Antimuscarinic effects of stereoisomers of tropicamide on rabbit iris sphincter.

The antimuscarinic activity of optical isomers of tropicamide were compared on the isolated rabbit iris sphincter. The increasing concentrations of both the (--)- and (+)-isomer shifted the dose-response curve of carbachol to the right in a parallel fashion. The competitive reversible muscarinic blocking effects of both isomers were confirmed by pA2 plots. The pA2 values from the nonpigmented irides for (--)- and (+)-tropicamide were 7.88 and 6.18, respectively. Thus the (+)-isomer has only 1/50 the blocking activity of the (--)-isomer. Although both isomers are slightly less active in the pigmented iris, the activity difference between the isomers was high. From the nonpigmented iris, the blocking effect of the active (--)-tropicamide was readily reversed by washing, whereas reversal of this isomer's effect from the pigmented iris was relatively slow.