The cardiovascular response to
5-hydroxytryptamine (5-HT) challenge has been previously described in cattle. Abrupt
bradycardia, followed by
tachycardia, triphasic systemic blood pressure response, and
pulmonary hypertension were the major changes elicited by
5-HT. The purpose of the present study was to determine whether the cardiovascular response to
5-HT in calves was attributable to 5-HT2 receptors. A specific
5-HT2 antagonist (
metrenperone, 0.05 mg/kg) was administered intramuscular to six unsedated Friesian calves 30 min before the animals were given a 5-min intravenous
5-HT infusion. Mean systemic arterial (SAP), mean pulmonary arterial (PAP), pulmonary capillary wedge (PW) pressures were obtained by means of fluid-filled
catheters, and cardiac output (CO) was measured by the thermodilution technique. Heart rate, stroke volume, systemic (SVR) and pulmonary (PVR) vascular resistances were calculated. Administration of
5-HT after
metrenperone induced a short-lasting period of severe
bradycardia followed by
tachycardia and increased CO. The systemic blood pressure response was exclusively hypotensive and associated with a decrease in SVR. Conversely, PAP, PW, and PVR were not modified by
5-HT administration. The results establish that
5-HT induced systemic as well as
pulmonary hypertension is mediated through the activation of type-2 serotonergic or
alpha-adrenergic receptors, or both. In contrast, neither apnoea,
bradycardia and
hypotension, nor the positive chronotropic effect induced by
5-HT in cattle are mediated through such receptors.