Activation of mast cells by systemic hypoxia, but not by local hypoxia, mediates increased leukocyte-endothelial adherence in cremaster venules

Abstract

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.

Renin released from mast cells activated by circulating MCP-1 initiates the microvascular phase of the systemic inflammation of alveolar hypoxia

“…This response, which has been observed directly in the mesentery (4, 5, 22, 30 -32) and skeletal muscle (6,10,21) and pial microcirculations (13), is not initiated by the low tissue PO 2 but rather by the rapid release into the circulation of a chemokine, monocyte chemoattractant protein-1 (MCP-1), also known as CCL2, from alveolar macrophages (AMO) activated by the low alveolar PO 2 (4). Circulating MCP-1 induces degranulation of perivascular MCs (4), which, in turn, leads to activation of the reninangiotensin system (RAS) and microvascular inflammation (10).…”

“…MCs isolated by this procedure exceed 95% in purity (4,5,18) as determined by light microscopy. Approximately 1 ϫ 10 6 MCs are usually harvested from one rat. The separated MCs were resuspended in 2 ml of DMEM with 10% serum containing penicillin (100 U/ml) and streptomycin (100 g/ml) and plated in a T-25 sterile flask at 37°C in 10% O 2-5% CO2-85% N2 for 45 min.…”

“…MCs did not adhere to the culture plate; rather, they were suspended in plasma, and the cell suspension was equilibrated with humidified gas mixtures (10% O 2-5% CO2-85% N2) via an 18-gauge needle placed on the flask's cap. Forty five minutes later, the suspension was divided into aliquots of 0.4 ml containing 0.4 ϫ 10 6 MCs each. The different pharmacological agents described in Experimental Protocol were administered at this step.…”

“…The possibility that the systemic inflammation is initiated by a mediator released from a distant site and transported by blood had been suggested by two critical findings: the first is a dissociation observed between tissue microvascular PO 2 and inflammation: MC degranulation and leukocyte recruitment occur only when alveolar PO 2 is reduced, independent of the systemic PO 2 (6,22). The second finding is that plasma from hypoxic rats evokes an inflammatory response in tissues of normoxic rats (16).…”

Monocyte Chemoattractant Protein–1 Released from Alveolar Macrophages Mediates the Systemic Inflammation of Acute Alveolar Hypoxia

“…The reduction of inspired partial pressure of oxygen (PO 2 ) in rats induces a rapid and widespread inflammatory response in the mesentery (6), skeletal muscle (7,8), and brain (9), as characterized by increased microvascular concentrations of reactive O 2 species (10), increased leukocyte endothelial adhesive interactions (6), leukocyte emigration (11), increased vascular permeability as evidenced by the extravasation of albumin in the presence of reduced blood flow (11-13), and the degranulation of perivascular mast cells (MCs) (7,8,14). The degranulation of MCs is a key event: the MC stabilizer cromolyn attenuates all of the markers of inflammation induced by alveolar hypoxia (14).…”