STRUCTURAL DETERMINANTS OF MAXIMAL O-2 TRANSPORT IN MUSCLES OF EXERCISING FOXES

The arctic blue fox (Alopex lagopus) has a specific maximal oxygen con sumption (V over dot (O2) max/Mb = 3.6 ml O-2 . s(-1). kg(-1)) that is approximately 1.6-fold greater than those of dogs and horses. The fox has one of the highest body mass specific skeletal muscle mitochondri al volumes (V(mt,m)/Mb = 44 cm(3) . kg(-1)) among mammalian athletic s pecies matching its higher V over dot (O2) max/Mb. The structural comp onents related to capillaries, such as specific capillary length densi ty (J(c)/Mb = 348 km . kg(-1)) and specific capillary volume (V(c)/Mb = 4.8 ml . kg(-1)), are not greater in the fox than in the larger athl etes. Because a greater specific muscle diffusing capacity for oxygen (DTO2/Mb) is not utilized by the fox to achieve a higher in V over dot (O2) max/Mb, a higher pressure difference for diffusion in the muscle capillaries is the alternative explanation for the fox's higher V ove r dot (O2) max/Mb. This mechanism is suggested by the fox's higher art erial and mixed venous capillary P-O2 (120 mm Hg and 37 mm Hg, respect ively) and its shorter mean muscle capillary transit time for blood (i (c) = 0.28 sec) compared to larger species.