Epinephrine synthesis in the rat iris

Epinephrine (E) alters blood flow, intraocular pressure and pupillary constriction. The rat iris contained E-forming activity that was moderately specific for a phenylethanolamine and was inhibited by the phenylethanolamine-N-methyltransferase (PNMT) inhibitor SKF 29661. Unilateral superior cervical ganglionectomy decreased iris norepinephrine (NE) 63%, but failed to lower PNMT activity or E in the iris. Removal of both adrenal medullae markedly lowered circulating E levels, but had no effect on iris E. Further treatment with 6-hydroxydopamine and reserpine greatly lowered iris NE levels, but failed to decrease either iris E or E forming activity. The rat iris has non-neuronal E-forming enzymes which appear to synthesize most of the E contained in the iris.