The role of 5-hydroxytryptamine in the feline response to intravenous infusion of live E. coli.

A standardized septic shock was induced in cats by intravenous infusion of a live E. coli bacteria strain. The bacterial infusion induced a rapid haemodynamic response characterized mainly by a pulmonary arterial hypertension and a late phase characterized by systemic hypotension and hypodynamic circulation. Systemic arterial, pulmonary arterial, portal venous, left atrial pressures, max inspiratory-expiratory pressure difference in the trachea, aortic and intestinal blood flows were monitored. Arterial blood samples were taken for recording the number of circulating platelets and white blood cells and for determining the acid-base balance. The effect of pretreatment with ketanserin, a specific 5-hydroxytryptamine2 (5-HT2)-receptor blocker on these haemodynamic reactions was studied. In short term experiments on non-bacteriaemic control cats, ketanserin prevented the pulmonary hypertension induced by intravenous 5-HT infusions but not the increase in intestinal blood flow. Ketanserin induced a reduction of total peripheral (including intestinal) vascular resistance to blood flow but had no effect on aortic blood flow. After infusion of bacteria, ketanserin pretreated cats were more hypotensive due to a relative peripheral dilatation of the resistance vessels. Ketanserin pretreatment had no effect on the pulmonary vascular reactions, the tracheal pressure difference or the number of circulating platelets or white blood cells. Thus, except for a more pronounced hypotension early after bacterial infusion, ketanserin pretreatment did not influence the haemodynamic response. It is concluded that 5-HT is not of significant importance in the pathogenesis of the haemodynamic reactions following experimental bacteraemia.