Bronchial hyperreactivity characterized by reversible bronchoconstriction in response to stimuli is a common sequela to bronchial inflammation and damage and a hallmark of feline bronchial disease. This study compared isometric contractile and relaxation responses of isolated intrapulmonary bronchioles (IPBs) from the caudal lung lobe of cats with adult heartworm infection versus uninfected cats.

Adult cats were infected with 100 stage L3 Dirofilaria immitis larvae (n = 7) or received a sham inoculation (n = 6) and the left caudal lung lobe was collected 9 months later. Isometric responses of IPB rings were studied via wire myographs. Contractions were induced by administration of acetylcholine followed by administration of the contractile agonists acetylcholine, histamine, and 5-hydroxytryptamine. Relaxation was evaluated by constricting IPB rings with 5-hydroxytryptamine, and concentration-response curves were generated from administration of sodium nitroprusside, isoproterenol, and substance P. Contractile responses to acetylcholine and 5-hydroxytryptamine were reduced in tissues from heartworm-infected cats as compared with control cat tissues, as was relaxation to isoproterenol. Relaxation to substance P was increased in tissues from heartworm-infected cats but was unchanged to sodium nitroprusside.

These results suggested that a hyperreactive response of the bronchiolar smooth muscle is not the primary mechanism of respiratory tract clinical signs but might be attributable to abnormal contractile function of the bronchioles and reduced clearance of mucus and inflammatory debris.

Commentary
This study illustrated that the mechanism of heartworm-associated respiratory disease is more complex than just inflammation and hyperreactivity/enhanced bronchoconstriction of the bronchioles and small airways. The respiratory tract signs often observed are more likely attributed to abnormal contractility and relaxation of the bronchioles and reduced clearance of inflammatory debris and mucus. This finding is important to note when treating cats with heartworm infection, as it explains why conventional therapy with tapering doses of steroids and bronchodilators can be unrewarding. Feline practitioners should pay close attention to future studies and new approaches for treating this disease.—Amara Estrada, DVM, DACVIM (Cardiology)

Source
Isometric responses of isolated intrapulmonary bronchioles from cats with and without adult heartworm infection. Wooldridge AA, Dillon AR, Tillson M, et al. AM J VET RES 73:439-446, 2012.