O2 extraction during progressive ischemia in canine skeletal muscle, J. Appl. Physiol. 79(4): 1351-1360, 1995.--O2 uptake (VO2) is defended during decreased O2 delivery (QO2) by an increase in the O2 extraction ratio (O2ER, VO2/QO2), presumably by recruitment of capillaries. This study tested the hypothesis that activity of the microvascular endothelium plays a necessary role in achievement of maximal O2ER. We pump perfused the vascularly isolated hindlimbs of 24 anesthetized and paralyzed dogs at progressively lower flows over a 90-min period. In eight dogs, hindlimb vascular endothelium was removed by injection of deoxycholate (DOC) into the perfusing artery before the ischemic challenge. DOC treatment resulted in loss of normal in vivo and in vitro endothelium-dependent dilatory responses to acetylcholine, but endothelium-independent vascular smooth muscle responses were intact. Eight other dogs were pretreated with nitro-L-arginine methyl ester plus indomethacin (L+I group) to block the synthesis of the vasodilators nitric oxide and prostacyclin. L+I and DOC treatment were associated with increases in hindlimb vascular resistance of 168 +/- 17 and 63 +/- 12%, respectively. O2ER at critical QO2 (QO2 at which VO2 begins to decrease) was 81 +/- 2% in eight control dogs, 66 +/- 6% in L+I, and 42 +/- 4% in DOC, indicating a significant O2 extraction defect in the two treatment groups. These data suggest that products of the vascular endothelium play an important role in the matching of O2 supply to demand during supply limitation in skeletal muscle.

The nitric oxide synthase (NOS) inhibitor N omega-nitro-L-arginine methyl ester (L-NAME) was used to determine whether the decrease in canine hindlimb blood flow (QL) with NOS inhibition would limit skeletal muscle O2 uptake (VO2). Arterial inflow and venous outflow from the hindlimb were isolated, and the paw was excluded from the circulation. Pump perfusion from the right femoral artery kept the hindlimb perfusion pressure near the auto-perfused level. Six anesthetized dogs received L-NAME (20 mg/kg i.v.), whereas another group of five dogs received the stereospecific enantiomer N omega-nitro-D-arginine methyl ester (D-NAME 20 mg/kg i.v.). Efficacy of NOS inhibition was tested with intra-arterial boluses of acetylcholine. QL was measured continuously, and whole body and hindlimb VO2 were measured 60 and 120 min after L-NAME or D-NAME. Whole body VO2 remained at control levels, but cardiac output decreased from 117 +/- 17 to 57 +/- 7 ml.kg-1.min-1 60 min after L-NAME (P < 0.05) and remained at that level for the duration of the experiment. Cardiac output was significantly higher in the D-NAME group than in the L-NAME group at 60 min. After L-NAME, QL fell 24% but VO2 increased from 5.2 +/- 0.4 to 7.4 +/- 0.6 ml.kg-1.min-1 (P < 0.05). No change in QL or VO2 occurred after D-NAME. NOS inhibition did not limit hindlimb VO2, despite decreases in blood flow.(ABSTRACT TRUNCATED AT 250 WORDS)

Our question was whether inhibition of nitric oxide [endothelium-derived relaxing factor (EDRF)/NO] production in an in situ vascularly isolated but innervated canine hindlimb would prevent hypoxic vasodilation or interfere with O2 extraction during ischemic (IH) or hypoxic hypoxia (HH). After a control period, we gave NG-nitro-L-arginine methyl ester (L-NAME, 20 mg/kg i.v.) to two of four groups of six dogs before a 30-min period of IH or HH. In IH, arterial inflow from a pump-membrane oxygenator system was lowered from 65 to 35 ml.min-1.kg-1 with PO2 maintained at approximately 110 Torr. In HH, PO2 was lowered from 107 to 28 Torr with flow at 78 ml.min-1.kg-1. Total O2 delivery was lowered to approximately 5 ml.min-1.kg-1 in all groups during hypoxia. Hindlimb vascular resistance (LVR) increased from 1.11 +/- 0.09 to 2.21 +/- 0.25 peripheral resistance units (PRU; P < 0.05) after L-NAME infusion and hindlimb O2 uptake increased from 3.9 +/- 0.2 to 4.5 +/- 0.3 ml.min-1.kg-1 (P < 0.05). In controls, LVR decreased from 1.10 +/- 0.06 to 0.63 +/- 0.04 PRU with HH (P < 0.05) and from 1.03 +/- 0.06 to 0.82 +/- 0.02 PRU (P = NS) with IH. In L-NAME-treated dogs, LVR decreased from 2.38 +/- 0.37 to 1.07 +/- 0.13 PRU with HH (P < 0.05) and from 2.04 +/- 0.29 to 1.41 +/- 0.13 PRU (P = NS) with IH. There were no differences in O2 extraction ratio (0.72) or in O2 uptake between groups during hypoxia.(ABSTRACT TRUNCATED AT 250 WORDS)

We and others have postulated that the most efficient oxygen extraction from a diminished oxygen supply is achieved when locally-generated vasodilators promote blood flow to areas of greater O2 demand while others remain under vasoconstrictor tone (Cain and Chapler, 1980; Granger and Shepherd, 1979). Previous experiments in our laboratory have shown that administration of Nω-nitro-L-arginine methyl ester (L-NAME), a nitric oxide synthase (NOS) inhibitor, doubles both total and hindlimb peripheral resistance in the anesthetized dog. The current study was undertaken to test the hypothesis that the greater levels of whole body and hindlimb resistance during NOS inhibition would limit the ability of the animal to effectively utilize the reduced oxygen supply available during severe hypoxic hypoxia (HH). In particular, we wished to assess the functional significance of endothelium derived relaxing factor on oxygen utilization and vascular resistance during severe hypoxic hypoxia in the whole body and skeletal muscle.

When anesthetized dogs were given the α-adrenergic blocking agent phenoxybenzamine and then made hypoxic, their ability to extract oxygen from a supply that was limiting to oxygen uptake was significantly less than in unblocked animals (Cain, 1978). This was evident by a lower slope of the line relating O2 uptake to oxygen delivery as O2 uptake became linearly dependent upon O2 delivery. The reason for the lesser efficiency in extracting oxygen by the α-blocked animals was postulated to depend upon the loss of vasoconstrictor tone. The hypothesis that was offered stated that a vigorous constrictor tone was necessary in hypoxia so that blood flow in excess to need would not occur in any organ system or tissue. The constrictor tone in areas where O2 demand exceeded O2 supply would then be offset by the production of vasodilator metabolites in proportion to the imbalance of supply and demand. In this manner, blood flow and O2 delivery would be matched to local O2 need so that O2 would not be shunted through areas that were overperfused relative to their O2 uptake.

Microembolization of peripheral vascular beds has been suggested as a mechanism for the pathological O2 supply dependency seen in adult respiratory distress syndrome (ARDS). To test the feasibility of this, we injected 19 million 14pm microspheres per 100 g of muscle into the arterial supply of the left hindlimb in anesthetized dogs (n = 8). Venous outflow from the limb was isolated and flow, O2 concentration, and blood gas tensions of arterial and venous blood were measured before and after embolization (EMB) and then during 60 minutes of hypoxia induced by ventilating the paralyzed animal with 9% O2-91 % N2. Another group (n = 8) was not embolized and was treated the same for comparison. Limb blood flow increased transiently with EMB but venous PO2 remained elevated (P < .05) even after blood flow returned to baseline. O2 uptake varied with O2 delivery after EMB during normoxia but not in the nonembolized group. Responses of blood flow and O2 extraction fraction lagged those in the non-embolized group during hypoxia. Critical O2 extraction fraction was always lower than in the nonembolized group during hypoxia, but the time course of adaptation was similar. The results were consonant with microembolization causing abnormal O2 supply dependency resembling that seen in ARDS.

We have examined the relative deficits in tension development and O2 uptake in contracting skeletal muscle during severe hypoxic hypoxia. Anesthetized mongrel dogs were ventilated to maintain an end-tidal PCO2 between 35 and 40 Torr. Venous outflow from the gastrocnemius muscle was measured using an electromagnetic flow probe. The tendon was cut and attached to a strain gauge. The muscle was stimulated to contract isometrically at 2 or 4 Hz for 20 min. Hypoxia (9% O2 in N2) was then imposed for 30 min, followed by 30 min of normoxia. Blood flow first increased in proportion to the contraction frequency and then increased further a similar amount in both groups during hypoxia. O2 extraction and blood flow reached maximal levels during hypoxia in the 2-Hz group. The further O2 deficit that was accumulated during 4 Hz and hypoxia was, therefore, a result of the greater discrepancy between O2 supply and demand. O2 uptake decreased more in hypoxia than did developed tension. These results are best explained by ATP supplementation from nonaerobic energy sources that was promoted by the free-flow condition of hypoxic hypoxia.

When systemic delivery of O2 (QO2 = QT X CaO2, where QT is cardiac output and CaO2 is arterial O2 content) is reduced by bleeding, the systemic O2 extraction ratio [ER = (CaO2 - CVO2)/CaO2, where CVO2 is venous O2 content] increases until a critical limit is reached below which O2 uptake (VO2) becomes limited by O2 delivery. During hypovolemia, reflex increases in mesenteric arterial tone may preferentially reduce gut blood flow so that the onset of O2 supply dependence occurs in the gut before other regions. We compared the critical O2 delivery (QO2c) and critical extraction ratio (ERc) of whole body and an isolated segment (30-50 g) of small bowel in seven anesthetized paralyzed dogs ventilated with room air. Systemic QO2 was reduced in stages by controlled hemorrhage as arterial O2 content was maintained, and systemic and gut VO2 and QO2 were measured at each stage. Body QO2c was 7.9 +/- 1.9 ml X kg-1 X min-1 (ERc = 0.69 +/- 0.12), whereas gut O2 supply dependency occurred when gut QO2 was 34.3 +/- 11.3 ml X min-1 X kg gut wt-1 (ERc = 0.63 +/- 0.09). O2 supply dependency in the gut occurred at a higher systemic QO2 (9.7 +/- 2.7) than whole-body QO2c (P less than 0.05). The extraction ratio at the final stage (maximal ER) was less in the gut (0.80 +/- 0.05) than whole body (0.87 +/- 0.06). Thus during reductions in systemic QO2, gut VO2 was maintained by increases in gut extraction of O2.(ABSTRACT TRUNCATED AT 250 WORDS)

The consequences of a decreased O2 supply to a contracting canine gastrocnemius muscle preparation were investigated during two forms of hypoxia: hypoxic hypoxia (HH) (n = 6) and CO hypoxia (COH) (n = 6). Muscle O2 uptake, blood flow, O2 extraction, and developed tension were measured at rest and at 1 twitch/s isometric contractions in normoxia and in hypoxia. No differences were observed between the two groups at rest. During contractions and hypoxia, however, O2 uptake decreased from the normoxic level in the COH group but not in the HH group. Blood flow increased in both groups during hypoxia, but more so in the COH group. O2 extraction increased further with hypoxia (P less than 0.05) during concentrations in the HH group but actually fell (P less than 0.05) in the COH group. The O2 uptake limitation during COH and contractions was associated with a lesser O2 extraction. The leftward shift in the oxyhemoglobin dissociation curve during COH may have impeded tissue O2 extraction. Other factors, however, such as decreased myoglobin function or perfusion heterogeneity must have contributed to the inability to utilize the O2 reserve more fully.

As a significant user of O2 at rest (20% of whole body), the gut may be subject to more severe limitation of O2 supply during global hypoxia than more vital areas because of preferential redistribution of blood flow. Accordingly, its accumulation of O2 deficit during hypoxia and its excess O2 use during normoxic recovery might be altered by extrinsic neural activity. We measured blood flow and O2 uptake in whole body (WB) and gut segments while anesthetized dogs were ventilated with 9% O2-91% N2 for 30 min followed by 30-min normoxic recovery. In six dogs extrinsic innervation to the gut segment was left intact and it was severed in another six animals. O2 deficit and excess were the accumulated differences from the normoxic O2 uptake for both gut and WB corrected for O2 stores changes. The intact gut, although only 4% body wt, incurred 22% of WB O2 deficit but contributed only 8% to WB O2 excess. The imbalance (gut excess was only 44% of gut deficit) implied that O2 using functions were curtailed during hypoxia without obligating an energy stores deficit. Denervation did not alter these quantitative relationships. Blood flow responses to transition between normoxia and hypoxia were only transiently altered. Extrinsic innervation apparently plays no major role in gut responses to WB hypoxia.

The distribution of whole-body O2 supply during severe hypoxia and recovery and its relation to the regional distribution of O2 deficit and repayment was studied. Mongrel dogs were anesthetized, paralyzed, and ventilated to maintain an end-tidal PCO2 between 35 and 40 Torr. In one group, the alpha- and beta-adrenergic receptors were blocked to eliminate neural and humoral adrenergic influences. In a second group, alpha-adrenergic receptors were stimulated to decrease O2 delivery by excessive vasoconstriction. In a third group, beta-adrenergic receptors were stimulated to increase O2 delivery. Whole-body and hindlimb muscle O2 uptake and vascular responses were measured during normoxic control, 15 or 30 min of severe hypoxia (9% O2 in N2), and 20 or 30 min of normoxic recovery, respectively. The whole-body O2 deficit and excess O2 uptake in recovery were partitioned into muscle and nonmuscle areas. The data showed that neural or humoral influences had little effect on the regional distribution of the total O2 deficit and O2 excess in recovery. The O2 deficit could be decreased somewhat by increasing delivery, but the amount of excess O2 used in recovery was unaffected. This suggested that the excess O2 use in recovery was more a function of an energy deficit during hypoxia and not an O2 deficit.

We wished to see whether aortic chemoreceptors and other vagal afferent traffic played an essential role in the circulatory adjustments to hypoxic hypoxia. Aortic chemoreceptors were denervated (AD) in one group (n = 6) of anesthetized dogs, bilateral cervical vagotomy (V) was done on a second group (n = 6), and a third group (n = 6) was sham-operated to serve as a control. Venous outflow from the left hindlimb was isolated. After a 20-min control period of ventilation with room air, the animals were ventilated for 60 min with 9% of O2 in N2. Arterial, mixed venous, and hindlimb venous blood samples were taken every 20 min. The cardiac output response to hypoxic hypoxia was attenuated at 40 and 60 min in both the AD and V groups (p less than 0.05). Hindlimb blood flow increased equally in all three groups during hypoxia. The pressor response at the onset of hypoxia (20 min) was abolished in the AD and V groups, but mean arterial pressure fell to similar levels in all three groups by 60 min of hypoxia. We concluded that reflex aortic chemoreceptor stimulation during hypoxia augmented cardiac output mostly by effects on the venous side of the circulation but played no role in skeletal muscle vascular responses to hypoxic hypoxia.

The importance of aortic chemoreceptors in the circulatory responses to severe carbon monoxide (CO) hypoxia was studied in anesthetized dogs. The aortic chemoreceptors were surgically denervated in eight dogs prior to the induction of CO hypoxia, with nine other dogs serving as intact controls. Values for both whole body and hindlimb blood flow, vascular resistance, and O 2 uptake were determined prior to and at 30 min of CO hypoxia in the two groups. Arterial O 2 content was reduced 65% using an in situ dialysis method to produce CO hypoxia. At 30 min of hypoxia, cardiac output increased but limb blood flow remained at prehypoxic levels in both groups. This indicated that aortic chemoreceptor input was not necessary for the increase in cardiac output during severe CO hypoxia, nor for the diversion of this increased flow to nonmuscle tissues. Limb O 2 uptake decreased during CO hypoxia in the aortic-denervated group but remained at prehypoxic levels in the intact group. The lower resting values for limb blood flow in the aortic-denervated animals required a greater level of O 2 extraction to maintain resting O 2 uptake. When CO hypoxia was superimposed upon this compensation, an O 2 supply limitation occurred because the limb failed to vasodilate even as maximal levels for O 2 extraction were approached.