In vivo functional implications of isoproterenol-mediated relaxation of isolated human iris sphincter.

The isolated human iris sphincter contracted with carbachol produced relaxation with an average ED50 of 1.30 x 10(-7) M for (-)-isoproterenol. The effect is sensitive to block by (-)-timolol with KB 3 x 10(-9) M. The magnitude of the average relaxation mediated by the beta-adrenoceptor agonist was only 31%. Lowering of the temperature did not potentiate the effect of (-)-isoproterenol to a significant extent. Thus, in contrast to bovine iris sphincter, where the beta-adrenoceptor mediated response of (-)-isoproterenol produces a maximum relaxation or complete mydriasis, the maximum beta-adrenoceptor mediated mydriasis of human iris may not be more than 1/3 of the maximum pupillary dilatation achieved by physiologic or pharmacologic procedures.