A prior bout of contractions speeds (V) over dotO2 and blood flow on-kinetics and reduces the (V) over dotO2 slow-component amplitude in canine skeletal muscle contracting in situ

Hernandez A, McDonald JR, Lai N, Gladden LB. A prior bout of contractions speeds (V) over dotO(2) and blood flow on-kinetics and reduces the (V) over dotO(2) slow-component amplitude in canine skeletal muscle contracting in situ. J Appl Physiol 108: 1169-1176, 2010. First published March 11, 2010; doi:10.1152/japplphysiol.01318.2009.-It was the purpose of this study to examine the effect of a priming contractile bout on oxygen uptake ((V) over dotO(2)) on-kinetics in highly oxidative skeletal muscle. Canine gastrocnemii (n = 12) were stimulated via their sciatic nerves (8 V, 0.2-ms duration, 50 Hz, 200-ms train) at a rate of 2 contractions/ 3 s (approximate to 70% peak (V) over dotO(2)) for two 2-min bouts, separated by 2 min of recovery. Blood flow was recorded with an ultrasonic flowmeter, and muscle oxygenation monitored via near-infrared spectroscopy. Compared with the first bout (bout 2 vs. bout 1), the (V) over dotO(2) primary time constant (mean +/- SD, 9.4 +/- 2.3 vs. 12.0 +/- 3.9 s) and slow-component amplitude (5.9 +/- 6.3 vs. 12.1 +/- 9.0 ml O-2.kg wet wt(-1).min(-1)) were significantly reduced (P < 0.05) during the second bout. Blood flow on-kinetics were significantly speeded during the second bout (time constant = 7.7 +/- 2.6 vs. 14.8 +/- 5.8 s), and O-2 extraction was greater at the onset of contractions (0.050 +/- 0.030 vs. 0.020 +/- 0.010 ml O-2/ml blood). Kinetics of muscle deoxygenation were significantly slower at the onset of the second bout (7.2 +/- 2.2 vs. 4.4 +/- 1.2 s), while relative oxyhemoglobin concentration was elevated throughout the second bout. These results suggest that better matching of O-2 delivery to (V) over dotO(2) speeds (V) over dotO(2) on-kinetics at this metabolic rate, but do not eliminate a potential role for enhanced metabolic activation. Additionally, altered motor unit recruitment at the onset of a second bout is not a prerequisite for reductions in the (V) over dotO(2) slow-component amplitude after a priming contractile bout in canine muscle in situ.