Role of convective O-2 delivery in determining Vo(2) on-kinetics in caninemuscle contracting at peak Vo(2)

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 m uscle did not limit O-2, uptake ((V) over dotO(2)) on-kinetics during trans itions 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 fo r transitions involving contractions of higher metabolic intensities. (V)ov er dotO(2), on-kinetics were determined in isolated canine gastrocnemius mu scles 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 simila r to 15-30 s before contractions at a constant level corresponding to the s teady-state value during contractions in Control (Fast O-2, Delivery). In F ast O-2, Delivery, adenosine was infused intra-arterially. Q was measured c ontinuously in the popliteal vein; arterial and popliteal venous O-2, conte nts 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), di fference between resting baseline and steady-state values during contractio ns was 24.9 +/- 1.6 (SE) s in Control and 18.5 +/- 1.8 s in Fast O-2, Deliv ery (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, defi cit and with less muscle fatigue. During transitions involving contractions at peak (V) over dotO(2),, convective O-2, delivery to muscle, together wi th an inertia of oxidative metabolism, contributes in determining the (V) o ver dotO(2), on-kinetics.