Stereoselectivity in beta-adrenomimetic and beta-adrenolytic actions of carteolol, a beta-adrenoceptor blocker with intrinsic sympathomimetic action in guinea-pig taenia caecum.

1. The beta-adrenomimetic and beta-adrenolytic activities of S(-) and R(+) isomers of carteolol, a beta-adrenergic partial agonist (a beta-adrenoceptor blocker with intrinsic sympathomimetic action) were tested in the guinea-pig taenia caecum. 2. The beta-adrenoceptor blocking activities (pA2 values) of S(-) and R(+) isomers of carteolol were significantly larger than the corresponding beta-adrenomimetic activities (pD2 values), supporting our views that beta-adrenoceptors contain two different binding sites; high and low affinity sites. 3. In beta-adrenoceptor blocking action S(-) carteolol was about 10 times as potent as R(+) carteolol while beta-adrenomimetic action of S(-) carteolol was about 2 times as potent as that of R(+) carteolol. Further, intrinsic activity for S(-) carteolol was slightly but significantly larger than that for R(+) carteolol. 4. These results suggest that the binding site for competitive antagonism between S(-) isoprenaline and S(-), R(+) and RS(+/-) carteolol is more stereoselective than the binding site to induce beta-adrenomimetic action.