Microvascular supply of skeletal muscle fibres. A microangiographic, histochemical and intravital microscopic study of hind limb muscles in the rat, rabbit and cat

Abstract

A vessel arrangement, similar to that found in tenuissimus, is present as a 'basic unit' even in the thicker hind limb muscles. The average cross- sectional area of this unit in the MG muscle is in the range of the total cross-sectional area of the tenuissimus (about 1 mm2). This value is about 80% greater in the biceps femoris and the LG muscles, but 50% less in the soleus. Generally, all fibers in the MG are 20-40% larger than the corresponding fibers in the tenuissimus, the biceps and the LG. The soleus has quite large SO fibers (about 70 μm in diameter) as compared to this type of fiber (25-45 μm) in the other muscles. Cat muscles with large muscle fibers have a higher number of capillaries per fiber (1-2) than muscles with a smaller average fiber diameter (relative value, less than 1 capillary). In spite of a larger diameter, the FG fibers are surrounded by fewer capillaries (1-3) as compared to the FOG, and the SO fibers (2-4). It seems most likely that the oxidative capacity of the fiber is more important than the fiber diameter as a regulatory factor for the fiber capillarization. The tenuissimus muscle does not differ from thicker muscles in respect of the vascular arrangement and the capillary supply of muscle fibers, although the tenuissimus seems to be of minor importance for locomotion. The rat EHP is a typical locomotor muscle and is also quite suitable for combined intravital and histochemical studies. FG fibers dominate in this muscle. The vascular bed has an arrangement similar to that of the tenuissimus. However, the capillaries have fewer interconnections, which is an advantage for evaluations of the TCSA. Chronic muscle stimulation of fast contracting muscles at 10 Hz results in widening of the capillaries, especially close to the collecting venules. The capillary density is also increased. These rearrangements of the capillary bed enhance the TCSA in the rat EHP by 46% after 14 days of stimulation. Growth of new capillaries in the stimulated muscles was observed as sprouts emanating from pre-existing capillaries. The proportion of sprouts to pre-existing capillaries is about 40% after 14 days of stimulation.