Endothelin receptor alterations in equine airway hyperreactivity

The purpose of this study was to evaluate the role of endothelin-1 (ET-1) and its receptors in the airway hyperreactivity of horses with obstructive pulmonary disease associated with summer pasture (SPAOPD). The right diaphragmatic lobe of the lung of 8 clinically healthy (unaffected) and 8 SPAOPD-affected horses was collected immediately after euthanasia. Bronchial rings (4 mm wide) were prepared and mounted in organ baths and attached to force transducers interfaced with a polygraph. Four rings were used to study each ET-1 receptor; 1 ring served as the control, and the other 3 were incubated with 10^-9, 10^-7, or 10^-5 M of either BQ-123, an ET_A-receptor antagonist, or IRL-1038, an ET_B-receptor antagonist. Cumulative concentrations (10^-8.5 to 10^-6 M) of ET-1 were applied to all rings. Using pooled pulmonary tissue from different regions of the lung, we performed a reverse-transcription polymerase chain reaction (RT-PCR) to determine ET_B-receptor gene expression. Although ET-1 caused concentration-dependent bronchial ring contraction in both groups of horses, the rings of SPAOPD-affected horses had significantly greater contraction than the rings of unaffected horses. Whereas ET_A-receptor blockade significantly increased the response to ET-1 in unaffected horses, ET_B-receptor blockade significantly decreased the response in affected horses. The pA₂ values showed a nonsignificant decrease in ET_A-receptor affinity and a significant increase in ET_B-receptor affinity in affected horses compared with unaffected horses. The ET_B-receptor mRNA expression of the pooled pulmonary tissue showed a nonsignificant increase in affected horses compared with unaffected horses. The airway hyperreactivity to ET-1 observed in the bronchial rings from the affected horses appears to be due in part to activation of pulmonary ET_B receptors, which appear to be inactive in unaffected horses.