Catch and the mechanism of action of 5-hydroxytryptamine on molluscan muscle: a speculation

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.

Catch-relaxing peptide (CARP) decreases the Ca-permeability of snail neuronal membrane

Effect of CARP was investigated on snail neurones. It was found that CARP decreased the slowly inactivating Ca- and the Ca-activated K-currents.

The possible role of serotonin in the rhythmicity of the crop of Aplysia dactylomela

The isolated perfused crop of Aplysia dactylomela was used to assess the effects of applied serotonin on 1st order small phasic contractions, 2nd order large tonic contractions, and 3rd order slow changes in tonus. Very low concentrations of serotonin may potentiate spontaneous contractions. Concentrations of the order of 10(-7) M serotonin may regularize "beating" of the crop by suppressing large tonic contractions while potentiating phasic contractions. Concentrations higher than 10(-5) M serotonin induce prolonged tonic contracture.

Alteration in biogenic amine levels in the snail Lymnaea acuminata by the latex of Euphorbia royleana

The latex of Euphorbia royleana which has high molluscicidal and anti-cholinesterase activity against Lymnaea acuminata reduced the levels of 5-hydroxytryptamine (5HT) and dopamine (DA) in the nervous tissue of L. acuminata. There was, however, no significant change in the level of 5-hydroxyindole acetic acid (5HIAA). The changes were found to be dependent on concentration of the latex extract. Similar changes were produced by both water and organic solvent extracts of the latex. The latex of E. royleana thus affects all the known neurotransmission mechanisms in the snail either separately or through a complex interaction between the different neurotransmitters. This may account for its high toxicity to snails.

The action of serotonin on calcium-45 efflux from the anterior byssal retractor muscle of Mytilus edulis

(45)Ca efflux was studied in resting anterior byssal retractor muscle. The data are described by a three-compartment system. The most rapidly exchanging compartment, with an average time constant of 7 min, contains about 0.9 mM Ca/liter muscle, and probably represents extracellular space. A second compartment, with a time constant of 83 +/- 5 min, contains 1.2 mM Ca/liter, and may represent a membrane calcium store. The presence of a third, or more, compartments, probably representing sarcoplasmic reticulum and contractile proteins, is indicated by the fact that the final time constant is 10 times the 83 min time constant of the second compartment. Serotonin (5HT), on initial application, increases (45)Ca efflux from this third compartment(s). This effect has a typical dose-response relationship with a maximum response appearing at 10(-7)M5HT. In addition, removal of 5HT causes a secondary increase in (45)Ca efflux which has a maximum at a 5HT concentration of 10(-7)M and declines at both higher and lower doses.

The action of 5-hydroxytryptamine on Mytilus smooth muscle

1. In the nerve-muscle preparation, where catch was characteristically minimal, 5-hydroxytryptamine (5-HT) had no effect on resting membrane potential, junction potentials, spikes or contraction.2. In muscle bundles, where catch was prominent, 5-HT did not change membrane potentials, but prolonged junction potentials and lowered the threshold for spike discharge and contraction.3. In muscle bundles, exposed to high concentrations of 5-HT, depolarization evoked repetitive spikes, while in low 5-HT, spikes were seldom fired even with much greater depolarization.4. In muscle bundles, the effective membrane resistance, R(eff.), decreased from 45-60 to 23-35 MOmega as 5-HT concentration was increased.5. It is suggested that 5-HT may facilitate spike discharge by lowering the internal free Ca(2+) concentration.

Factors influencing contraction and catch in Mytilus smooth muscle

1. Conditions are defined which determine the level of catch after acetylcholine stimulation of Mytilus muscle.2. Catch tension in dissected muscle is absent when connexions with ganglia are intact.3. Catch tension is absent at temperatures above 30 degrees C.4. Catch tension decreases when intervals between stimuli are increased.5. Increasing concentrations of 5-hydroxytryptamine (5-HT) from 10(-8)M to 10(-6)M quantitatively decreases catch tension.6. The length-tension curve of ganglion-free Mytilus muscle bundles suggests that catch tension varies in proportion to the tension developed in contraction.7. External Ca(2+) concentration has no selective influence on catch.8. All factors which reduce catch also increase muscle excitability, suggesting that catch may depend on a mechanism controlling the intracellular concentration of an activator such as Ca(2+).