EFFECT OF 5-HYDROXYTRYPTAMINE TYPE-2 RECEPTOR BLOCKADE ON PULMONARY-FUNCTION IN CALVES WITH EXPERIMENTALLY-INDUCED PASTEURELLA-HAEMOLYTICA PNEUMONIA

Objective - To evaluate a 5-hydroxytryptamine type-2 receptor antagoni st, metrenperone (MET), in alleviating respiratory distress associated with experimentally induced Pasteurella haemolytica pneumonia in feed lot calves. Design - Double-blind controlled clinical trial. Animals - 30 healthy 6- to 8-month-old Hereford-type calves (250 to 450 kg). Pr ocedure - Initial measurements were made of rectal temperature (RT), a rterial blood gas (ABG) tensions, and pulmonary mechanics. Calves were then infected with P haemolytica in logarithmic phase of growth by in tratracheal inoculation. 18 hours later, determination of RT and ABG t ensions, and pulmonary function testing were repeated and calves were selected for inclusion in the study on the basis of having 2 of the fo llowing. respiratory rate > 50 breaths/min, RT > 40 C, or Pa-O2 > 20 m m of Hg below the baseline value. MET (0.1 mg/kg of body weight, IM) o r an equivalent vehicle dose was when administered. RT, ABG, and pulmo nary mechanics measurements were repealed at 0.5, 1, 2, 3, 4, 6, 12, a nd 24 hours after treatment. Calves were then euthanatized, and gross necropsy scoring and histologic examination were performed on the lung s. Results - Injection with P haemolytica caused significant increases in RT and respiratory rate, and reduction in Pa-O2, Pa-CO2, and tidal volume 18 hours after inoculation. MET-treated calves had significant ly reduced rectal temperature between 1 and 12 hours, compared with ve hicle-treated calves. In addition, MET-treated calves had reduced resp iratory rate with concomitantly increased tidal volume between 0.5 and 2 hours after treatment, compared with vehicle-treated calves. Necrop sy revealed acute lobar bronchopneumonia in all 30 calves, but there w as no difference in necropsy score between treatment groups. Conclusio ns and Clinical Relevance - MET may have an antipyretic effect on calv es with pneumonia caused by Phaemolytica. Its influence on pulmonary m echanics was minimal however, and it did nor induce lung lesions in th e short term.