Action of beta-adrenoceptor antagonists on the response to isoprenaline in the oestrogen dominated rabbit uterus.

1. Log dose-response curves to isoprenaline from spontaneously contracting muscle strips from rabbit uterus have been obtained. The effects of the alpha-adrenoceptor antagonists phentolamine and phenoxybenzamine, the beta- adrenoceptor antagonists propranolol and practolol, and (+)-propranolol on the log dose-response curves were studied. 2. Phentolamine 5.3 times 10- minus 7 M had a stimulating effect on the muscle strips, moving the log dose-response curve of isoprenaline to the right. Phenoxybenzamine 2.9 times 10- minus 5 M had no effect on the curves. 3. Propranolol 3.4 times 10- minus 6 M had no effect on the curves from circular muscle strips, with either phentolamine 5.4 times 10- minus 7 M or with phenoxybenzamine 2.9 times 10- minus 5 M as alpha-blocker. The curves from the longitudinal muscle strips were shifted somewhat to the right, the same shift, however, being obtained with (+)-propranolol 3.4 times 10- minus 6 M. 4. Practolol 3.8 times 10- minus 6, 3.8 times 10- minus 5 and 3.8 times 10- minus 4 M was without effect on the curves, either in the circular or in the longitudinal strips. 5. It is concluded that neither propranolol at 3.4 times 10- minus 6 M nor practolol act as beta-adrenoceptor antagonists in oestrogen dominated rabbit uterus. In all other tissues investigated, propranolol or practolol block the effect of isoprenaline. Whether the effect of isoprenaline in this tissue may be termed a beta-effect, is then a question of definition. In addition there is no evidence for any extraneuronal uptake mechanisms for isoprenaline. 6. Variations in sensitivity to beta-stimulation with the time of the year were observed, the sensitivity being greatest in the winter and lowest in the summer.