VO(2) on-kinetics in isolated canine muscle in situ during slowed convective O(2) delivery.

The purpose of this study was to examine O(2) uptake (Vo(2)) on-kinetics when the spontaneous blood flow (and therefore O(2) delivery) on-response was slowed by 25 and 50 s. The isolated gastrocnemius muscle complex (GS) in situ was studied in six anesthetized dogs during transitions from rest to a submaximal metabolic rate (≈50-70% of peak Vo(2)). Four trials were performed: 1) a pretrial in which resting and steady-state blood flows were established, 2) a control trial in which the blood flow on-kinetics mean response time (MRT) was set at 20 s (CT20), 3) an experimental trial in which the blood flow on-kinetics MRT was set at 45 s (EX45), and 4) an experimental trial in which the blood flow on-kinetics MRT was set at 70 s (EX70). Slowing O(2) delivery via slowing blood flow on-kinetics resulted in a linear slowing of the Vo(2) on-kinetics response (R = 0.96). Average MRT values for CT20, EX45, and EX70 Vo(2) on-kinetics were (means ± SD) 17 ± 2, 23 ± 4, and 26 ± 3 s, respectively (P < 0.05 among all). During these transitions, slowing blood flow resulted in greater muscle deoxygenation (as indicated by near-infrared spectroscopy), suggesting that lower intracellular Po(2) values were reached. In this oxidative muscle, Vo(2) and O(2) delivery were closely matched during the transition period from rest to steady-state contractions. In conjunction with our previous work showing that speeding O(2) delivery did not alter Vo(2) on-kinetics under similar conditions, it appears that spontaneously perfused skeletal muscle operates at the nexus of sufficient and insufficient O(2) delivery in the transition from rest to contractions.