title: Excitation-contraction coupling in muscles of the Norway lobster Nephrops norvegicus
Creators: name: Holmes, Janet
Publisher: name: University of Glasgow
Year: 1997
Subjects: subject: Physiology.

description: Crustacean muscle fibres are classified into tonic and phasic types. Within this classification a further subdivision of fibres can be made on the basis of histochemical and biochemical techniques, and electron microscopy. While some muscles are composed of fibres with uniform morphological and physiological properties (homogenous muscles), others muscles comprise mixed populations of fibres with different properties (heterogeneous muscles). Crustacean muscles have been used extensively as a model system in the study of the excitation-contraction (EC) coupling mechanism, but few studies have related the properties of EC coupling to the differences between fibre types, mainly due to difficulties in fibre type identification following physiological measurements. Working on the heterogeneous population of S1 and S2 fibres found in the lateral and medial bundles of the superficial flexor (SF) muscle of the Norway lobster, Nephrops norvegicus overcomes this problem of relating structure to function since the fibre types have been identified on the basis of several criteria. This project uses the SF muscle system to investigate the EC coupling properties of two slow fibre phenotypes. Experiments on membrane-intact fibres of the SF muscle, both as intact nerve-muscle preparations and as single fibres, demonstrate that the EC coupling mechanism in the medial and lateral bundles of the SF muscle is Ca2+-induced Ca2+-release (CICR) since both the flux of Ca2+ ions across the sarcolemma and the release of Ca2+ ions from the sarcoplasmic reticulum (SR) are essential for muscle contraction, and since the external Ca2+ ions themselves cannot directly activate the myofibrillar proteins (indicating the involvement of an intermediate step-release of Ca2+ from the SR). Evidence for this includes: (1) an inhibition of the subsequent steps of CICR on removal of external Ca2+ from the saline, (2) depletion of the SR of Ca2+ by repeated application of caffeine, and (3) the application of pharmacological agents: tetracaine blocks the release of Ca2+ ions from the SR. and 2,3-butanedione monoxime acts in a similar manner to caffeine to induce the release of Ca2+ ions from the SR in the absence of external Ca2+. Based on experiments conducted on the whole muscle bundle preparation, the medial and lateral muscle bundles express similar EC coupling properties. However, studies using the two electrode voltage clamp technique indicate that the S1 and S2 fibre types express different populations of sarcolemmal ion channels. Differences were observed in both the inward Ca2+ currents the outward K+ currents. The heavy metal ion Mn2+ has been used as a tool to investigate the involvement of Ca2+ ions in the different steps of EC coupling, both at the level of the sarcolemmal ion channels and in the activation of the myofibrillar proteins. Experiments conducted on the nerve-muscle preparation show that Mn2+ at high concentrations (above) inhibits the neuronally-evoked muscle force and the force evoked by electrical field stimulation in a dose-dependent manner, and at concentrations above 10 mM blocks the K+-induced force. (Abstract shortened by ProQuest.).