A kinetic model of muscle contraction and its oscillatory characteristics

Summary A set of five differential equations has been found which gives a satisfactory account of the isotonic and isometric properties of striated muscle. Four of these differential equations give an equally satisfactory account of the results of length-drive experiments with sinusoidal variation of length. In this case, the fifth equation (of motion) is redundant. These sets of equations predict a number of results not yet measured relating to the superposition of oscillatory length changes on isotonic contraction.The equations predict correctly the variation of tension with time when the amplitude of the driven oscillation increases beyond the region where it can be treated as a perturbation, and the deviation of the mean tension per cycle from the steady-state tension for isotonic contraction with superimposed oscillations in length or velocity. The equations can be derived rigorously from a more complex set of eight equations which themselves are formulated from the basic principles of chemical physics, the theory of molecular force fields and radiationless transitions. The reduced model may be consistent with many other molecular theories and its predictive success does not prove the correctness or otherwise of the level 1 assumptions of the seven-state theory. By the same token, macroscopic mechanical experiments of the type presently carried out cannot give information on level 1 questions such as the existence or otherwise of binding of crossbridges to the thin filament. The experimental kinetic results can be described with or without this assumption. The theory needs considerable development in so far as it does not consider elastic elements at all at present, nor have detailed conclusions yet been extracted from the equations for the case of stretching, except for isotonic steady states where agreement is encouraging.     

Involvement of small GTPases in the regulation of smooth muscle contraction

Neurohumoral stimulation of smooth muscle leads to an increased responsiveness of the myofilaments to Ca²⁺. This review provides a summary of the data that suggest that the signalling from the membrane-bound serpentine receptors to the contractile apparatus leading to the increase in Ca²⁺-sensitivity requires the activation of the Ras-related low molecular mass GTPase Rho. In smooth muscle permeabilized with α-toxin or β-escin, the increase in force elicited by different agonists at fixed [Ca²⁺] (Ca²⁺-sensitization) can be inhibited by bacterial toxins (EDIN, and exoenzyme C3) which ADP-ribosylate and inactivate Rho proteins. Moreover, the agonist-induced increase in Ca²⁺-sensitivity can be mimicked by constitutively active recombinant Rho proteins. The physiological relevance of this mechanism is suggested by the fact that toxins that are internalized into intact cells [toxin B from C. difficile and a chimeric toxin (DC3B) consisting of C3 and the (non-catalytic) B fragment of diphteria toxin (inhibit the tonic phase of an agonist-induced contraction. Toxin B inhibits contraction without affecting the intracellular Ca²⁺-transient determined with fura-2. However, it inhibits phosphorylation of the regulatory light chains of myosin (MLC). Rho has been suggested to activate a Rho-associated kinase which in turn phosphorylates the myosin binding subunit of the myosin light chain phosphatase. This would lead to an increase in phosphorylation of MLC and hence of force at constant Ca²⁺. The Ca²⁺-sensitizing effect of agonists is also inhibited by tyrosine kinase inhibitors. This suggests the possibility that in smooth muscle, like in non-muscle cells, there is a cross-talk between Rho and tyrosine kinases.     

The role of tropomyosin-troponin in the regulation of skeletal muscle contraction

Summary Steric blocking of actin-myosin interaction by tropomyosin has been a working hypothesis in the study of the regulation of skeletal muscle contraction, yet the simple movement of actin-associated tropomyosin from a myosin-blocking position (relaxation) to a nonblocking position (contraction) cannot adequately account for all of the biophysical and biochemical observations which have been made to date. Ambiguous assignment of tropomyosin positions on actin during contraction, due in part to the limited resolution of reconstruction techniques, may also hint at a real lack of clearcut on and off positioning of tropomyosin and tropomyosin-troponin complex. Recent biochemical evidence suggests processes relatively independent of tropomyosin-troponin may have a governing effect on contraction, involving kinetic constraints on actin-myosin interaction influenced by the binding of ATP and the intermediates of ATP hydrolysis. Based on our current understanding put forth in this review, it is clear that regulatory interactions in muscle contraction do not consist solely of steric effects but involve kinetic factors as well. Where the latter are being defined in systems reconstituted from purified proteins and their fragments, the steric components of regulation are most clearly observed in studies of structurally more intact physiologic systems (e.g. intact or skinned whole muscle fibres). The fine detail of the processes and their interplay remains an intriguing question. Likewise, the precise physical relationship of myosin with actin in the crossbridge cycle continues to elude definition. Refinement of several methodologies (X-ray crystallography, three-dimensional reconstruction, time-resolved X-ray diffraction) will increase the potential for detailing the molecular basis of the regulation of muscle contraction.     

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 14.5nm) 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.     

The effect of adrenalin infusion on the regulation of glycogenolysis in human muscle during isometric contraction

The regulation of glycogenolysis in human muscle during isometric contraction without and with adrenalin infusion has been investigated. The content of cAMP in muscle increased three-fold during the infusion. Total glycogen phosphorylase and synthetase activities were unchanged during contraction without and with adrenalin infusion. The fraction of phosphorylase in the a form was in resting muscle 26% and at the end of contraction 24%. During adrenalin infusion phosphorylase a increased to 80%. Contraction during continued infusion resulted in a decrease of phsophorylase a to 42%, despite persistently increased cAMP content in muscle. The activity of synthetase I decreased to about half of the initial value during adrenalin infusion and contraction both without and with the infusion. The rate of glycogenolysis in muscle during contraction was not significantly changed by the infusion. Phosphocreatine (PCr) decreased during the contraction and the decrease was similar without and with adrenalin infusion. The amount of inorganic phosphate (P₁) accumulated in muscle during contraction was lower when adrenalin was given due to a greater accumulation of hexosemonophosphates. It is concluded that the rate of glycogenolysis in muscle during contraction without and with adrenalin infusion is a function both of phosphorylase in the form a form and the availability of Pj at the active site of the enzyme.     

The effect of adrenaline infusion on the regulation of glycogenolysis in human muscle during isometric contraction

The regulation of glycogenolysis in human muscle during isometric contraction without and with adrenaline infusion has been investigated. The content of cAMP in muscle increased three-fold during the infusion. Total glycogen phosphorylase and synthetase activities were unchanged during contraction without and with adrenaline infusion. The fraction of phosphorylase in the a form was in resting muscle 26% and at the end of contraction 24%. During adrenaline infusion phosphorylase a increased to 80%. Contraction during continued infusion resulted in a decrease of phosphorylase a to 42%, despite persistently increased cAMP content in muscle. The activity of synthetase I decreased to about half of the initial value during adrenaline infusion and contraction both without and with the infusion. The rate of glycogenolysis in muscle during contraction was not significantly changed by the infusion. Phosphocreatine (PCr) decreased during the contraction and the decrease was similar without and with adrenaline infusion. The amount of inorganic phosphate (Pi) accumulated in muscle during contraction was lower when adrenaline was given due to a greater accumulation of hexose-monophosphates. It is concluded that the rate of glycogenolysis in muscle during contraction without and with adrenaline infusion is a function both of phosphorylase in the form a form and the availability of Pi at the active site of the enzyme.     

Ca2+-dependent and Ca2+-independent regulation of smooth muscle contraction

An increase in the cytosolic Ca²⁺ concentration is a prerequisite in activation of contractile activity of smooth muscle. The shape of the Ca²⁺-signal is determined by spatial distribution and kinetics of Ca²⁺-binding sites in the cell. The increase in cytosolic Ca²⁺ activates myosin light chain kinase (MLCK) which in turn phosphorylates the regulatory light chains of myosin II. This Ca²⁺-dependent MLC₂₀ phosphorylation is modulated in a Ca²⁺-independent manner by inhibiting the constitutive active myosin light chain phosphatase mediated by the monomeric GTPase Rho and the Rho-associated kinase as well as protein kinase C or by increasing its activity through cGMP. Furthermore, the activity of MLCK may be decreased due to phosphorylation by CaM kinase II and perhaps p21 activated protein kinase. Hence, smooth muscle tone appears to be regulated by a network of activating and inactivating intracellular signaling cascades which not only show a temporal but also a spatial activation pattern.     

The regulation of muscle glycogen: the granule and its proteins

Despite decades of studying muscle glycogen in many metabolic situations, surprisingly little is known regarding its regulation. Glycogen is a dynamic and vital metabolic fuel that has very limited energetic capacity. Thus its regulation is highly complex and multifaceted. The stores in muscle are not homogeneous and there appear to be various metabolic pools. Each granule is capable of independent regulation and fundamental aspects of the regulation appear to be associated with a complex set of proteins (some are enzymes and others serve scaffolding roles) that associate both with the granule and with each other in a dynamic fashion. The regulation includes altered phosphorylation status and often translocation as well. The understanding of the roles and the regulation of glycogenin, protein phosphatase 1, glycogen targeting proteins, laforin and malin are in their infancy. These various processes appear to be the mechanisms that give the glycogen granule precise, yet dynamic regulation.