O2 DELIVERY AT VO2MAX AND OXIDATIVE CAPACITY IN MUSCLES OF STANDARD-BRED HORSES

The purpose of this study was to describe the relationships between 16 physiological, biochemical, and morphological variables presumed to relate to the oxidative capacity in quadriceps muscles or muscle parts in Standardbred horses. The variables included O2 delivery (blood flow) and mean capillary transit time (MTT) during treadmill locomotion at whole animal maximal O2 consumption (VO2max, 134 +/- 2 ml . min-1 . kg-1), capillary density and capillary-to-fiber ratio, myoglobin concentration, oxidative enzyme activities, glycolytic enzyme activities, fiber type populations, and fiber size. These components of muscle metabolic capacity were found to be interrelated to varying degrees using correlation matrix analysis, with lactate dehydrogenase activity showing the most significant correlations (n = 14) with other variables. Most of the ''oxidative'' variables occurred in the highest quantities in the deepest muscle of the group (vastus intermedius) and in the deepest parts of the other quadriceps muscles where the highest proportions of type I fibers were localized. The highest blood flow measured with microspheres in the muscle group during exercise was in vastus intermedius muscle (145 ml . min-1 . 100 g-1), and the lowest was in the superficial part of rectus femoris muscle (32 ml . min-1 . 100 g-1). Average muscle blood flow during exercise at whole animal VO2max was 116 ml . min-1 . 100 g-1. Because skeletal muscle comprised 43% of total body mass (453 +/- 34 kg), total muscle blood flow was estimated at 226 l/min, which was approximately 78% of total cardiac output (288 l/min). MTT for blood in the capillaries (MTT = estimated capillary volume/blood flow) of the quadriceps muscles ranged from 0.52 to 1.75 s, with the lower MTTs occurring in the deeper more oxidative muscles. Muscle O2 delivery during heavy exercise is closely related to other oxidative variables, whereas O2 delivery during standing is not, suggesting that the various structural and metabolic components of the muscles' oxidative systems are designed to support metabolism during maximal aerobic exercise.