Effect of serum from dogs infected with Dirofilaria immitis on endothelium-dependent relaxation of rat aorta in vitro.

Endothelium-dependent relaxation of canine femoral artery in vivo is depressed in dogs infected with Dirofilaria immitis (heartworms). In vitro, endothelium-dependent relaxation of aorta from rat is depressed in the presence of adult heartworms or heartworm-conditioned media. The depression of relaxation is attributable, in part, to a low molecular weight, biologically active product that is released by the adult parasites. Because heartworms reside in the right heart and pulmonary arteries, biologically active factors produced by the parasites could circulate and alter endothelial cell function. The hypothesis that filarial factors in serum from heartworm-infected dogs depress endothelium-dependent relaxation was tested. Rings of thoracic aorta from rats were constricted by use of norepinephrine, and cumulative dose-response relationships to methacholine and nitroglycerin were evaluated in the presence of serum from heartworm-infected dogs or serum from noninfected (control) dogs. Nitroglycerin relaxation was not different; however, methacholine relaxation was significantly depressed in rings exposed to serum from heartworm-infected dogs when compared with that of controls. These results supported the hypothesis suggested that circulating filarial factors have the potential to influence the behavior of any endothelial cell surface.