OBJECTIVE To determine whether iris sphincter and other tissues of the iris-ciliary body secrete adrenomedullin (ADM), a novel hypotensive peptide that is classified into the calcitonin gene-related peptide (CGRP) family and to determine the binding sites for ADM and compare the effects of ADM and CGRP in the absence and presence of their receptor antagonists on cAMP formation and relaxation in the iris sphincter. METHODS Sphincter muscle was incubated in Krebs-Ringer bicarbonate buffer in the absence and presence of ADM for 10 minutes. Accumulation of cAMP in the tissue extract was determined by radioimmunoassay (RIA). The binding of [125I]ADM to iris sphincter membranes was carried out by rapid filtration. Distribution of ADM in the ocular tissues was determined by RIA. Changes in muscle tension were recorded isometrically. RESULTS Immunoreactive ADM was present in all tissues of the cat iris-ciliary body. In the isolated cat iris sphincter, ADM increased cAMP accumulation in a time- (t1/2 = 2.2 minutes) and concentration- (EC50 = 13 nM) dependent manner, and this effect was sixfold more efficacious than CGRP. ADM, CGRP, vasoactive intestinal peptide, prostaglandin E2, isoproterenol, and forskolin increased cAMP formation in cat sphincter by 12.5-, 2-, 2.2-, 1-, 2.6-, and 2.4-fold, respectively. The rank of the effects of ADM on cAMP formation in iris sphincter isolated from different animal species was in the following order: cat > dog > bovine > human > rabbit. In the cat iris sphincter, the CGRP antagonist, CGRP(8 to 37), was more effective than the ADM antagonist, ADM (26 to 52), in inhibiting both ADM- and CGRP-induced cAMP formation. ADM and CGRP inhibited carbachol-induced contraction in a concentration-dependent manner with IC50 values of 10 and 90 nM, respectively. Both ADM and CGRP displaced the binding of [125I]ADM to sphincter membranes effectively, with IC50 values of 0.81 and 1.15 nM, respectively. CONCLUSIONS In iris sphincter isolated from cat and other mammalian species including human, ADM is a much more efficacious activator of adenylate cyclase and a much more effective relaxant than CGRP. Its biological effects may be due to direct involvement of ADM receptors, but also to activation of CGRP receptors. Activation of ADM receptors by the peptide leads to concentration-dependent increases in cAMP accumulation and subsequent inhibition (relaxation) of smooth muscle contraction. These findings suggest a role for ADM as a local modulator of smooth muscle tone. A possible function for this potent hypotensive peptide in the regulation of intraocular pressure remains to be investigated.