Respiration Physiology, 81 (1990) 291-302 291 Elsevier RESP 01693 Maximal diffusion-distance within skeletal muscle can be estimated from mitochondrial distributions Richard L. Londraville and Bruce D. Sidell University of Maine, Department of Zoology and Center for Marine Studies, Murray Hall, Orono, Maine. U.S.A. Abstract. Mitochondrial volume density (V,[mit]) distributions were measured with a test pattern of concentric rings centered upon randomly chosen capillaries in oxidative skeletal muscle ceils of two Antarctic fishes, Trematomus newnesi and Notothenia gibberifrons. Vv(mit ) in both species was highest in the ring closest to the capillary, minimal further from the capillary (at a distance that was characteristic for each species), and rose in the annuli furthest from the capillary. Plots of Vv(mit) against total area between each ring and the central capillary fit the form of a second-order polynomial (r2> 0.9). If Po2 or blood-borne metabolite concentration predicates the pattern of Vv(mit ) distribution, minimal Vv(mit ) is at the same position as the minimum in concentration or gaseous partial pressure of capillary-supplied commodities. This minimum is the boundary between cylinders of tissue being supplied by adjacent capillaries, and thus delineates the maximal diffusion-distance for capillary-supplied commodities. Maximal diffusion-distance (#m) for T. newnesi - 26.23 + 1.64; N. gibberifrons = 21.45 + 0.51. For 02, maximal diffusion-distance con- ventionally is referred to as Krogh's radius, R. With an easily obtained estimate of numerical capillary density, these R values can be used to calculate a capillary tortuosity constant (c[k,0]) and capillary length density (Jr [c,f]). c(k,0) values were also determined using an established method, and R and Jv(c,f) values calculated from these values did not significantly differ from values determined from mitochondrial distribu- tions. Mitochondrial distribution analysis may more accurately reflect changes in capillary blood flow and heterogeneity of diffusion and solubility constants within muscle than currently existing techniques. Similar distributions of V,,(mit) reported for several species of vertebrates suggest wide applicability of the method. Animal, antarctic fish; Capillaries, density in skeletal muscle, tortuosity; Diffusion, of 02 in tissue; Mito- chondria, distribution of- in skeletal muscle; Skeletal muscle, diffusion distance, capillary supply Several methods exists to estimate maximal diffusion-distances for capillary-supplied commodities within skeletal muscle, including the capillary-domains method (Hoofd et al., 1985), the closest individual method (Kayar et al., 1982), and the concentric circle method (Kayar and Banchero, 1982). All of these techniques are based upon August Krogh's pioneering model of oxygen delivery to striated muscle cells (Krogh, 1919). Correspondence to: B.D. Sidell, University of Maine, Department of Zoology, Murray Hall, Orono, ME 04469, U.S.A. 0034-5687/90/$03.50 © 1990 Elsevier Science Publishers B.V. (Biomedical Division)