Strips of bovine intrapulmonary artery (IPA), intrapulmonary vein (IPV), trachealis muscle and parenchyma were mounted for recording of isometric tension and responses to several vascular smooth muscle relaxants were measured and compared. Although nitroglycerin (GTN) and sodium nitroprusside (SNP) induced concentration-related (10(-11) to 10(-5)) relaxation (up to 80-100%) in IPA, IPV and trachealis muscle, 10(-5) M GTN or SNP induced only 4-5% relaxation in parenchyma. In contrast, all four tissues relaxed well in response to isoproterenol (10(-11) to 10(-5) M), forskolin (10(-5) M), 8-bromo-cyclic GMP (10(-4) M) and 1-methyl-3-isobutylxanthine (10(-5) M). When compared to IPA, IPV and trachealis muscle, the marked hyporesponsiveness of parenchyma to GTN and SNP suggests that the contractile cells which are primarily responsible for potassium-induced tone in bovine parenchymal strips differ markedly from smooth muscle cells in trachealis muscle and large, central IPA and IPV in their responsiveness to the vasodilators, GTN and SNP.
Animals Cattle