Vo2 On-kinetics In Isolated Canine Muscle In Situ During Slowed Convective O2 Delivery

Abstract

Numerous studies have measured heart rate, cardiac output, and limb blood flow as proxies for exercising muscle blood flow; and pulmonary oxygen uptake (VO2) and limb O2 as proxies for exercising muscle VO2. However, relatively few studies have characterized O2 delivery on-kinetics and VO2 on-kinetics directly across a contracting muscle. Furthermore, to our knowledge, no one has altered only the time course of the transition between metabolic rates, leaving resting and steady state O2 delivery unchanged. PURPOSE: To slow the blood flow on-kinetics (via pump perfusion control), and thereby O2 delivery on-kinetics to a contracting isolated muscle in situ and determine the resultant VO2 on-kinetics. METHODS: A step change in metabolic rate was elicited by stimulating canine gastrocnemius-plantaris muscles (n=6) via their sciatic nerves (6-8 V, 0.2 ms duration, 50 Hz, 200 ms train) at a rate of 0.5 Hz. With arterial O2 concentration maintained constant, in random order trials, blood flow tau was set at a control transition speed between rest and contractions (determined in pilot work) of 20 sec (CT20) or was slowed to tau values of 45 sec (EX45) and 70 sec (EX70). Resting and steady state (4 min of contractions) blood flows were not altered between the transition conditions. An indwelling venous oximeter continuously measured venous O2 saturation, from which a-vO2 difference was determined. VO2 was calculated on a contraction-by-contraction basis from a-vO2 difference and blood flow. RESULTS: VO2 average mean response time (MRT = tau + time delay) values for EX45 and EX70 were significantly different from each other and from CT20 (mean±S.D.; CT20=19.9±3.8, EX45=26.3±5.9, and EX70=31.7±4.1 sec). VO2 on-response MRT values were linearly related to the blood flow on-response MRT values (R=0.99997). CONCLUSIONS: These results suggest that in this highly-oxidative muscle, blood flow and thereby O2 delivery is finely matched to O2 demand in the transition from rest to contractions at a submaximal metabolic rate, and that there is little "buffer" of blood flow/O2 delivery during this transition.