PAF, rather than Histamine, Participates in Mouse Anaphylactic Hypotension

We determined the roles of platelet-activating factor (PAF) and histamine in anaphylactic hypotension in ovalbumin-sensitized anesthetized BALB/c mice. The effects of PAF and histamine on hemodynamic variables were studied by measuring the systemic arterial (Psa), portal venous (Ppv) and central venous (Pcv) pressures. Intravenous PAF evoked a biphasic Psa response, an initial rapid and transient drop followed by marked hypotension, accompanied by a decrease in Pcv. Histamine caused only mild systemic hypotension. Both agents similarly increased Ppv by approximately 4 cm H2O at high doses. After an injection of antigen, Psa initially increased slightly and then decreased from the baseline of 94 ± 1 mm Hg to 46 ± 1 mm Hg at 10 min after antigen administration, with Pcv decreasing by 2.5 cm H2O. Ppv increased by 3.5 cm H2O at 5 min after antigen injection. Pretreatment with either CV-6209 (PAF receptor antagonist, 1 mg/kg) or diphenhydramine (histamine H1 receptor antagonist, 20 mg/kg) significantly attenuated an antigen-induced decrease in Psa. The inhibitory action of CV-6209 was greater than that of diphenhydramine, and the combination of these 2 antagonists almost completely inhibited the anaphylactic hypotension. In contrast, the antigen-induced increase in Ppv was attenuated by CV-6209 alone but augmented by diphenhydramine. It is concluded that anaphylactic hypotension is mainly mediated by PAF and, to a lesser extent, by histamine in anesthetized BALB/c mice.