Modulation of bronchial responsiveness in horses by phenylbutazone and furosemide.

Effects of phenylbutazone (PBZ) and furosemide (FUR) on the respiratory tract of horses were evaluated, focusing on bronchial responsiveness. Four healthy Thoroughbreds were used and data were analyzed by use of a Latin square design. Histamine provocation tests (0.5, 1, 2, and 4 micrograms/min, i.v.) were done: (1) without prior treatment with PBZ or FUR, (2) 30 minutes after administration of PBZ (8 mg/kg, i.v.), (3) 1 hour after administration of FUR (1 mg/kg, i.v.), and (4) after administration of PBZ plus FUR. Pulmonary function tests (dynamic compliance, resistance, respiratory frequency, and tidal volume) and heart rate were monitored throughout the experiments. Phenylbutazone did not influence basal pulmonary function test results, whereas FUR caused a significant (P < 0.05) increase in dynamic compliance and decrease in resistance. Histamine infusion resulted in a dose-dependent decrease in dynamic compliance and a dose-dependent increase in resistance, respiratory frequency, and heart rate. Phenylbutazone administration significantly (P < 0.05) attenuated most of the changes induced by histamine, whereas FUR had less protective action. Administration of PBZ plus FUR before administration of histamine was less effective than administration of PBZ alone.