Region-specific adaptations in determinants of rat skeletal muscle oxygenation to chronic hypoxia.

Chronic exposure to hypoxia is associated with muscle atrophy (i.e., a reduction in muscle fiber cross-sectional area), reduced oxidative capacity, and capillary growth. It is controversial whether these changes are muscle and fiber type specific. We hypothesized that different regions of the same muscle would also respond differently to chronic hypoxia. To investigate this, we compared the deep (oxidative) and superficial (glycolytic) region of the plantaris muscle of eight male rats exposed to 4 wk of hypobaric hypoxia (410 mmHg, Po(2): 11.5 kPa) with those of nine normoxic rats. Hematocrit was higher in chronic hypoxic than control rats (59% vs. 50%, P < 0.001). Using histochemistry, we observed 10% fiber atrophy (P < 0.05) in both regions of the muscle but no shift in the fiber type composition and myoglobin concentration of the fibers. In hypoxic rats, succinate dehydrogenase (SDH) activity was elevated in fibers of each type in the superficial region (25%, P < 0.05) but not in the deep region, whereas in the deep region but not the superficial region the number of capillaries supplying a fiber was elevated (14%, P < 0.05). Model calculations showed that the region-specific alterations in fiber size, SDH activity, and capillary supply to a fiber prevented the occurrence of anoxic areas in the deep region but not in the superficial region. Inclusion of reported acclimatization-induced increases in mean capillary oxygen pressure attenuated the development of anoxic tissue areas in the superficial region of the muscle. We conclude that the determinants of tissue oxygenation show region-specific adaptations, resulting in a marked differential effect on tissue Po(2).

Oxygen transport and the function of myoglobin. Theoretical model and experiments in chicken gizzard smooth muscle.

We studied the steady-state oxygen transfer across thin layers of respiring chicken gizzard smooth muscle and compared three models for oxygen consumption with respect to their influence on the facilitation of oxygen diffusion by myoglobin. These models assumed zero-order, Michaelis-Menten or exponential kinetics. The transport equation was solved for these models with simultaneous oxygen facilitation assuming chemical equilibrium between oxygen and myoglobin. Experimental flux data were obtained in two situations: a) high oxygen pressure throughout the layer of tissue providing maximum oxygen consumption and oxygen permeability, and b) anoxic conditions in part of the layer and with submaximal oxygen consumption and desaturation of myoglobin. Measurements in the presence of functional myoglobin were compared with data obtained after abolishing the transport function of myoglobin by application of 1 kPa carbon monoxide. It was found that oxygen consumption interferes with the facilitation effect. The oxygen pressure at half maximum oxygen consumption in the Michaelis-Menten model was 0.3 +/- 0.1 (S.E) kPa. The facilitation of the oxygen transport by myoglobin was 50 to 100% of the maximum value to be expected on the basis of the prevailing myoglobin concentration.

Nonequilibrium facilitated transport of carbon dioxide in bicarbonate and bovine albumin solutions.

The nonlinear diffusion-reaction equations describing the nonequilibrium transport of CO2 through flat layers of complex but homogeneous aqueous media were solved by an approximate analytical method called the "Combined Damköhler Number" (CDN) technique. Unlike other approximate analytical solutions, the CDN technique is valid for the full range of Damköhler numbers, i.e., for any layer thickness. The present theoretical treatment uses as a basis the equilibrium approach of Stroeve, Hoofd, and Kreuzer which accounts for any species in the solution except for possible carbamate formation (binding of CO2 by the protein). The nonequilibrium model developed here for CO2 transport is the most general technique currently available in the literature. Theoretical results were compared to experimental data from the literature for diffusion in bicarbonate and albumin solutions and were generally in good agreement. Results obtained from numerical calculations were also compared and were found to be in excellent agreement with the CDN results.

Facilitated carbon dioxide transport in bovine albumin solutions.

Steady-state CO2 diffusion in the presence of a CO2 gradient was measured in thin layers of bovine albumin solutions containing different amounts of buffer base, added as NaHCO3, and/or of NaCl. In the same solutions, electrical potentials due to the CO2 diffusion across the layers were measured. Addition of carbonic anhydrase induced a chemical reaction equilibrium to exist for the CO2 reaction system, and led to equilibrium values for facilitated CO2 transport due to a bicarbonate flux and to maximum values for the diffusion potential. The diffusion potentials are generated due to the large differences in the ionic mobilities of albumin and other ionic species such as bicarbonate. The diffusion potential markedly reduces the facilitated CO2 flux. The presence of sodium chloride had no significant effect on the CO2 transport rate. The total mass transfer rates of CO2 in albumin solutions were considerably lower than those found by Stroeve and Ziegler (23) in hemoglobin solutions at identical concentrations of buffer base.

Differing patterns of capillary distribution in fish and mammalian skeletal muscle.

The heterogeneity of capillary supply to muscles of different metabolic capacity and fibre size was assessed in slow and fast muscles from a fish and a mammal. The area surrounding each capillary delineated by equidistant boundaries from adjacent vessels, the capillary domain, was derived from morphometric analysis of histological sections. This 2-D integration of intercapillary distances may reveal heterogeneity of supply that is hidden by a global approach, especially when compared with the more usual 0- and 1-D indices of capillarisation. Mean radii of the equivalent Kroghian tissue cylinders (R) and heterogeneity of their lognormal distribution, represented by the logarithmic standard deviation (LogSD), were calculated. In eel slow muscle there was a 35-fold greater capillary density (CD) than fast muscle (698 vs 20 mm-2) although heterogeneity of capillary spacing was similar (LogSD congruent to 0.06). The difference in CD between slow and fast muscles of rat was less pronounced, but there was significantly lower heterogeneity in the aerobic tissue (LogSD = 0.08 vs 0.10) corresponding to a range in domain area of around 350-2300 microns 2 and 400-2900 microns 2, respectively. The overall capillary to fibre ratio (C:F) is inappropriate for sparse networks where many fibres lack direct capillary contact. The cumulative fraction of individual domains overlapping a muscle fibre (local capillary to fibre ratio, LCFR) plotted against fibre area showed the best correlation of any index in all tissue and was strongest in both fish muscles (r = 0.9), indicating a functionally homologous spatial distribution of capillaries with respect to muscle fibres in tissue of widely differing oxidative capacity. These data suggest that maximal oxygen supply to, or metabolite removal from, muscle fibres is not restricted to contiguous capillaries but also involves those remote from the fibre surface.

Non-steady-state O(2) diffusion in metmyoglobin solutions studied in a diffusion chamber.

In this study, we studied the "passive" diffusion through myoglobin solutions by determining the oxygen diffusion coefficient (DO(2)) and the oxygen permeability (permeability O(2)) of metmyoglobin (metMb) solutions (3-33 g. 100 mL(-1)) at 25 degrees C. These oxygen diffusion parameters were determined in a diffusion chamber using a non-steady-state method and were also determined of albumin solutions (4-32 g. 100 mL(-1)) and distilled water for comparison. From these parameters, the oxygen solubility (alphaO(2)) could be calculated, because alphaO(2) = permeabilityO(2)/DO(2). Both DO(2) and permeabilityO(2) decreased with increasing metMb and albumin concentration. The values of DO(2), permeabilityO(2) and alphaO(2) of both metMb solutions and albumin solutions were comparable with literature values of methemoglobin (metHb) and serum protein solutions. The values of the metMb solutions can be used in following studies of facilitated oxygen diffusion through myoglobin solutions.

The relationship between capillarisation and fibre types during compensatory hypertrophy of the plantaris muscle in the rat.

Compensatory hypertrophy of the plantaris muscle was obtained by denervation of its synergists. This hypertrophy is characterised by a 32% increase in muscle mass. The muscle consists of type I and IIa (oxidative), and IIb (glycolytic) fibres. Fibres of all types were enlarged in hypertrophied muscles and the proportion of type I fibres was increased. We investigated the capillarisation after hypertrophy as related to fibre type. In order to obtain this information a new technique was used, capable of estimating not only the traditional overall capillary density (CD) but also an index of heterogeneity in capillary spacing (LogSD), the 'local capillary to fibre ratio' (LCFR), obtained separately for each muscle fibre type, and finally a capillary density for each respective fibre type, the 'capillary fibre density' (CFD). It was found in both control and hypertrophied muscles that CD was higher in the deep (few IIb fibres) than in the superficial part of the muscle (considerable number of IIb fibres). The LogSD was lower, indicating less heterogeneity, in the deep than in the superficial part of the muscle. The LCFR and CFD of each fibre type was greater in the deep than in the superficial region of both control and hypertrophied muscles. Furthermore the CFD and LCFR were larger in type I and IIa fibres than in IIb fibres in each region of control and hypertrophied muscles. In hypertrophied muscles the CD was not significantly different from that of control muscles. However, LCFR of all fibre types was increased significantly in hypertrophied muscles as compared with controls, demonstrating capillary proliferation. The decreased CFD of type I and IIa fibres in the deep region of hypertrophied muscles as compared with controls suggests that here the capillary proliferation lags behind the increase in muscle mass. Endurance training had no significant effects for any region in any of the indices that were used.

Metabolic capacity, fibre type area and capillarization of rat plantaris muscle. Effects of age, overload and training and relationship with fatigue resistance.

1. The influences of age (5, 13 and 25-month-old rats), overload as obtained by denervation of synergists, and training on the metabolic capacity, relative muscle cross-sectional area occupied by each fibre type, capillarization and fatigue resistance of the rat m. plantaris were investigated. 2. Creatine kinase, phosphorylase and citrate synthase activities were lower in muscles of 25 than in those of 13-month-old rats (P < 0.001). 3. Overload resulted in an increased relative area of type I and IIa fibres at all ages (P = 0.001). 4. Capillary density decreased with overload and increasing age (P < 0.001). 5. Fatigue resistance was higher in muscles of 13 than in those of 5-month-old rats (P < 0.05), and increased with overload (P < 0.05) at all ages. 6. Fatigue resistance of the whole muscle was not closely related to its oxidative capacity in contrast to what is generally found for single fibres or motor units.