Role of convective O2 delivery in determining Vo2 on-kinetics in canine muscle contracting at peak Vo2

A previous study (Grassi B, Gladden LB, Samaja M, Stary CM, and Hogan MC, J Appl Physiol 85: 1394-1403, 1998) showed that convective O-2 delivery to muscle did not limit O-2, uptake ((V) over dotO(2)) on-kinetics during transitions from rest to contractions at similar to 60% of peak (V) over dotO(2). The present study aimed to determine whether this finding is also true for transitions involving contractions of higher metabolic intensities. (V)over dotO(2), on-kinetics were determined in isolated canine gastrocnemius muscles in situ (n = 5) during transitions from rest to 4 min of electrically stimulated isometric tetanic contractions corresponding to the muscle peak (V) over dotO(2),. Two conditions were compared: 1) spontaneous adjustment of muscle blood flow ($) (Control) and 2) pump-perfused Q, adjusted similar to 15-30 s before contractions at a constant level corresponding to the steady-state value during contractions in Control (Fast O-2, Delivery). In Fast O-2, Delivery, adenosine was infused intra-arterially. Q was measured continuously in the popliteal vein; arterial and popliteal venous O-2, contents were measured at rest and at 5- to 7-s intervals during the transition. Muscle (V) over dotO(2), was determined as Q times the arteriovenous blood O-2, content difference. The time to reach 63% of the (V) over dotO(2), difference between resting baseline and steady-state values during contractions was 24.9 +/- 1.6 (SE) s in Control and 18.5 +/- 1.8 s in Fast O-2, Delivery (P < 0.05). Faster (V) over dotO(2), on-kinetics in Fast O-2, Delivery was associated with an similar to 30% reduction in the calculated O-2, deficit and with less muscle fatigue. During transitions involving contractions at peak (V) over dotO(2),, convective O-2, delivery to muscle, together with an inertia of oxidative metabolism, contributes in determining the (V) over dotO(2), on-kinetics.