The crude extract (CE) from M. velutina (0.2-0.8 mg/ml), antagonized in a dose-dependent and reversible manner bradykinin (BK)- and lysyl-BK-induced contractions of the guinea-pig urinary bladder in vitro. 2. Schild plots yielded nominal pA2 values (as g/ml) of 3.5 and 3.7, but the slopes were significantly greater than unity. 3. At concentrations 2-10-fold higher, the CE failed to affect responses to acetylcholine or histamine. 4. In electrically-stimulated tissue, BK, L-BK and M-L-BK potentiated nerve-mediated twitch responses and increased muscular tone. The B1 agonist [des-Arg9]-BK, was inactive up to 10 microM. 5. The CE from M. velutina (0.4 mg/ml) inhibited both actions of the kinins.
