The oxygen dependence of respiration was obtained in situ in microscopic regions of rat spinotrapezius muscle for different levels of metabolic activity produced by electrical stimulation at rates from 0.5 to 8 Hz. The rate of O consumption (V̇o) was measured with phosphorescence quenching microscopy (PQM) as the rate of O disappearance in a muscle with rapid flow arrest. The phosphorescent oxygen probe was loaded into the interstitial space of the muscle to give O tension (Po) in the interstitium. A set of sigmoid curves relating the Po dependence of V̇o was obtained with a Po-dependent region below a characteristic Po (~30 mmHg) and a Po-independent region above this Po. The V̇o(Po) plots were fit by the Hill equation containing O demand (rest to 8 Hz: 216 ± 26 to 636 ± 77 nl O/cm s) and the Po value corresponding to O demand/2 (rest to 8 Hz: 22 ± 4 to 11 ± 1 mmHg). The initial Po and V̇o pairs of values measured at the moment of flow arrest formed a straight line, determining the rate of oxygen supply. This line had a negative slope, equal to the oxygen conductance for the O supply chain. For each level of tissue blood flow the set of possible values of Po and V̇o consists of the intersection points between this O supply line and the set of V̇o curves. An electrical analogy for the intraorgan O supply and consumption is an inverting transistor amplifier, which allows the use of graphic analysis methods for prediction of the behavior of the oxygen processing system in organs. NEW & NOTEWORTHY The sigmoidal shape of curves describing oxygen dependence of muscle respiration varies from basal to maximal workload and characterizes the oxidative metabolism of muscle. The rate of O supply depends on extracellular O tension and is determined by the overall oxygen conductance in the muscle. The dynamics of oxygen consumption is determined by the supply line that intersects the oxygen demand curves. An electrical analogy for the oxygen supply/consumption system is an inverting transistor amplifier.

Leukoreduction of blood used for transfusion alleviates febrile transfusion reactions, graft versus host disease and alloimmunization to leukocyte antigen. However, the actual clinical benefit of leukoreduction in terms of microcirculatory tissue O2 delivery after packed red blood cell (pRBC) transfusion has not been investigated. As such, the aim of this study was to determine the effects of non-leukoreduced (NLR) and leukoreduced (LR) fresh pRBC transfusion on interstitial oxygenation in anesthetized male Sprague-Dawley rats. Interstitial fluid PO2 and arteriolar diameters in spinotrapezius muscle preparations were monitored before and after transfusion with NLR- or LR-pRBCs. The major findings were that (1) transfusion of NLR-pRBCs significantly decreased interstitial oxygenation whereas transfusion of LR-pRBCs did not, and (2) transfusion with LR-pRBCs elicited a substantially greater increase in arterial blood pressure (ABP) than did transfusion with NLR-pRBCs. These changes in PO2 and ABP were not associated with changes in the diameters of resistance arterioles in the spinotrapezius muscle. These data suggest that transfusion of fresh NLR-pRBCs may negatively affect tissue oxygenation via enhanced leukocyte influx and decreased O2 delivery. They also suggest that leukocytes diminish the capability of transfused pRBCs to increase cardiac output. As such, transfusion of LR-pRBCs may be less deleterious on tissue PO2 levels than NLR-pRBCs although a concomitantly greater increase in ABP may accompany transfusion of LR-pRBCs.

A continuous supply of O2 to cells in a mammalian organism is necessary to maintain normal physiological function and the microcirculation is especially important in this matter as it is the site of O2 exchange. Under conditions of active/functional hyperemia following muscle contraction, the O2 transport system in older subjects does not appear to respond as rapidly and to the same degree as in younger subjects. With aging, elements of the O2 transport and regulatory system are changed in such a way that matching O2 supply to O2 demand does not work as well in older as in younger subjects. With regard to the O2 demand component of the overall system, in vitro studies have shown that oxygen consumption (VO2) remains constant until a critical partial pressure (Pcrit) of O2 (<1 mmHg) is reached. This knowledge was predicated on the assumption that the in vitro studies mimicked conditions in living tissue. More recently, however, in vivo studies have shown that VO2 varies over a much wider range of O2 partial pressure in the interstitial fluid (PISFO2). An intravital microscopic approach is being used to show that developmental changes in the O2 transport system begin earlier than previously thought. Significant and rapid changes in the microvascular network of skeletal muscle have been observed during the first few weeks of postnatal development. Three different developmental groups of male Sprague‐Dawley rats (2, 4, and 6 months) were used to investigate early changes in the O2 demand component of the transport system by measuring VO2 and the PISFO2 dependence of VO2 under resting conditions. VO2 of the spinotrapezius muscle was measured with a quasi‐continuous, flash‐synchronized, rapidly pressurized airbag system to briefly arrest blood flow and determine the rate of decrease of oxygen tension (dPO2/dt) in the ISF bathing the skeletal muscle fibers. Changes in PISFO2 of the skeletal muscle were measured using phosphorescence quenching microscopy. To understand the PO2 dependence of VO2, VO2 data were plotted versus PO2 and the results analyzed using a generalization of Michaelis‐Menten kinetics. This yielded information about both maximum VO2 (Vmax) and the PO2 at half Vmax (P50). Experiments were carried out on 16 animals with average weights (mean ± SEM) from 280 ± 10, 408 ± 17, to 454 ± 6 grams at 2 mo, 4 mo, and 6 mo, respectively. Average Vmax (mean ± SEM) was 138 ± 9, 148 ± 15, to 122 ± 6 nL O2/cm3·s for 2 mo, 4 mo, and 6 mo, respectively. Between 2 mo and 6 mo, there was a downward trend with Vmax which was not significant. Average P50 (mean ± SEM) was 11 ± 1, 15 ± 1, to 21 ± 1 mmHg for 2 mo, 4 mo, and 6 mo, respectively. There was a significant (p < 0.05) increase in P50 from 2 mo to 6 mo which indicates that as the developmental age of the animal increases, there is an increased sensitivity of O2 consumption to changes in PISFO2. The findings are consistent with published data indicating that VO2 depends on PISFO2 over a much wider physiological range than previously thought. It is clear that as the rat develops, it consumes less O2 which may be ascribed to some combination of changes in the O2 delivery system and mitochondrial function.Support or Funding InformationSupport from Department of Physiology and BiophysicsThis abstract is from the Experimental Biology 2018 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.

The involvement of oxygen in reactive hyperemia following brief periods of local flow arrest was investigated by measuring PO2 under conditions of different ischemic durations of 5, 15, 30, 60, and 300 sec. Spinotrapezius muscles of anesthetized male Sprague‐Dawley rats were used; the muscle was covered with a gas barrier film to isolate it from atmospheric oxygen. Well perfused sites were chosen with one or more arterioles and a capillary network. Phosphorescence quenching microscopy was used to measure interstitial PO2 with a topically applied phosphorescent probe. Localized ischemia was produced by rapidly inflating an airbag over the muscle; reperfusion was initiated by deflating the bag. Baseline PO2 varied between 52 and 60 mmHg. During ischemia, PO2 dropped to 45 mmHg for 5 s ischemia down to 3 mmHg for >;30 s ischemia. Maximum PO2 during the reperfusion phase ranged from 52 to 71 mmHg. The time required for PO2 recovery to baseline ranged from 9 (5 s ischemia) to 122 sec (300 s ischemia). These data exhibited a positive correlation between ischemic duration and the time for PO2 recovery, as well as to the maximum PO2 reached during reperfusion. The data will be used to establish a relationship between ischemic duration and tissue oxygen consumption (VO2) during reperfusion.Support: NHLBI grant HL18292

The effects of brief ischemia (5, 15, 30, and 60 s duration; Isch) on interstitial PO2 (PISFO2), arteriolar PO2 (PaO2), blood flow (Q) and O2 consumption (VO2) during the recovery period was observed in exteriorized spinotrapezius muscles of 29 male Sprague‐Dawley rats (284 ± 20 g) for intravital microscopy. Phosphorescence quenching microscopy was used to obtain PISFO2, PaO2 and VO2. Arteriolar blood flow was calculated from arteriolar diameter and centerline RBC velocity obtained from an Optical Doppler Velocimeter. Ischemia was initiated by rapidly pressurizing an airbag attached to the microscope objective for a selected ischemic duration. The average baseline value of arteriolar blood flow was 5.8±0.8 µl/min; as expected, reactive hyperemia was observed during the recovery from Isch. Flow recovery (dQ/dt) was fastest [3.88±0.64 (µl·min‐1)/s] after 60 seconds of Isch. Average baseline PISFO2 was 62±4 mmHg; PISFO2 decreased to 75%, 34%, 10% and 2% of baseline following 5, 15, 30 and 60 s of Isch, respectively. Up to 30 s of ischemia, the rate of recovery of PISFO2 steadily increased as the ischemic duration increased; however, it dropped off following 60 s of ischemia. The drop in the rate of recovery of PISFO2 despite an increase in flow recovery following 60 s of ischemia is suggestive of a period of imbalance during which the rate at which the tissue was consuming oxygen temporarily exceed the rate at which it could be supplied. The rate of recovery of PISFO2 was fastest (4.2±0.7 mmHg/s) following 30 s of Isch. Average baseline VO2 was 120±19 nl O2/cm3·s and it increased above baseline during the early part of the recovery period. The time course of recovery of PISFO2 represented a balance between O2 delivery, proportional to local arteriolar blood flow, and local VO2. Grant Funding Source: NHLBI grant HL18292