Microscopic Study of PO 2 and V̇O 2 Kinetics in Contracting Rat Spinotrapezius Muscle

Transitions from rest to exercise and back in muscles are accompanied by changes in O 2 consumption (V̇O 2 ). Transients in metabolism have been studied using Fick's principle, based on O 2 mass conservation in steady state. We used phosphorescence quenching, with periodic short‐term compression of spinotrapezius muscle, to determine kinetics of interstitial PO 2 and V̇O 2 in 300 µm regions under 1, 2 and 4 Hz stimulation. During steady state contractions PO 2 decreased (~5 mmHg/Hz) from 1 to 4 Hz stimulation. During steady state contractions V̇O 2 increased from rest by a factor of 1.54 at 1Hz, 2.42 at 2 Hz, but only 1.32 at 4 Hz. Sorting kinetic curves based on their functional response to steady contractions (i.e., PO 2 > or <30 mmHg) revealed two groups of muscle regions with different V̇O 2 kinetics. In muscle regions with PO 2 >30 mmHg during contraction, V̇O 2 increased more than 2‐fold compared to rest. In the other data set with stationary work PO 2 <30 mmHg,V̇O 2 exceeded the resting level by only 18%. A new finding is the maintenance of a high post‐contraction V̇O 2 , while PO 2 starts to recover immediately after the end of contraction. In regions with PO 2 >30 mmHg, the high V̇O 2 state extended for 20 s, followed by a decrease with time constant τ = 45 s. In the data set with PO 2 <30 mmHg the post‐contraction V̇O 2 increased nearly twofold for ~ 40 s before a slow decline (τ = 74 s) to baseline. The phenomenon of extended duration of high V̇O 2 and its slow decline may be lost in an analysis using Fick's principle. An electrical analog model was applied for the analysis of the kinetic curves. A mechanistic hypothesis about the kinetic curves is proposed, which is an alternative to the traditional bi‐exponential approach. Support: AHA #14GRNT20380551