The histochemical demonstration of myoglobin in muscle spindles

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

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.

Myoglobin concentration in single type I and type II muscle fibres in man

Histochemical visualization of myoglobin in a benzidine peroxidase reaction suggests that human skeletal muscle fibres are differentiated into fibres having either high or a low myoglobin content. In the present study myoglobin was quantified in single human muscle fibres after being classified as either type I ("slow-twitch") or type II ("fast twitch"). Samples were obtained from m. quadriceps femoris in 4 healthy untrained male subjects using the needle biopsy technique. After freeze-drying, individual fibres were dissected out and classified as either type I or type II by a myofibrillar ATPase stain. Myoglobin analyses were performed on these single fibres by a radioimmunoassay. The myoglobin concentrations were found to be significantly higher in type I than in type II muscle fibres in all 4 subjects and were, on the average, of the same magnitude as found in larger (mixed muscle) samples. The myoglobin concentration ratio between type I and type II fibres ranged from 1.4 to 1.7.

The blood supply of muscle spindles in some mammals and the pigeon

The blood supply of muscle spindles was studied in serial cross sections in macaque, cat, rabbit, guinea pig, mouse and pigeon muscles which had been incubated in a medium containing 3,3' diaminobenzidine. Lumina of blood vessels were recognized by the reaction product that was localized within erythrocytes. The outer capsule was well vascularized, but few or no capillaries were seen in the periaxial space. The inner spindle capsule, which closely invests the axial bundle, was rarely contacted by periaxial capillaries at the equator and juxtequator. Capillaries occurred more frequently adjacent to intrafusal fibers at the polar region and beyond the end of the outer capsule. Shorter diffusion distances and, usually, higher capillary densities were found at the polar region than at the spindle midsection. This suggests that transcapillary exchange at the polar segment is nearer to conditions prevalent in extrafusal muscle than elsewhere in the spindle, provided the inner and outer capsules are not less permeable at the poles than at the midsection. Differences in blood supply among mammalian species appear to be related to receptor size.