Journal
JOURNAL OF PHYSIOLOGY-LONDON
Volume 539, Issue 3, Pages 927-934Publisher
WILEY
DOI: 10.1113/jphysiol.2001.013165
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In humans, pulmonary oxygen uptake ((V)over dot(O2)) kinetics may be speeded by prior exercise in the heavy domain. This 'speeding' arises potentially as the result of an increased muscle O-2 delivery ((Q)over dot(O2)) and/or a more rapid elevation of oxidative phosphorylation. We adapted phosphorescence quenching techniques to determine the (Q)over dot(O2)-to-O-2 utilization ((Q)over dot(O2)/(V)over dot(O2))(_)characteristics via microvascular O-2 pressure (P-O2m) measurements across sequential bouts of contractions in rat spinotrapezius muscle. Spinotrapezius muscles from female Sprague-Dawley rats (n = 6) were electrically stimulated (1 Hz twitch, 3-5 V) for two 3 min bouts (ST1 and ST2) separated by 10 min rest. P-O2,P-m responses were analysed using an exponential + time delay (TD) model. There was no significant difference in baseline and DeltaP(O2,m) between ST1 and ST2 (28.5 +/- 2.6 vs. 27.9 +/- 2.4 mmHg, and 13.9 +/- 1.8 vs. 14.1 +/- 1.3 mmHg, respectively). The TD was reduced significantly in the second contraction bout (ST1, 12.2 +/- 1.9; ST2, 5.7 +/- 2.2 s, P < 0.05), whereas the time constant of the exponential P-O2,P-m, decrease was unchanged (ST1, 16.3 +/- 2.6; ST2, 17.6 +/- 2.7 s, P > 0.1). The shortened TD found in ST2 led to a reduced time to reach 63% of the final response of ST,2 compared to ST1 (ST1, 28.3 +/- 3.0; ST2, 20.2 +/- 1.8 s, P < 0.05). The speeding of the overall response in the absence of an elevated P-O2,P-m baseline (which had it occurred would indicate an elevated (Q)over dot(O2)/(V)over dot(O2)) or muscle blood flow suggests that some intracellular process (es) (e.g. more rapid increase in oxidative phosphorylation) may be responsible for the increased speed of P-O2,P-m kinetics after prior contractions under these conditions.
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