Systemic hypoxia produces an inflammatory response characterized by increases in reactive O(2) species (ROS), venular leukocyte-endothelial adherence and emigration, and vascular permeability. Inflammation is typically initiated by mediators released from activated perivascular cells that generate the chemotactic gradient responsible for extravascular leukocyte accumulation. These experiments were directed to study the possible participation of mast cells in hypoxia-induced microvascular inflammation. Mast cell degranulation, ROS levels, leukocyte adherence and emigration, and vascular permeability were studied in the mesenteric microcirculation by using intravital microscopy of anesthetized rats. The main findings were 1) activation of mast cells with compound 48/80 in normoxia produced microvascular effects similar, but not identical, to those of hypoxia; 2) systemic hypoxia resulted in rapid mast cell degranulation; 3) blockade of mast cell degranulation with cromolyn prevented or attenuated the hypoxia-induced increases in ROS, leukocyte adherence/emigration, and vascular permeability; and 4) mast cell degranulation during hypoxia was prevented by administration of the antioxidant lipoic acid and of nitric oxide. These results show that mast cells play a key role in hypoxia-induced inflammation and suggest that alterations in the ROS-nitric oxide balance may be involved in mast cell activation during hypoxia.

We recently demonstrated that systemic hypoxia during reduced inspired PO(2) produces a rapid increase in leukocyte adherence to rat mesenteric venules. Evidence suggests that the mechanism of this response involves decreased nitric oxide (NO) levels. One possible pathway for NO depletion could involve increased reactive oxygen species (ROS) generation resulting in inactivation of NO. The overall goal of the present study was to examine the role of ROS in promoting leukocyte-endothelial adherence during systemic hypoxia. Experiments were designed to 1) evaluate changes in ROS generation in the mesenteric microcirculation during systemic hypoxia, 2) determine how the ROS signal changes when PO(2) levels return to normal after a period of systemic hypoxia, 3) assess the effect of antioxidants on ROS generation during hypoxia, and 4) utilize antioxidants to examine the functional relationship between ROS generation and leukocyte adherence during hypoxia. The major findings from this study are that systemic hypoxia increases ROS generation within the mesenteric microcirculation and that antioxidants prevent the increase in leukocyte-endothelial adhesive interactions observed in hypoxia.

Alveolar hypoxia produces widespread systemic inflammation in rats. The inflammation appears to be triggered by activation of mast cells by a mediator released from alveolar macrophages, not by the reduced systemic partial pressure of oxygen (PO 2 ). If this is correct, the following should apply: (1) neither mast cells nor tissue macrophages should be directly activated by hypoxia; and (2) mast cells should be activated when in contact with hypoxic alveolar macrophages, but not with hypoxic tissue macrophages. We sought here to determine whether hypoxia activates isolated alveolar macrophages, peritoneal macrophages, and peritoneal mast cells, and to study the response of the microcirculation to supernatants of these cultures. Rat mesenteric microcirculation intravital microscopy was combined with primary cultures of alveolar macrophages, peritoneal macrophages, and peritoneal mast cells. Supernatant of hypoxic alveolar macrophages, but not of hypoxic peritoneal macrophages, produced inflammation in mesentery. Hypoxia induced a respiratory burst in alveolar, but not peritoneal macrophages. Cultured peritoneal mast cells did not degranulate with hypoxia. Immersion of mast cells in supernatant of hypoxic alveolar macrophages, but not in supernatant of hypoxic peritoneal macrophages, induced mast cell degranulation. Hypoxia induced release of monocyte chemoattractant protein-1, a mast cell secretagogue, from alveolar, but not peritoneal macrophages or mast cells. We conclude that a mediator released by hypoxic alveolar macrophages activates mast cells and triggers systemic inflammation. Reduced systemic PO 2 and activation of tissue macrophages do not play a role in this phenomenon. The inflammation could contribute to systemic effects of diseases featuring alveolar hypoxia.Keywords: hypoxia; systemic inflammation; alveolar macrophages; mast cells; monocyte chemoattractant protein Alveolar hypoxia, induced by reduction of inspired PO 2 , initiates a rapid and widespread inflammatory response in mesentery (1), skeletal muscle (2, 3), and brain (4) of rats. The inflammation is characterized by increased microvascular levels of reactive oxygen species (ROS) (5), perivascular mast cell degranulation (2, 3, 6), increased leukocyte-endothelial adhesive interactions (1), and extravasation of albumin (7).Studies in the cremaster microcirculation suggest that the inflammation elicited by alveolar hypoxia is not triggered by the reduction of cremaster PO 2 , but rather by a mediator released from a distant site and transported by the circulation. This idea is supported by two lines of evidence: first, selective reduction of cremaster PO 2 does not produce mast cell degranulation and inflammation in the cremaster microcirculation unless alveolar PO 2 is also reduced (2, 3); second, plasma obtained from hypoxic rats applied to the normoxic cremaster produces an inflammatory response similar to that elicited by alveolar hypoxia (8). The response to hypoxic rat plasma is not due to inflammatory mediators released into plasma by...

Although the effects of ischemia-reperfusion have received considerable attention, few studies have directly evaluated the microcirculatory response to systemic hypoxia. The overall objective of this study was to assess the effect of environmental hypoxia on adhesive interactions of circulating leukocytes with rat mesenteric venules by using intravital microscopy. Experiments were designed to 1) characterize the adhesive interactions of circulating leukocytes to venules during acute hypoxia produced by a reduction in inspired PO(2), 2) evaluate the role of nitric oxide in these adhesive interactions, 3) determine whether the effect of hypoxia on leukocyte adhesive interactions differs between acclimatized and nonacclimatized rats, and 4) assess whether compensatory changes in nitric oxide formation contribute to this difference. The results showed that acute hypoxia promotes leukocyte-endothelial adherence in mesenteric venules of nonacclimatized rats. The mechanism of this response is consistent with depletion of nitric oxide within the microcirculation. In contrast, no leukocyte-endothelial adherence occurred during hypoxia in rats acclimatized to hypobaric hypoxia. The results are consistent with increased nitric oxide formation due to expression of inducible nitric oxide synthase during the acclimatization period. Further studies are needed to establish the cause of nitric oxide depletion during acute hypoxia as well as to define the compensatory responses that attenuate hypoxia-induced leukocyte-endothelial adherence in the microvasculature of acclimatized rats.

To attempt to explain the difference in intrinsic (untrained) endurance running capacity in rats selectively bred over seven generations for either low (LCR) or high running capacity (HCR), the relationship among skeletal muscle capillarity, fiber composition, enzyme activity, and O(2) transport was studied. Ten females from each group [body wt: 228 g (HCR), 247 g (LCR); P = 0.03] were studied at 25 wk of age. Peak normoxic maximum O(2) consumption and muscle O(2) conductance were previously reported to be 12 and 33% higher, respectively, in HCR, despite similar ventilation, arterial O(2) saturation, and a cardiac output that was <10% greater in HCR compared with LCR. Total capillary and fiber number in the medial gastrocnemius were similar in HCR and LCR, but, because fiber area was 37% lower in HCR, the number of capillaries per unit area (or mass) of muscle was higher in HCR by 32% (P < 0.001). A positive correlation (r = 0.92) was seen between capillary density and muscle O(2) conductance. Skeletal muscle enzymes citrate synthase and beta-hydroxyacyl-CoA dehydrogenase were both approximately 40% higher (P < 0.001) in HCR (12.4 +/- 0.7 vs. 8.7 +/- 0.4 and 3.4 +/- 0.2 vs. 2.4 +/- 0.2 mmol. kg(-1). min(-1), respectively), whereas phosphofructokinase was significantly (P = 0.02) lower in HCR (27.8 +/- 1.2 vs. 35.2 +/- 2.5 mmol. kg(-1). min(-1)) and hexokinase was the same (0.65 +/- 0.04 vs. 0.65 +/- 0.03 mmol. kg(-1). min(-1)). Resting muscle ATP, phosphocreatine, and glycogen contents were not different between groups. Taken together, these data suggest that, in rats selectively bred for high-endurance exercise capacity, most of the adaptations for improved O(2) utilization occur peripherally in the skeletal muscles and not in differences at the level of the heart or lung.

O(2) transport during maximal exercise was studied in rats bred for extremes of exercise endurance, to determine whether maximal O(2) uptake (VO(2 max)) was different in high- (HCR) and low-capacity runners (LCR) and, if so, which were the phenotypes responsible for the difference. VO(2 max) was determined in five HCR and six LCR female rats by use of a progressive treadmill exercise protocol at inspired PO(2) of approximately 145 (normoxia) and approximately 70 Torr (hypoxia). Normoxic VO(2 max) (in ml. min(-1). kg(-1)) was 64.4 +/- 0.4 and 57.6 +/- 1.5 (P < 0.05), whereas VO(2 max) in hypoxia was 42.7 +/- 0.8 and 35.3 +/- 1.5 (P < 0.05) in HCR and LCR, respectively. Lack of significant differences between HCR and LCR in alveolar ventilation, alveolar-to-arterial PO(2) difference, or lung O(2) diffusing capacity indicated that neither ventilation nor efficacy of gas exchange contributed to the difference in VO(2 max) between groups. Maximal rate of blood O(2) convection (cardiac output times arterial blood O(2) content) was also similar in both groups. The major difference observed was in capillary-to-tissue O(2) transfer: both the O(2) extraction ratio (0.81 +/- 0.002 in HCR, 0.74 +/- 0.009 in LCR, P < 0.001) and the tissue diffusion capacity (1.18 +/- 0.09 in HCR and 0.92 +/- 0.05 ml. min(-1). kg(-1). Torr(-1) in LCR, P < 0.01) were significantly higher in HCR. The data indicate that selective breeding for exercise endurance resulted in higher VO(2 max) mostly associated with a higher transfer of O(2) at the tissue level.

We recently observed that acute systemic hypoxia produces rapid increases in leukocyte adherence in the mesenteric microcirculation of the anesthetized rat Wood JG, Johnson JS, Mattioli LF, and Gonzalez NC. J Appl Physiol 87: 1734-1740, 1999; Wood JG, Mattioli LF, and Gonzalez NC. J Appl Physiol 87: 873-881, 1999. Hypoxia-induced leukocyte adherence is associated with an increase in reactive oxygen species (ROS) generation and is attenuated by antioxidants or interventions that increase tissue levels of nitric oxide (NO). These results suggest that the acute effects of hypoxia on leukocyte-endothelial interactions are caused by a change in the ROS-NO balance. The present experiments were designed to extend our observations of the initial microcirculatory response to hypoxia; specifically, we wanted to determine whether the response to systemic hypoxia involves increased microvascular permeability and leukocyte emigration and whether ROS generation and decreased NO levels contribute to these responses. At this time, there is conflicting evidence, from in vitro studies, regarding the effect of hypoxia on these indexes of vascular function. Our studies were carried out in the physiological setting of the conscious animal, in which a prolonged hypoxic exposure is possible without the adverse effects that may develop under anesthesia. The central observation of these studies is that conscious animals exposed for 4 h to environmental hypoxia show increased microvascular permeability and emigration of leukocytes into the extravascular space of the mesenteric circulation. Furthermore, these events are dependent on increased ROS generation and, possibly, a subsequent decrease in tissue NO levels during systemic hypoxia. Our results show that systemic hypoxia profoundly affects vascular endothelial function through changes in the ROS-NO balance in the conscious animal.

Systemic hypoxia, produced by lowering inspired Po2, induces a rapid inflammation in several microcirculations, including cremaster muscle. Mast cell activation is a necessary element of this response. Selective reduction of cremaster microvascular Po2 (PmO2) with normal systemic arterial Po2 (PaO2; cremaster hypoxia/systemic normoxia), however, does not elicit increased leukocyte-endothelial adherence (LEA) in cremaster venules. This could be due to a short time of leukocyte exposure to the hypoxic cremaster environment. Conversely, LEA increases when PaO2 is lowered, while cremaster PmO2 remains high (cremaster normoxia/systemic hypoxia). An alternative explanation of these results is that a mediator released from a central site during systemic hypoxia initiates the inflammatory cascade. We hypothesized that if this is the case, cremaster mast cells would be activated during cremaster normoxia/systemic hypoxia, but not during cremaster hypoxia/systemic normoxia. The microcirculation of rat cremaster muscles was visualized by using intravital microscopy. Cremaster PmO2 was measured with a phosphorescence quenching method. Cremaster hypoxia/systemic normoxia (PmO2 7 +/- 1 Torr, PaO2 87 +/- 2 Torr) did not increase LEA; however, topical application of the mast cell activator compound 48/80 under these conditions did increase LEA. The effect of compound 48/80 on LEA was blocked by topical cromolyn, a mast cell stabilizer. LEA increased during cremaster normoxia/systemic hypoxia, (PmO2 64 +/- 5 Torr, PaO2 33 +/- 2 Torr); this increase was blocked by topical cromolyn. The results suggest that mast cell stimulation occurs only when PaO2 is reduced, independent of cremaster PmO2, and support the idea of a mediator that is released during systemic hypoxia and initiates the inflammatory cascade.