We compared the effects of genistein with its structural derivatives daidzein and equol on excitation of pulmonary artery and vein. The concentration of genistein necessary to inhibit contractions evoked by U46619 (1nM-100 microM) ranged from 10 to 100 microM. Genistein (55 microM) reduced KCl-responses by approximately 50% and essentially abolished those evoked by U46619. Daidzein was much less effective against either agonist, and equol was ineffective against U46619. A23187-evoked contractions were markedly reduced by all 3 isoflavones, but caffeine-evoked contractions were not. Using the Western blot technique, we found many proteins were tyrosine phosphorylated within 30 seconds after stimulation with U46619, reaching a peak at 120 seconds and then falling at 300 seconds. One band at 110 kD was increased nearly 300% above baseline, while 3 others ranging from 60 to 80 kD were more than doubled in intensity. Genistein had little effect on baseline levels of phosphorylation but largely prevented the U46619-induced change; daidzein was much less effective in this respect, and equol did not significantly affect this phosphorylation. We conclude that these isoflavones provide powerful tools in the study of excitation-contraction coupling of pulmonary vasculature and that inhibition of tyrosine kinase activity may be useful clinically against pulmonary hypertension.