimulation, whereas higher concentrations (10(-5) g/ml.) undermine peak tension.4. Frequencies exceeding 40 cycles evoke a contraction accompanied by tension remnant, which is eliminated with 5-HT.5. Dropwise addition of Ca on a trypsin window in the muscle induces a latency relaxation before onset of Ca-contracture.