Carbon Dioxide Concentration Alters the Dynamics of Oxygen-Mediated Capillary Blood Flow Response s in Skeletal Muscle
Authors: Folkins Alexander , Russell McEvoy Gaylene , Fraser Graham 
Published: May 2024

Objective: We sought to quantify how different muscle carbon dioxide concentrations ([CO2]) alter the dynamics of oxygen-mediated blood flow responses in skeletal muscle capillaries. Methods: 6 male Sprague Dawley rats (164-215g) were anaesthetized, instrumented for systemic monitoring, and mechanically ventilated. The extensor digitorum longus muscle was isolated via blunt dissection, externalized, and reflected over a microfluidic gas exchange chamber mounted in the stage of an inverted microscope. Intravital video of capillary blood flow was recorded during 4-minute O2 challenges consisting of a 1-minute baseline at 7% oxygen concentration ([O2]), followed by 3 minutes at 2% [O2]. Before recordings, the muscle was equilibrated to constant background [CO2] of either 2%, 5%, or 8% for 5 minutes. Recordings were analyzed offline using custom MATLAB software. The time transients () of capillary hemodynamic responses were determined using a least-squared regression fit (LSRF) to single- and double-exponential models. All animal protocols were approved by Memorial University’s Animal Care Committee. Results: Baseline red blood cell (RBC) velocity was significantly different at 5% [CO2] (213.2±179.4 μm/s) compared to 2% [CO2] (177.7±162.6 μm/s, p=0.0016), and 8% [CO2] (250.3±193.6 μm/s, p<0.0001). RBC oxygen saturations (SO2) were significantly different beyond 3 s post-baseline in all [CO2] conditions ( p<0.02, =3.2-4.3s). Minimum RBC SO2 was 52%, 46%, and 38% below baseline at 12s, 14s, and 12s for 2%, 5%, and 8% [CO2] conditions, respectively. RBC velocity was significantly increased during 2% [CO2] by 3 s post-baseline (177.7±162.6 vs 188.7±168.6 μm/s, p<0.0001), during 5% [CO2] by 6 s post-baseline (213.2±179.4 vs 224.2±189.9 μm/s, p=0.0190), and during 8% [CO2] by 12 s post-baseline (250.3±193.6 vs 267.0±199.9 μm/s, p=0.0062). Time transients of the fast component of the oxygen-mediated velocity response, as determined by LSRF, were 1.9s for 2% [CO2], 4.8s for 5% [CO2], and 9.8s for 8% [CO2]. Velocity was increased at the end of the O2 challenge by 39%, 33%, and 36% during 2%, 5%, and 8% [CO2] conditions, respectively. Baseline RBC supply rate (SR) was significantly different at 5% [CO2] (13.5±18.9 cells/s) compared to 2% [CO2] (9.5±15.3 cells/s, p=0.0090), and 8% [CO2] (19.0±23.6 cells/s, p<0.0001). At 2% [CO2], SR increased significantly by 9 s post-baseline (9.5±15.2 vs 10.9±16.9 cells/s, p=0.0001), by 10 s post-baseline for 5% [CO2] (13.5±18.9 vs 14.9±20.6 cells/s, p=0.0099), and by 13 s post-baseline during 8% [CO2] (19.0±23.6 vs 21.1±25.2 cells/s, p=0.0204). The fast component of the oxygen-mediated SR response had a of 2.7s for 2% [CO2], 4.5s for 5% [CO2], and 6.9s for 8% [CO2] as determined by LSRF. Overall SR was increased by 74%, 51%, and 39% at the end of the O2 challenge during 2%, 5%, and 8% [CO2], respectively. As well, capillary hematocrits were significantly different during baseline at 5% [CO2] (21.4±16.1%) compared to 2% [CO2] (18.2±22.2%, p=0.0113), and 8% [CO2] (27.2±21.4%, p<0.0001). Conclusion: We have quantified the fast component of oxygen-mediated capillary hemodynamic responses to an acute decrease in skeletal muscle [O2]. The time transients of O2-mediated responses vary with tissue [CO2] such that increasing CO2 results in slower response dynamics. As well, increasing background [CO2] provokes higher capillary RBC velocity, SR, and hematocrit both at baseline and during low O2 challenges as expected. This project was supported by a Natural Sciences and Engineering Research Council of Canada Discovery Grant awarded to GMF. Student support was provided by Memorial University's School of Graduate Studies and Faculty of Medicine. This is the full abstract presented at the American Physiology Summit 2024 meeting and is only available in HTML format. There are no additional versions or additional content available for this abstract. Physiology was not involved in the peer review process.