Perfused capillary surface area in postural and locomotor skeletal muscle

A measure was made of the perfused capillary surface area per gram of tissue (Sf) in postural, anterior latissimus dorsi (ALD), and locomotor, posterior latissimus dorsi (PLD), skeletal muscles in chickens. The animals were anesthetized with L.A. Thesia and the ALD and PLD muscles were prepared for observation using a modification of the method developed by R. E. Klabunde and P. C. Johnson (1977, Amer. J. Physiol.232, H411–417). Sf was determined from the relationship Sf = πdLθf, where d = capillary diameter, L = capillary length, and θf = number of perfused capillaries per gram. Capillary lengths and diameters were measured directly through the microscope. Estimates of θf were measured in two ways: by a flow method (θQf), and by an histochemical method (θHf). It was observed that the average capillary diameter in PLD was the same as in ALD muscles, but the average capillary length in PLD was two times longer than in ALD muscles. Both the flow and histochemical methods yielded values of θf that gave similar values of Sf for PLD muscles (SQf = 24.7 ± 20.0 cm2/g, SHf = 13.7 ± 14.4 cm2/g), but different values for ALD muscles (SQf = 9.8 ± 6.8 cm2/g, SHf = 42.8 ± 19.1 cm2/g). The difference between SQf and SHf for ALD muscles could be explained by the more irregular and longer perfusion path observed in postural versus locomotor muscle. The flow method appears to underestimate Sf in capillary beds with irregular perfusion paths. The results indicate that only a small fraction of the total capillary bed was perfused in resting skeletal muscle and that SHf for ALD was approximately three times larger than SHf for PLD.