APStracts 2:0450A, 1995.

Muscle o2 uptake kinetics in humans: implications for metabolic control. Grassi, Bruno, David C. Poole, Russell S. Richardson, Douglas R. Knight, B. Kipp Erickson, and Peter D. Wagner. Department of Medicine - 0623, University of California at San Diego, La Jolla, CA 92093 - 0623; Section of Physiology - ITBA, National Research Council, I-20131 Milano, Italy

APStracts 2:0450A, 1995.

Muscle O2 uptake (VO2m) kinetics in response to an augmented energetic requirement (on- transition) has never been directly determined in humans. We have developed a constant-infusion thermodilution technique which allowed rapid measurements of leg blood flow (Qleg) and, in conjunction with frequent serial measurement of arteriovenous O2 difference [(Ca-Cv)O2leg] across the leg, permitted the determination of the O2 uptake of the leg (VO2leg) at 3-4 s time intervals. VO2leg kinetics during the on- transition was taken as a close approximation of VO2m kinetics. Alveolar VO2 (VO2alv), Qleg, leg O2 delivery [(Q . CaO2)leg], (Ca-Cv)O2leg and VO2leg kinetics were determined in 6 trained subjects (age 22.8 +/- 4.4 years [x +/- SD]; VO2max 59.1 +/- 5.3 ml . kg-1 . min-1) during the transition from unloaded pedalling (U) to a work load (L) (183 +/- 20 watt) well below the previously determined ventilatory threshold. For all variables, two distinct phases were recognized. During the first 10 -15 s of L (phase I) VO2alv, Qleg, and (Q . CaO2)leg increased rapidly, whereas VO2leg increased only slightly and (Ca-Cv)O2leg actually decreased. Following phase I, all variables showed a monoexponential increase (phase II), with similar time courses [slightly faster for (Ca-Cv)O2leg]. Considering both phases, the half times of the responses among variables were not significantly different: 25.5 +/- 2.6 s for VO2alv, 26.6 +/- 7.6 s for Qleg, 26.9 +/- 8.3 s for (Q . CaO2)leg, 23.5 +/- 1.3 s for (Ca-Cv)O2leg, and 27.9 +/- 5.7 s for VO2leg. It is concluded that during the on -transition the kinetics of VO2alv and VO2leg, as measured with these methods, appear to be closely similar. The analysis of the early phase (first 10-15 s) of the on- transition indicates that bulk delivery of O2 to the working muscles is not limiting VO2leg kinetics. However, the present results cannot discriminate between maldistribution of Q / VO2 versus inertia of the intracellular oxidative machinery as the limiting factor.

Received 21 December 1994; accepted in final form 3 October 1995.
APS Manuscript Number A1292-4.
Article publication pending Journal of Applied Physiology.
ISSN 1080-4757 Copyright 1995 The American Physiological Society.
Published in APStracts on 6 November 95