Effects of prior contractions on muscle microvascular oxygen pressure at onset of subsequent contractions

Abstract

In humans, pulmonary oxygen uptake (.V(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(O2)) and/or a more rapid elevation of oxidative phosphorylation. We adapted phosphorescence quenching techniques to determine the.Q(O2)-to-O(2) utilization (.Q(O2)/.V(O2)) characteristics via microvascular O(2) pressure (P(O2,m)) 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 (ST(1) and ST(2)) separated by 10 min rest. P(O2,m) responses were analysed using an exponential + time delay (TD) model. There was no significant difference in baseline and DeltaP(O2,m) between ST(1) and ST(2) (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 (ST(1), 12.2 +/- 1.9; ST(2), 5.7 +/- 2.2 s, P < 0.05), whereas the time constant of the exponential P(O2,m) decrease was unchanged (ST(1), 16.3 +/- 2.6; ST(2), 17.6 +/- 2.7 s, P > 0.1). The shortened TD found in ST(2) led to a reduced time to reach 63 % of the final response of ST(2) compared to ST(1) (ST(1), 28.3 +/- 3.0; ST(2), 20.2 +/- 1.8 s, P < 0.05). The speeding of the overall response in the absence of an elevated P(O2,m) baseline (which had it occurred would indicate an elevated.Q(O2)/.V(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,m) kinetics after prior contractions under these conditions.

Figure 1. Comparison of pre-contractions baseline P_O2,m (left) and change in P_O2,m from rest to contractions (right) for the first (ST₁) and second (ST₂) contraction bouts

No change in pre-contracting baseline P_O2,m between first and second bouts reflects a similar Q_O2/V̇_O2 prior to the two contraction bouts.

Figure 2. Comparison of P_O2,m dynamics in first (ST₁) and second (ST₂) contraction bouts for an individual muscle from onset of stimulation (time 0)

Note the shorter time delay observed across the second contractions transient with relatively no change in the primary (exponential) component of the response. Smoothed lines represent model fits.

Figure 3. Mean response data for first (ST₁) and second (ST₂) contraction bouts

Note the significantly reduced time delay in the second contraction bout leading to a reduced time to 63 % of the final response (t₆₃). *P < 0.05.

Figure 4. Theoretical responses in P_O2,m across the rest-to-contraction transition for three muscles with differentQ_O2-to-V̇_O2 dynamics

Responses ‘a’ and ‘b’ reflect data from Behnke et al. (2001). Responses close to ‘b’ and ‘c’ were observed in the present investigation for ST₁ and ST₂, respectively. τ denotes the time constant of the response.