Impairment of sympathetic pulmonary vasoconstriction by 3-methylindole in cattle.

Sixteen Holstein cattle allotted into 4 groups (4 cattle/group) were each given a single oral dosage of 0.2 g of 3-methylindole (3MI)/kg of body weight. The groups were killed at 12, 24, 48, and 72 hours, respectively, after 3MI administration. Comparison of clinical signs, pathologic pulmonary lesions, and in vitro pulmonary artery responses to pharmacologic stimuli was made between the 4 treated groups and 8 control Holstein cattle of similar age. Clinical signs of pulmonary distress first appeared 8 to 12 hours after 3MI administration. After 20 hours, clinical signs included dyspnea, moderate depression, and a marked expiratory grunt. A partial remission of these clinical signs was seen between 30 and 45 hours after 3MI administration. After remission, the cattle had clinical signs of severe dyspnea and depression and expiratory grunts were more pronounced. Pathologic pulmonary lesions, including heavy rubbery lungs, dilated interlobular septae, and subplural air bullae characteristic of pulmonary edema and interstitial emphysema were observed. The lungs of treated cattle did not collapse when the thorax was incised at necropsy. In vitro pulmonary artery strips contracted dose dependently to norepinephrine (NE). Group I tissues (12 hours after 3MI administration) responded similarly to control samples. Group II tissues (24 hours after 3MI administration) had a significant inhibition (P less than 0.05) in response to NE stimulation as compared with controls.