Effects of muscle contraction on cytochrome a,a3 redox state.

The relationships among mitochondrial O2 availability, O2 delivery, and lactate formation in exercising skeletal muscle remain unclear. Some data suggest that muscle O2 provision is sufficient at maximal O2 consumption (VO2max) to challenge the concept of a mitochondrial O2 limitation at VO2max. The relationships among VO2, mitochondrial O2 availability, and net lactate production were studied over a wide range of exercise intensities. Using near-infrared spectroscopy, the oxidation-reduction state of cytochrome a,a3 was monitored in the canine gracilis in vivo. Twenty adult dogs were anesthetized with alpha-chloralose, intubated, and mechanically ventilated on room air. Five-minute stimulation periods at rates of 2, 3, 4, 5, 7, 8, 10, or 12 stimuli/s were performed. VO2max generally was achieved at a stimulation rate of 8 stimuli/s; mean VO2max was 0.12 +/- 0.09 (SE) ml.min-1 x g-1. The concentration of oxidized mitochondrial cytochrome a,a3 decreased at all work loads relative to resting state and demonstrated a near-linear relationship with muscle VO2 (r2 = 0.99). Muscle lactate efflux and the lactate-pyruvate ratio also were correlated positively with cytochrome a,a3 reduction, suggesting a common regulatory mechanism coupling the processes of aerobic glycolysis and oxidative phosphorylation. At VO2max, the corresponding cytochrome oxidation was not significantly different from that observed at death. Thus, in the gracilis maximal exercise leads to near-complete reduction of cytochrome a,a3 secondary to deficient O2 provision. We conclude that VO2max is limited primarily by O2 delivery to this muscle and not by other factors limiting mitochondrial ATP production or substrate oxidation.

Cerebral oxygen availability by NIR spectroscopy during transient hypoxia in humans.

The effects of mild hypoxia on brain oxyhemoglobin, cytochrome a,a3 redox status, and cerebral blood volume were studied using near-infrared spectroscopy in eight healthy volunteers. Incremental hypoxia reaching 70% arterial O2 saturation was produced in normocapnia [end-tidal PCO2 (PETCO2) 36.9 +/- 2.6 to 34.9 +/- 3.4 Torr] or hypocapnia (PETCO2 32.8 +/- 0.6 to 23.7 +/- 0.6 Torr) by an 8-min rebreathing technique and regulation of inspired CO2. Normocapnic hypoxia was characterized by progressive reductions in arterial PO2 (PaO2, 89.1 +/- 3.5 to 34.1 +/- 0.1 Torr) with stable PETCO2, arterial PCO2 (PaCO2), and arterial pH and resulted in increases in heart rate (35%) systolic blood pressure (14%), and minute ventilation (5-fold). Hypocapnic hypoxia resulted in progressively decreasing PaO2 (100.2 +/- 3.6 to 28.9 +/- 0.1 Torr), with progressive reduction in PaCO2 (39.0 +/- 1.6 to 27.3 +/- 1.9 Torr), and an increase in arterial pH (7.41 +/- 0.02 to 7.53 +/- 0.03), heart rate (61%), and ventilation (3-fold). In the brain, hypoxia resulted in a steady decline of cerebral oxyhemoglobin content and a decrease in oxidized cytochrome a,a3. Significantly greater loss of oxidized cytochrome a,a3 occurred for a given decrease in oxyhemoglobin during hypocapnic hypoxia relative to normocapnic hypoxia. Total blood volume response during hypoxia also was significantly attenuated by hypocapnia, because the increase in volume was only half that of normocapnic subjects. We conclude that cytochrome a,a3 oxidation level in vivo decreases at mild levels of hypoxia. PaCO is an important determinant of brain oxygenation, because it modulates ventilatory, cardiovascular, and cerebral O2 delivery responses to hypoxia.

Comparison of systemic oxygen delivery and uptake with NIR spectroscopy of brain during normovolemic hemodilution in the rabbit.

Incremental hyperoxic normovolemic hemodilution was utilized to progressively decrease oxygen delivery (DO2) in anesthetized rabbits. At decreasing DO2, we compared systemic responses related to the adequacy of DO2, i.e. mixed venous oxygen saturation (SvO2), oxygen consumption (VO2), and arterial lactate concentrations, to near infrared spectroscopy (NIRS) of the brain, a regional measure of intracellular oxygen availability. We sought concomitantly to define critical SvO2 and DO2, beyond which whole body VO2 begins to decline and arterial lactate concentrations increase. NIR Spectroscopy provided the means to test the hypothesis that systemic indicators of inadequate DO2 would not accurately reflect the oxygenation of a critical organ such as the brain. In thirteen rabbits anesthetized with fentanyl, paralyzed and artificially ventilated at an FIO2 of 0.60, hemodilution produced an early decrease in mixed venous oxygen saturation. When mixed venous oxygen saturation decreased below approximately 50%, arterial lactate concentrations began to increase significantly. Further decreases in oxygen delivery precipitated a decline in systemic VO2. Finally, NIRS revealed an increase in the reduction level of brain cytochrome a,a3 after systemic parameters of oxygen delivery had been altered. Analysis of the data indicated that falling SvO2 predicted inadequate DO2 to tissue during early hemodilution under narcotic/relaxant anesthesia and that the brain showed evidence of intracellular hypoxia only after systemic parameters such as SvO2 were affected markedly.

Cerebrocortical oxygenation and ventilatory response during sustained hypoxia.

Cerebrocortical oxygenation was monitored in 8 healthy adults during exposure to sustained isocapnic hypoxia. Subjects were maintained at an arterial oxygen saturation (SaO2) of 80% for 12 min with a rebreathing circuit while cerebrocortical oxygenation was assessed non-invasively using near-infrared (NIR) spectroscopy to measure changes in the oxidation state of cytochrome a,a3 (Cyt a,a3) and changes in cortical blood volume (tBV). During sustained hypoxia, subjects demonstrated a biphasic ventilatory response. The mean minute ventilation (VE) peak response was 255% of baseline at an average of 3.4 +/- 0.5 min (mean +/- SE) after the initiation of hypoxia. A subsequent significant attenuation of VE to 163% (P less than 0.05) of baseline occurred after an additional 8.6 min. NIR monitoring revealed a significant (P less than 0.05) decrease in oxidized Cyt a,a3 as well as a significant (P less than 0.05) increase in tissue blood volume (tBV) at the time of peak VE. Both Cyt a,a3 and tBV remained stable during the remainder of the hypoxic period, despite attenuation of VE during sustained hypoxia. The data suggest that cerebrocortical oxygenation and blood flow remain constant when the ventilatory attenuation is observed during sustained hypoxia.

Eicosanoids and the hemodynamic course of live Escherichia coli-induced sepsis in baboons.

Time-related changes in eicosanoid release and hemodynamic parameters were characterized in baboons during the early development of sepsis induced by intravenous (i.v.) infusion of live Escherichia coli (4 x 10(10) organisms/kg) in baboons. Plasma levels of thromboxane B2 (TxB2), a stable metabolite of thromboxane A2 (TxA2), rose rapidly in arterial, venous, and pulmonary arterial blood after infusion of live E. coli, attaining maximal increases at 30 min and returning to control values by 60 min. In contrast, plasma concentrations of 6-keto-PGF1 alpha rose slowly after infusion, reaching peak concentrations at 120 min, then slowly returned to control values between 4 and 5 hr after infusion of live E. coli. Hemodynamic values remained stable during the first 2 hr after infusion, although early changes in cellular energy metabolism and incipient hemodynamic failure were inferred from pyrexia, tachycardia, and metabolic acidosis. At 3 hr, signs of further hemodynamic compromise developed, including increased venous PCO2, reduced pulmonary capillary wedge pressure, and reduced stroke volume, followed by gradual increases in systemic and pulmonary vascular resistance. These factors coincided with progressive reductions in cardiac output and deteriorating circulatory efficiency. The time course of events following infusion of live E. coli indicates that alterations in cellular energy provision occurred early (within 1 hr), whereas central hemodynamic parameters decayed much more slowly. Additionally, TxA2 and PGI2 appear related to the early events in the development of sepsis as their release preceded cardiocirculatory failure.

Altered mitochondrial redox responses in gram negative septic shock in primates.

Gram negative sepsis causes changes in oxygen supply-demand relationships. We have used a primate model of hyperdynamic gram negative sepsis produced by intravenous infusion of Escherichia coli (E. coli) to evaluate sepsis-induced alterations in mitochondrial oxidation-reduction (redox) state in muscle in vivo. The redox state of cytochrome a,a3, the terminal member of the intramitochondrial respiratory chain, was assessed in the intact forearm by near-infrared (NIR) spectroscopy. The muscle NIR data were compared to routine measures of oxygen delivery (DO2) and oxygen consumption (VO2). After E. coli infusion and fluid resuscitation, DO2 and VO2 showed minimal changes through 24 hr of sepsis. In contrast, changes in cytochrome a,a3 redox state evaluated by NIR occurred within a few hours and were progressive. Mitochondrial functional responses were correlated with structural changes observed on serial muscle biopsies. Gross morphological changes in muscle mitochondria were present in some animals as early as 12 hr, and, in most animals, by 24 hr. The morphologic changes were consistent with decreases in oxidative capacity as suggested by NIR spectroscopy. The NIR data also suggest that two mechanisms are operating to explain abnormalities in oxygen metabolism and mitochondrial function in lethal sepsis. These mechanisms include an early defect in oxygen provision to mitochondria that is followed by a progressive loss in functional cytochrome a,a3 in the muscle.