Comparative responses of bronchial rings to mediators of airway hyperreactivity in healthy horses and those affected with summer pasture-associated obstructive pulmonary disease

Objective-To compare responses of bronchial rings obtained from healthy hor ses and horses affected with summer pasture-associated obstructive pulmonar y disease (SPAOPD) to selected mediators of airway hyperreactivity in vitro . Sample Population-Bronchial rings from 6 healthy horses and 6 horses affect ed with SPAOPD. Procedure-Bronchial rings obtained from each group of horses were mounted i n organ baths and attached to force transducers interfaced with a polygraph . After applying 2g of tension, each ring was allowed to equilibrate for 45 minutes in Tyrode's solution at 37 C. Cumulative concentration-response re lationships to graded concentrations of selected mediators (10(-8) to 10(-4 ) M) were determined and analyzed for significance at each concentration. Results-Acetylcholine, histamine, 5-hydroxytryptamine, and leukotriene Dq i nduced concentration-dependent contractile responses in bronchial rings. Pr ostaglandin F-2 alpha induced weak and inconsistent contractile responses. The other 2 agents, norepinephrine and substance P, did not induce concentr ation-dependent responses. Considering the overall group-drug effect, acety lcholine, histamine, 5-hydroxytn/ptamine, and leukotriene D-4 were effectiv e in inducing consistent concentration-dependent contractile responses in b oth groups. Only 5-hydroxytryptamine and histamine induced significant resp onses in contractility between groups. The response of bronchial rings from horses with SPAOPD to 5-hydroxytryptamine was significantly greater than t hose from control horses, whereas the response to histamine was significant ly lower. Significant responses were evident at concentrations ranging from 10(-6) to 10(-4) M for both drugs. Conclusions and Clinical Relevance-Because the airways of horses with SPAOP D had increased responsiveness to 5-hydroxytryptamine in vitro, treatment m odalities using 5-hydroxytryptamine antagonists should be investigated to a ddress this phenomenon.