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 antagonist, metrenperone (MET), in alleviating respiratory distress associated with experimentally induced Pasteurella haemolytica pneumonia in feedlot calves.


METHODS
Double-blind controlled clinical trial.


METHODS
30 healthy 6- to 8-month-old Hereford-type calves (250 to 450 kg).


METHODS
Initial measurements were made of rectal temperature (RT), arterial blood gas (ABG) tensions, and pulmonary mechanics. Calves were then infected with P haemolytica in logarithmic phase of growth by intratracheal inoculation. 18 hours later, determination of RT and ABG tensions, and pulmonary function testing were repeated and calves were selected for inclusion in the study on the basis of having 2 of the following: respiratory rate > 50 breaths/min, RT > 40 C, or PaO2 > 20 mm of Hg below the baseline value. MET (0.1 mg/kg of body weight, IM) or an equivalent vehicle dose was then administered. RT, ABG, and pulmonary mechanics measurements were repeated at 0.5, 1, 2, 3, 4, 6, 12, and 24 hours after treatment. Calves were then euthanatized, and gross necropsy scoring and histologic examination were performed on the lungs.


RESULTS
Infection with P haemolytica caused significant increases in RT and respiratory rate, and reduction in PaO2, PaCO2, and tidal volume 18 hours after inoculation. MET-treated calves and significantly reduced rectal temperature between 1 and 12 hours, compared with vehicle-treated calves. In addition, MET-treated calves had reduced respiratory rate with concomitantly increased tidal volume between 0.5 and 2 hours after treatment, compared with vehicle-treated calves. Necropsy revealed acute lobar bronchopneumonia in all 30 calves, but there was no difference in necropsy score between treatment groups.


CONCLUSIONS
MET may have an antipyretic effect on calves with pneumonia caused by P haemolytica. Its influence on pulmonary mechanics was minimal however, and it did not induce lung lesions in the short term.
Hare
J E
JE
Department of Biomedical Sciences, Ontario Veterinary College, University of Guelph, Canada.
Tesarowski
D B
DB
Dawe
G E
GE
Vlaminck
K
K
Shewen
P E
PE
Viel
L
L
D000818:Animals; D001066:Appetite / Q000187:drug effects; D001831:Body Temperature; D002245:Carbon Dioxide / Q000097:blood; D002417:Cattle; D008168:Lung / Q000187:drug effects / Q000473:pathology / Q000503:physiopathology*; D016978:Mannheimia haemolytica; D010100:Oxygen / Q000097:blood; D010313:Partial Pressure; D010326:Pasteurella Infections / Q000473:pathology / Q000503:physiopathology*; D010880:Piperidines / Q000494:pharmacology*; D011985:Receptors, Serotonin / Q000502:physiology*; D012119:Respiration; D012129:Respiratory Function Tests / Q000662:veterinary*; D012702:Serotonin Antagonists / Q000494:pharmacology*; D013990:Tidal Volume / Q000187:drug effects; D013997:Time Factors
D010880:Piperidines; D011985:Receptors, Serotonin; D012702:Serotonin Antagonists; D002245:Carbon Dioxide; D010100:Oxygen; C046881:metrenperone