Relaxing and contracting activities of heartworm extract on isolated canine abdominal aorta

Effect of adult heartworm (HW) crude extract on isolated canine abdominal a ortic strips precontractcd with noradrenaline was examined by recording iso metric changes in tension. HW extract caused contraction of the aortic stri p at a low concentration (LC) and its relaxation at a high concentration (H C). In aortic strips without endothelium, LC extract elicited a contraction similar to that in the strips with endothelium, whereas HC extract failed to produce any relaxation but instead produced a contraction. The relaxing effect of HC extract was blocked after treatment with 300 muM N-G-nitro-L-a rginine methyl ester hydrochloride, with reversal by additional treatment w ith 3 mM L-arginine. It was also markedly reduced or abolished after treatm ent with 3 muM oxyhemoglobin or 1 muM methylene blue. Fractionation of HW e xtract by high-performance liquid chromatography revealed that the relaxing and contracting activities are due to different substances in the extract. The results indicate that HW extract contains 2 different vasoactive subst ances, 1 causing contraction of canine abdominal aorta via a direct action on the smooth muscle, and the other its relaxation indirectly by releasing nitric oxide from endothelial cells. These vasoactive substances might play a role in HW extract-induced shock in dogs, and in the pathogenesis of HW infection.