Oxygen flux from capillary to mitochondria: integration of contemporary discoveries

Abstract

Resting humans transport ~ 100 quintillion (10¹⁸) oxygen (O₂) molecules every second to tissues for consumption. The final, short distance (< 50 µm) from capillary to the most distant mitochondria, in skeletal muscle where exercising O₂ demands may increase 100-fold, challenges our understanding of O₂ transport. To power cellular energetics O₂ reaches its muscle mitochondrial target by dissociating from hemoglobin, crossing the red cell membrane, plasma, endothelial surface layer, endothelial cell, interstitial space, myocyte sarcolemma and a variable expanse of cytoplasm before traversing the mitochondrial outer/inner membranes and reacting with reduced cytochrome c and protons. This past century our understanding of O₂'s passage across the body's final O₂ frontier has been completely revised. This review considers the latest structural and functional data, challenging the following entrenched notions: (1) That O₂ moves freely across blood cell membranes. (2) The Krogh-Erlang model whereby O₂ pressure decreases systematically from capillary to mitochondria. (3) Whether intramyocyte diffusion distances matter. (4) That mitochondria are separate organelles rather than coordinated and highly plastic syncytia. (5) The roles of free versus myoglobin-facilitated O₂ diffusion. (6) That myocytes develop anoxic loci. These questions, and the intriguing notions that (1) cellular membranes, including interconnected mitochondrial membranes, act as low resistance conduits for O₂, lipids and H⁺-electrochemical transport and (2) that myoglobin oxy/deoxygenation state controls mitochondrial oxidative function via nitric oxide, challenge established tenets of muscle metabolic control. These elements redefine muscle O₂ transport models essential for the development of effective therapeutic countermeasures to pathological decrements in O₂ supply and physical performance.

Keywords: Aquaporin channels; Capillary function; Exercise limitation; Mitochondrial structure; Muscle anoxia; Myoglobin; Perfusive and diffusive oxygen conductances; Rhesus channels.

Figure 1.

Oxygen (blue arrows) moves down its pressure (PO₂) gradient from red blood cells in the capillaries to the mitochondrial reticulum in skeletal muscle fibers to power contractile function and energetics. From rest-maximal exercise the O₂ flux may increase up to 100-fold.

Figure 2.. Perfusive and Diffusive Oxygen Transport Determine Oxygen Utilization.

The maximal amount of oxygen utilized by skeletal muscle during exercise (V̇O₂max) is determined by perfusive [curved line, Fick principle: V̇O₂ = muscle blood flow (Q̇) x fractional oxygen extraction (Ca-CvO₂)] and diffusive O₂ [straight line, Fick’s Law: V̇O₂ = diffusing capacity for oxygen (DO₂) x oxygen partial pressure (PO₂)] transport. Conflating perfusive and diffusive O₂ transport in health (left panel) provides the framework to understanding the physiological adaptations to each component (right panel) that result from exercise training (i.e. spatially and temporally improved bulk blood flow and red blood cell distribution, DO₂, elevated PO₂, etc.) and/or disease conditions (i.e. impaired blood flow distribution, reduced percentage of capillaries supporting red blood cell flux, impaired O₂ extraction, DO₂, maintained/lowered PO₂, etc.) and how they determine/alter V̇O₂max (Poole et al. 2012, 2018; Hirai et al. 2015).

Figure 3.. Traditional (theoretical) and Contemporary Perspectives on Vascular-muscle Oxygen Pressures.

The Krogh-Erlang model (top) postulates that the partial pressure of oxygen (PO₂) decreases systematically from the capillary to distant mitochondria, whereby some distant tissue regions are anoxic, and that capillaries are recruited upon contractions/exercise onset to prevent further tissue anoxia. Contemporary measurements of PO₂ in vascular, interstitial, and intramyocyte compartments (bottom) utilize technological advances (phosphorescence quenching, proton nuclear magnetic resonance spectroscopy (P-NMR), cryomicrospectrophotometric, PO₂ microelectrodes, etc.) and elegant experimental designs. These measurements have not resolved the presence of anoxic loci or steep intramyocyte PO₂ gradients (nor do low interstitial PO₂’s allow for such), regardless of distance from capillaries. Note that distinct PO₂ gradients exist between compartments (i.e. microvascular-interstitial and interstitial-intramyocyte) reflecting the driving pressure necessary to generate the required O₂ flux density across the apparent resistances provided by each compartment as O₂ moves down its pressure gradient from red blood cell (RBC) to mitochondrial reticulum to power muscle energetics (Hirai et al. 2015,2019; Poole et al. 2020).

Figure 3.. Traditional (theoretical) and Contemporary Perspectives on Vascular-muscle Oxygen Pressures.

The Krogh-Erlang model (top) postulates that the partial pressure of oxygen (PO₂) decreases systematically from the capillary to distant mitochondria, whereby some distant tissue regions are anoxic, and that capillaries are recruited upon contractions/exercise onset to prevent further tissue anoxia. Contemporary measurements of PO₂ in vascular, interstitial, and intramyocyte compartments (bottom) utilize technological advances (phosphorescence quenching, proton nuclear magnetic resonance spectroscopy (P-NMR), cryomicrospectrophotometric, PO₂ microelectrodes, etc.) and elegant experimental designs. These measurements have not resolved the presence of anoxic loci or steep intramyocyte PO₂ gradients (nor do low interstitial PO₂’s allow for such), regardless of distance from capillaries. Note that distinct PO₂ gradients exist between compartments (i.e. microvascular-interstitial and interstitial-intramyocyte) reflecting the driving pressure necessary to generate the required O₂ flux density across the apparent resistances provided by each compartment as O₂ moves down its pressure gradient from red blood cell (RBC) to mitochondrial reticulum to power muscle energetics (Hirai et al. 2015,2019; Poole et al. 2020).

Figure 4.. Red Blood Cell-to-Intramyocyte Oxygen Transport/Diffusion.

To support the metabolic demands of skeletal muscle tissue, which increases exponentially during exercise, O₂ bound to haemoglobin in red blood cells (RBCs) is transported down its pressure gradient at the RBC-capillary interface (PmvO₂, microvascular partial pressure for O₂) across plasma, the glycocalyx, capillary endothelial cells, and into the interstitial space. Recent evidence supports that, in addition to simple diffusion, pressure-gradient driven movement of O₂ across the RBC plasma membrane may occur through aquaporin (AQP-1) and rhesus (RhAg) channels (Zhao et al. 2020; Michenkova et al. 2021). As there are no carriers for O₂ between RBCs and the sarcolemma (CFR, carrier free region), the differences in available surface area for O₂ flux mandates that the RBC-interstitial O₂ flux density is greater than interstitial-intramyocyte O₂ flux density, and necessitates a substantial driving pressure (i.e. Δ(PmvO₂ > PisO₂), interstitial space PO₂) to overcome the apparent resistance to O₂ flux between compartments. It is now appreciated that subsarcolemmal mitochondria are interconnected with deep-tissue mitochondria (Glancy et al. 2015; Vincent et al. 2019) and surrounded by myoglobin in the cytoplasm. When saturated with O₂, for example at rest, oxymyoglobin constitutes a functionally-depleted O₂-carrier region (FDCR) that, at the onset of exercise/contractions, becomes partially desaturated (i.e. deoxymyoglobin) serving to more effectively transport O₂ (increased diffusing capacity) to the mitochondria in deep tissue (see text and Figure 6 for further details, rev. Honig et al. 1997).

Figure 5.. Krogh vs. Hill Models of O₂ Delivery.

The Krogh (left) versus the Hill solid cylinder (right) models of myocyte O₂ delivery (blue arrows) that was conceived initially by Ellis et al. (1983). For the Krogh model, straight, parallel capillaries each supply a cylindrical volume of muscle such that the tissue volume increases with increasing distance from the capillary. Subsequent recognition that capillaries may, especially in contracted or shortened muscles, be extremely tortuous combined with the presence of O₂ fluxes (from capillaries and adjacent fibers) within the interstitial space support the Hill geometry. Note that this is a far more efficient O₂ delivery (blue arrows) mechanism, in part, because the tissue volume supplied decreases with increasing distance from the capillary.

Figure 6.. Contemporary View of Myoglobin-mediated Oxidative Regulation.

Top: At rest, myoglobin molecules are mostly saturated with O₂ and create a functionally depleted O₂-carrier region (FDCR) which establishes a low diffusing capacity for O₂ (DO₂). However, at the onset of contractions/exercise, deoxymyoglobin levels increase dramatically and support metabolic demands (increases in O₂ utilization → ATP production) by elevating DO₂. Bottom: Myoglobin molecules may also serve as O₂ sensors, locally regulating the cytochrome c oxidase activity within the electron transport chain of mitochondria via oxy- and deoxymyoglobin’s role in nitric oxide (NO) metabolism. Importantly, in regions with greater O₂ availability (i.e., higher PO₂), NO is scavenged to the inactive nitrate (NO₃^-) form in the presence of oxymyoglobin, which allows cytochrome c oxidase to function properly. However, should regions of especially low O₂ availability (i.e. low PO₂) develop, deoxymyoglobin reduces nitrite (NO₂-) to NO, whereby NO inhibits cyctochrome c oxidase activity and preserves O₂ locally: Preventing development of anoxic loci especially during heavy- and severe-intensity exercise (based upon Clanton, 2019).

Figure 7.. Summary of Mechanisms for Capillary→Mitochondria O₂ Flux in Skeletal Muscle.

1. O₂ (blue arrows) moves down its pressure gradient from the capillary/capillaries into the interstitial space. 2. Interstitial O₂ transport. 3. O₂ tracks along phospholipid bilayer of sarcolemma. 4. O₂ moving by simple diffusion through cytosol. 5. Myoglobin- facilitated O₂ diffusion. 6. H⁺-electrochemical gradient and perhaps O₂ track across contiguous mitochondrial reticulum.