C5a/C5a_des-Arg-induced increase in blood pressure in the guinea pig: role of thromboxane

Cleavage of the complement protein C5 by activation of the complement system yields a low molecular weight fragment C5a. Knowledge of the alterations in blood pressure induced by C5a as well as the mediators responsible for the blood pressure changes may provide information concerning the potential role of C5a in the adverse hemodynamic responses associated with complement activation. The purpose of this study was to characterize changes in mean arterial pressure in the guinea pig after intravenous challenge with a combination of guinea pig C5a plus C5a_des-Arg(C5a/C5a_des-Arg) and determine the mediators responsible for the transient increase in blood pressure which was observed. Mean arterial pressure was monitored in mechanically ventilated pentobarbital-anesthetized guinea pigs. Intravenous injection of C5a/C5a_des-Arg consistently caused a marked but transient rise in blood pressure. A transient hypotensive response was also seen with injection of markedly higher doses of guinea pig C5a/C5a_des-Arg. Various pharmacological antagonists were used to determine the mediators responsible for the increase in blood pressure induced by guinea pig C5a/C5a_des-Arg. We found that the LTD₄ antagonist L-649,923 did not inhibit the transient rise in blood pressure. However, the cyclooxygenase inhibitor indomethacin inhibited the C5a/C5a_des-Arg-induced pressor response as did the thromboxane synthetase inhibitor U-63557A and the thromboxane receptor antagonist SQ 29,548. In addition, the C5a/C5a_des-Arg-induced pressor response was not inhibited by the H₁ antagonist pyrilamine, but was inhibited in part by the α-adrenergic antagonist phentolamine. Also, the response was reduced in animals depleted of circulating platelets or white blood cells. Thus, the results of our studies suggest that intravenously injected guinea pig C5a/C5a_des-Arg causes release of the vasoconstrictor thromboxane, most likely from circulating white blood cells or platelets, resulting in a transient rise in blood pressure in the guinea pig. In addition, release of catecholamines may contribute to the pressor response observed.