Comparison of interactions of R-(+)- and S-(−)-isomers of β-adrenergic partial agonists, befunolol and carteolol, with high affinity site of β-adrenoceptors in isolated rabbit ciliary body and guinea-pig taenia caeci

Abstract

The stereoselectivities of β-adrenergic partial agonists for the high affinity binding site of β-adrenoceptors in the rabbit ciliary body and the guinea-pig taenia caeci were studied. The pA2 values of the S-(−)-isomers of befunolol and carteolol against S-(−)-isoprenaline, which were calculated from the shift of each concentration–response curve in increasing cyclic AMP levels, were significantly larger than those of the R-(+)-isomers in the guinea-pig taenia caeci, while the pA2 values of the S-(−)-isomers were not significantly larger than those of the R-(+)-isomers in the rabbit ciliary body. The pKi values determined from the binding experiments were in good agreement with the pA2 values from the increases in cyclic AMP levels. These results suggest that the high affinity binding site of β-adrenoceptors in the guinea-pig taenia caeci may be able to discriminate stereoselectively between the R-(+)- and S-(−)-isomers, while in the rabbit ciliary body there is no stereoselectivity between the two enantiomers.Key words: stereoselectivity, β-adrenoceptor, partial agonist, rabbit ciliary body, guinea-pig taenia caeci.