Delayed vagal withdrawal slows circulatory but not oxygen uptake responses at work increase

V̇o2 on-kinetics in isolated canine muscle in situ during slowed convective O2 delivery

The purpose of this study was to examine O2 uptake (V̇o2) on-kinetics when the spontaneous blood flow (and therefore O2 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 V̇o2). 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 O2 delivery via slowing blood flow on-kinetics resulted in a linear slowing of the V̇o2 on-kinetics response ( R = 0.96). Average MRT values for CT20, EX45, and EX70 V̇o2 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 Po2 values were reached. In this oxidative muscle, V̇o2 and O2 delivery were closely matched during the transition period from rest to steady-state contractions. In conjunction with our previous work showing that speeding O2 delivery did not alter V̇o2 on-kinetics under similar conditions, it appears that spontaneously perfused skeletal muscle operates at the nexus of sufficient and insufficient O2 delivery in the transition from rest to contractions.

Kinetics of oxygen uptake during arm cranking with the legs inactive or exercising at moderate intensities

The purpose of this study was to compare the kinetics of oxygen uptake (VO(2)) during arm cranking with the legs inactive or exercising. Each subject (n = 8) performed three exercise protocols: 6-min arm cranking at an intensity of 60% of peak oxygen uptake (VO(2peak), AC(60)) and 6-min combined arm cranking and leg cycling in which AC(60) was added to on-going leg cycling at an intensity of 20% or 40% of VO(2peak) (LC(20) and LC(40): AC(60)LC(20) and AC(60)LC(40), respectively). After the onset of arm cranking, VO(2) tended to increase until the end of arm cranking in all of the three exercise modes. The amplitudes of this increase in VO(2) were 0.98 (0.18), 0.93 (0.16) and 0.84 (0.12) l.min(-1) during AC(60), AC(60)LC(20) and AC(60)LC(40), respectively, and there were significant differences between values for each exercise. The data are presented as means and standard deviations. There were no significant differences in the effective VO(2) time constant, partial O(2) deficit, and the difference between the values of VO(2) measured at 3 and 6 min in the three exercise modes. The present results indicate that the amplitude of the increase in VO(2) is reduced during arm cranking with the legs exercising, that this reduction becomes greater with increases in the intensity of leg cycling, and that the rate of increase in VO(2) is not affected by the additional muscle mass of the legs exercising below moderate intensities. The decrease in the amplitude of increase in VO(2) might be caused by reduction in oxygen supply to the exercising arms due to large muscle mass and/or overlaps of activity of stabilizing muscles during combined arm and leg exercise.

Facial cooling-induced bradycardia does not slow pulmonary V.O2 kinetics at the onset of high-intensity exercise

The mechanism(s) underlying the attenuation of the slow component of pulmonary O2 uptake (Vo2) by prior heavy-intensity exercise is (are) poorly understood but may be ascribed to either an intramuscular-metabolic or a circulatory modification resulting from "priming" exercise. We investigated the effects of altering the circulatory dynamics by delayed vagal withdrawal to the circulation induced by the cold face stimulation (CFS) on the Vo2 kinetics during repeated bouts of heavy-intensity cycling exercise. Five healthy subjects (aged 21-43 yr) volunteered to participate in this study and initially performed two consecutive 6-min leg cycling exercise bouts (work rate: 50% of the difference between lactate threshold and maximal Vo2) separated by 6-min baseline rest without CFS as a control (N1 and N2). CFS was then applied separately, by gel-filled cold compresses to the face for 2-min spanning the rest-exercise transition, to each of the first bout (CFS1) or second bout (CFS2) of repeated heavy-intensity exercise. In the control protocol, Vo2 responses in N2 showed a facilitated adaptation compared with those in N1, mainly attributable to the reduction of slow component. CFS application successfully slowed and delayed the heart rate (HR) kinetics (P < 0.05) on transition to exercise [HR time constant; N1: 55.6 +/- 16.0 (SD) vs. CFS1: 69.0 +/- 12.8 s and N2: 55.5 +/- 11.8 vs. CFS2: 64.0 +/- 17.5 s]; however, it did not affect the "primary" Vo2 kinetics [Vo2 time constant; N1: 23.7 +/- 7.9 (SD) vs. CFS1: 20.9 +/- 3.8 s, and N2: 23.3 +/- 10.3 vs. CFS2: 17.4 +/- 6.3 s]. In conclusion, increased vagal withdrawal delayed and slowed the circulatory response but did not alter the Vo2 kinetics at the onset of supra-lactate threshold cycling exercise. As the facilitation of Vo2 subsequent to prior heavy leg cycling exercise is not attenuated by slowing the central circulation, it seems unlikely that this facilitation is exclusively determined by a blood flow-related mechanism.