Measurement of myoglobin diffusivity in the myoplasm of frog skeletal muscle fibres.

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347

The heart and those striated muscles that contract for long periods, having available almost limitless oxygen, operate in sustained steady states of low sarcoplasmic oxygen pressure that resist change in response to changing muscle work or oxygen supply. Most of the oxygen pressure drop from the erythrocyte to the mitochondrion occurs across the capillary wall. Within the sarcoplasm, myoglobin, a mobile carrier of oxygen, is developed in response to mitochondrial demand and augments the flow of oxygen to the mitochondria. Myoglobin-facilitated oxygen diffusion, perhaps by virtue of reduction of dimensionality of diffusion from three dimensions towards two dimensions in the narrow spaces available between mitochondria, is rapid relative to other parameters of cell respiration. Consequently, intracellular gradients of oxygen pressure are shallow, and sarcoplasmic oxygen pressure is nearly the same everywhere. Sarcoplasmic oxygen pressure, buffered near 0.33·kPa (2.5·torr; equivalent to approximately 4·µmol·l -1 oxygen) by equilibrium with myoglobin, falls close to the operational Km of cytochrome oxidase for oxygen, and any small increment in sarcoplasmic oxygen pressure will be countered by increased oxygen utilization. The concentration of nitric oxide within the myocyte results from a balance of endogenous synthesis and removal by oxymyoglobin-catalyzed dioxygenation to the innocuous nitrate. Oxymyoglobin, by controlling sarcoplasmic nitric oxide concentration, helps assure the steady state in which inflow of oxygen into the myocyte equals the rate of oxygen consumption.Key words: myoglobin, oxygen, facilitated diffusion, dimensionality in diffusion, heart, red skeletal muscle, nitric oxide, mitochondria, cytochrome oxidase, Krogh cylinder. Here, we describe myoglobin-augmented oxygen supply to heart and red muscle, taking into account their threedimensional structures and the elevated concentration of myoglobin in the cytoplasmic domain to which it is restricted and recognizing the large area of mitochondrial surface available for oxygen diffusion. [A mathematical formulation of oxygen diffusion in the cardiac myocyte will be presented elsewhere.] Heart and muscle, having available an almost unlimited supply of oxygen, actually operate at controlled low oxygen pressure, at or near 0.33·kPa (2.5·torr), where myoglobin is about half-saturated with oxygen. Partial saturation of myoglobin enables oxymyoglobin to play a pivotal role; by converting endogenous nitric oxide to the innocuous nitrate, oxymyoglobin controls the level of nitric oxide (NO) within the cell. This, in turn, may control both the rate of capillary oxygen delivery to the cell and the rate of oxygen utilization by cytochrome oxidase. Summary Introduction Review Myoglobin function reassessed Formulations of oxygen diffusion in musclePresent descriptions of oxygen diffusion/transport in tissues originate from the studies of Krogh, Hill and Jeffries Wyman, to whom this essay is dedicated. Krogh (1919a,b) and later Hill (1928) considered that oxyge...

Section: Diffusivity Of Myoglobinmentioning

confidence: 83%

Myoglobin function reassessed

Wittenberg

2003

460

347

“…In addition, it is possible that intracellular convection associated with contraction may enhance intracellular transport (Hochachka, 1999;Suarez et al, 2003). There has been limited experimental testing of this hypothesis, although the D of several proteins was not increased by muscle contraction or passive stretching and shortening (Baylor and Pape, 1988;Papadopoulos et al, 2000). To our knowledge the influence of contraction on the diffusion of small metabolites has not been addressed, and further study in this area is warranted.…”

Section: The Intracellular Environment Of Muscle Has Characteristics mentioning

confidence: 91%

“…The diffusion of Mb in skeletal muscle has been of particular interest because of its role in reversibly binding O 2 and therefore serving as a temporary O 2 store and facilitating O 2 diffusion (reviewed in Wittenberg and Wittenberg, 2003). Several studies have found that D of Mb in muscle fibers is about 1/6 to 1/10 of that in aqueous solution (Baylor and Pape, 1988;Jurgens et al, 1994;Papadopoulos et al, 1995;Papadopoulos et al, 2000;Papadopoulos et al, 2001). Measurements of protein diffusion in skeletal muscle from frog (Maughan and Lord, 1988;Maughan and Godt, 1999) and cultured mammalian fibers (Arrio-Dupont et al, 1997;Arrio-Dupont et al, 2000) yielded D values that were minimally 1/3 lower than that in water, and in many cases much more dramatically reduced.…”

Section: The Intracellular Environment Of Muscle Has Characteristics mentioning

confidence: 99%

Molecules in motion: influences of diffusion on metabolic structure and function in skeletal muscle

Kinsey

Locke

Dillaman

2011

When examining a single tissue type, D is comparatively invariant over physiological time scales, so the extent to which diffusion influences cell structure and function is largely governed by the interaction between diffusion distance and reaction flux rate. In skeletal muscle fibers rates of reaction fluxes and diffusion distances can vary over several orders of magnitude. For instance, aerobic metabolic rate in white muscle from fishes and crustaceans Accepted 25 August 2010 Summary Metabolic processes are often represented as a group of metabolites that interact through enzymatic reactions, thus forming a network of linked biochemical pathways. Implicit in this view is that diffusion of metabolites to and from enzymes is very fast compared with reaction rates, and metabolic fluxes are therefore almost exclusively dictated by catalytic properties. However, diffusion may exert greater control over the rates of reactions through: (1) an increase in reaction rates; (2) an increase in diffusion distances; or (3) a decrease in the relevant diffusion coefficients. It is therefore not surprising that skeletal muscle fibers have long been the focus of reaction-diffusion analyses because they have high and variable rates of ATP turnover, long diffusion distances, and hindered metabolite diffusion due to an abundance of intracellular barriers. Examination of the diversity of skeletal muscle fiber designs found in animals provides insights into the role that diffusion plays in governing both rates of metabolic fluxes and cellular organization. Experimental measurements of metabolic fluxes, diffusion distances and diffusion coefficients, coupled with reaction-diffusion mathematical models in a range of muscle types has started to reveal some general principles guiding muscle structure and metabolic function. Foremost among these is that metabolic processes in muscles do, in fact, appear to be largely reaction controlled and are not greatly limited by diffusion. However, the influence of diffusion is apparent in patterns of fiber growth and metabolic organization that appear to result from selective pressure to maintain reaction control of metabolism in muscle.

“…The first measurement of Mb diffusion in an intact muscle cell was that of Baylor and Pape, who microinjected MetMb into frog muscle fibers and measured spectrophotometrically the MetMb concentration profile along the longitudinal fiber axis at various times (Baylor and Pape, 1988 (Jürgens et al, 1994). The diffusion distances observed by them were 70-500m in the longitudinal direction.…”

Section: Spectrophotometric Determination Of Mb Translational Diffusionmentioning

confidence: 99%

Myoglobin's old and new clothes: from molecular structure to function in living cells

Gros

Wittenberg

Jue

2010

102

SUMMARY Myoglobin, a mobile carrier of oxygen, is without a doubt an important player central to the physiological function of heart and skeletal muscle. Recently, researchers have surmounted technical challenges to measure Mb diffusion in the living cell. Their observations have stimulated a discussion about the relative contribution made by Mb-facilitated diffusion to the total oxygen flux. The calculation of the relative contribution, however, depends upon assumptions, the cell model and cell architecture, cell bioenergetics, oxygen supply and demand. The analysis suggests that important differences can be observed whether steady-state or transient conditions are considered. This article reviews the current evidence underlying the evaluation of the biophysical parameters of myoglobin-facilitated oxygen diffusion in cells, specifically the intracellular concentration of myoglobin, the intracellular diffusion coefficient of myoglobin and the intracellular myoglobin oxygen saturation. The review considers the role of myoglobin in oxygen transport in vertebrate heart and skeletal muscle, in the diving seal during apnea as well as the role of the analogous leghemoglobin of plants. The possible role of myoglobin in intracellular fatty acid transport is addressed. Finally, the recent measurements of myoglobin diffusion inside muscle cells are discussed in terms of their implications for cytoarchitecture and microviscosity in these cells and the identification of intracellular impediments to the diffusion of proteins inside cells. The recent experimental data then help to refine our understanding of Mb function and establish a basis for future investigation.