Mediators of Anaphylaxis

Inhibitory effects of azelastine on nasal allergic responses in sensitized guinea pigs

The in vivo effects of the antiallergic drug azelastine were investigated in sensitized guinea pigs. Topical administration of antigen into the nasal cavity produced an increase in nasal vascular permeability together with an increase in both the histamine and leukotriene C4 (LTC4) concentrations of nasal lavage fluid. Pre-treatment with azelastine significantly inhibited both the LTC4 release and the increase in nasal vascular permeability. These results suggest that azelastine inhibits the release of antigen-induced leukotrienes and increases nasal vascular permeability in vivo.

The pathophysiological role of kinin and chemical mediators on experimental allergic rhinitis

In order to clarify the pathophysiological role of the chemical mediators, the releases of kinins, histamine and leukotriene C4(LTC4) into the nasal cavity were measured following nasal allergic challenge in ovalbumin(OA)-sensitized guinea pigs, or following nasal stimulation with one of these chemical mediators in OA-non-sensitized animals. In sensitized animals, an increased vascular permeability of nasal mucosa was recognized immediately after antigenic stimulation and lasted for 60-90 minutes. Releases of kinins and LTC4 into the nasal lavage fluid augmented not only immediately after the antigenic challenge, but also during 60 to 90 minutes after the stimulation. Release of histamine into the nasal lavage fluid was observed only immediately after the antigenic stimulation. In non-sensitized guinea pigs, nasal stimulation with bradykinin accelerated nasal vascular permeability. Nasal stimulation with histamine or LTC4 resulted in increase of nasal vascular permeability and of kinins concentration in the nasal lavage fluid. These results suggest that kinins might be concerned with the immediate and later vascular permeability during the allergic response.

Metabolism of platelet activating factor (1-alkyl-2-acetyl-sn-glycero-3-phosphocholine) and 1-alkyl-2-acetyl-sn-glycerol by human endothelial cells

The metabolism of platelet activating factor (1-[1,2-3H]alkyl-2-acetyl-sn-glycero-3-phosphocholine) and 1-[1,2-3H]alkyl-2-acetyl-sn-glycerol was studied in cultures of human umbilical vein endothelial cells. Human endothelial cells deacetylated 1-[1,2-3H]alkyl-2-acetyl-sn-glycero-3-phosphocholine to the corresponding lyso compound (1-[1,2-3H]alkyl-2-lyso-sn-glycerol-3-phosphocholine) and a portion was converted to 1-[1,2-3H]alkyl-2-acyl(long-chain)-sn-glycero-3-phosphocholine. Lyso platelet activating factor (lyso-PAF) (1-[1,2-3H]alkyl-2-lyso-sn-glycero-3-phosphocholine) was detected in the media very early during the incubation and the amount remained higher than the level of the lyso product observed in the cells. Cellular levels of 1-[1,2-3H]alkyl-2-lyso-sn-glycero-3-phosphocholine were significantly higher than the acylated product (1-[1,2-3H]alkyl-2-acyl(long-chain)-sn-glycero-3-phosphocholine) at all times during the 60-min incubation period, which suggests that the ratio of acetylhydrolase to acyltransferase activities is greater in endothelial cells than in most other cells. When endothelial cells were incubated with 1-[1,2-3H]alkyl-2-acetyl-sn-glycerol, a known precursor of PAF, 1-[1,2-3H]alkyl-sn-glycerol was the major metabolite formed (greater than 95% of the 3H-labeled metabolites during 20- and 40-min incubations). At least a portion of the acetate was removed from 1-[1,2-3H]alkyl-2-acetyl-sn-glycerol by a hydrolytic factor released from the endothelial cells into the medium during the incubations. Only negligible amounts of the total cellular radioactivity (0.2%) was incorporated into platelet activating factor (1-[1,2-3H]alkyl-2-acetyl-sn-glycero-3-phosphocholine); therefore, it is unlikely that the previously observed hypotensive activity of 1-alkyl-2-acetyl-sn-glycerols can be explained on the basis of the conversion to platelet activating factor (1-alkyl-2-acetyl-sn-glycero-3-phosphocholine) by endothelial cells. Results of this investigation indicate that endothelial cells play an important role in PAF catabolism. Undoubtedly, the endothelium is important in the regulation of PAF levels in the vascular system.

Conversion of 1-alkyl-2-acetyl-sn-glycerols to platelet activating factor and related phospholipids by rabbit platelets

The metabolic pathway for 1-alkyl-2-acetyl-sn-glycerols, a recently discovered biologically active neutral lipid class, was elucidated in experiments conducted with rabbit platelets. The total lipid extract obtained from platelets incubated with 1-[1,2-(3)H]alkyl-2-acetyl-sn-glycerols or 1-alkyl-2-[3H]acetyl-sn-glycerols contained at least six metabolic products. The six metabolites, identified on the basis of chemical and enzymatic reactions combined with thin-layer or high-performance liquid chromatographic analyses, corresponded to 1-alkyl-sn-glycerols, 1-alkyl-2-acetyl-sn-glycero-3-phosphates, 1-alkyl-2-acyl (long-chain)-sn-glycero-3-phosphoethanolamines, 1-alkyl-2-acetyl-sn-glycero-3-phosphoethanolamines, 1-alkyl-2-acetyl-sn-glycero-3-phosphoethanolamines, 1-alkyl-2-acyl(long-chain)-sn-glycero-3-phosphocholines, and 1-alkyl-2-acetyl-sn-glycero-3-phosphocholines (platelet activating factor). These results indicate that the metabolic pathway for enzymatic activities: choline- and ethanolamine- phosphotransferases, acetyl-hydrolase, an acyltransferase, and a phosphotransferase. The step responsible for the biosynthesis of platelet activating factor would appear to be the most important reaction in this pathway and this product could explain the hypotensive activities previously described for alkylacetyl-(or propionyl)-glycerols. Of particular interest was the preference exhibited for the utilization of the 1-hexadecyl-2-acetyl-sn-glycerol species in the formation of platelet activating factor.