Muscle structure and theories of contraction.

Assay of proteins in the presence of interfering materials.

One can trace to Archimedes the idea that in a system consisting of two additive components which are mixed but the densities of which are known ( d l , d 2 ) , the determination of the density of the system ( D ) allows one to calculate the proportional masses of the two components. Let’s denote these components as W1 and W2.S Then, in a system with total weight W = W1 + Wz, the general equation for calculating component W1 expressed as a fraction (w1) of the total body weight is:

Densitometric analysis of body composition: revision of some quantitative assumptions*

The molecular origin of standard metabolic rate and thermogenesis in mammals is examined. It is pointed out that there are important differences and distinctions between the cellular reactions that 1) couple to oxygen consumption, 2) uncouple metabolism, 3) hydrolyze ATP, 4) control metabolic rate, 5) regulate metabolic rate, 6) produce heat, and 7) dissipate free energy. The quantitative contribution of different cellular reactions to these processes is assessed in mammals. We estimate that approximately 90% of mammalian oxygen consumption in the standard state is mitochondrial, of which approximately 20% is uncoupled by the mitochondrial proton leak and 80% is coupled to ATP synthesis. The consequences of the significant contribution of proton leak to standard metabolic rate for tissue P-to-O ratio, heat production, and free energy dissipation by oxidative phosphorylation and the estimated contribution of ATP-consuming processes to tissue oxygen consumption rate are discussed. Of the 80% of oxygen consumption coupled to ATP synthesis, approximately 25-30% is used by protein synthesis, 19-28% by the Na(+)-K(+)-ATPase, 4-8% by the Ca2(+)-ATPase, 2-8% by the actinomyosin ATPase, 7-10% by gluconeogenesis, and 3% by ureagenesis, with mRNA synthesis and substrate cycling also making significant contributions. The main cellular reactions that uncouple standard energy metabolism are the Na+, K+, H+, and Ca2+ channels and leaks of cell membranes and protein breakdown. Cellular metabolic rate is controlled by a number of processes including metabolic demand and substrate supply. The differences in standard metabolic rate between animals of different body mass and phylogeny appear to be due to proportionate changes in the whole of energy metabolism. Heat is produced by some reactions and taken up by others but is mainly produced by the reactions of mitochondrial respiration, oxidative phosphorylation, and proton leak on the inner mitochondrial membrane. Free energy is dissipated by all cellular reactions, but the major contributions are by the ATP-utilizing reactions and the uncoupling reactions. The functions and evolutionary significance of standard metabolic rate are discussed.

Cellular energy utilization and molecular origin of standard metabolic rate in mammals

Calcium ion and muscle contraction.

Segmental flexibility of the S-1 moiety of myosin.

On the molecular weight of myosin. II.

Analytical description of the effects of modifiers and of enzyme multivalency upon the steady state catalyzed reaction rate

This study provides additional evidence that creatine, an end product of contraction unique to muscle, is involved in the control of muscle protein synthesis. Creatine is shown to stimulate selectively the rate of synthesis of two major contractile proteins, actin and myosin heavy chain, in cultures of differentiating skeletal muscle. Creatine affects only the rate of synthesis and not the rate of degradation. Several creatine analogs are as effective as creatine in stimulating muscle protein synthesis, creatinine and amino acids such as arginine and glycine are not. Creatine stimulates myosin heavy chain synthesis twofold in cultures of embryonic muscle grown in either normal or dialyzed media.

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Specificity of creatine in the control of muscle protein synthesis

These experiments provide evidence that creatine, an end product of contraction unique to muscle, is involved in the control of muscle-protein synthesis. Skeletal muscle cells formed both in vitro and in vivo synthesize myosin heavy chain faster when supplied creatine in vitro. The response is apparent within four hours after addition of creatine to the culture medium, and is dependent on concentration over a range of 10-100 μM creatine. The effect seems to be selective for cell-specific proteins(s), since the rate of total protein synthesis is unaffected.

Manuel F. Morales

Papers

Segmental flexibility of the S-1 moiety of myosin.

On the molecular weight of myosin. II.

Analytical description of the effects of modifiers and of enzyme multivalency upon the steady state catalyzed reaction rate

Specificity of creatine in the control of muscle protein synthesis

Creatine and the Control of Myosin Synthesis in Differentiating Skeletal Muscle