Drug responses of adenylate cyclase in iris-ciliary body determined by adenine labelling

The intracellular adenine nucleotide pool of rabbit iris-ciliary body was labelled by uptake of 3H-adenine in vitro. A variety of agents were tested for their ability to stimulate or inhibit the incorporation of radioactivity into cyclic AMP formed from ATP labelled with 3H-adenine. Isoproterenol, vasoactive intestinal peptide, forskolin, and prostaglandin E2 stimulated incorporation of label 3-10-fold in 15-20 min compared with paired tissues not treated with hormone, whereas histamine, serotonin, substance P, and bradykinin were inactive. Clonidine, alpha-methylnorepinephrine, and dopamine decreased the rate of incorporation of label into the cyclic-AMP pool in tissues that showed high spontaneous basal rates. In low-basal tissues these drugs were inactive by themselves but clonidine and alpha-methylnorepinephrine blocked the stimulation effected by isoproterenol. The findings indicate that several receptor-coupled adenylate cyclase systems are present in ICB and that dual adrenergic control of adenylate cyclase through positive and negative coupling of adrenergic receptors probably occurs. The negatively coupled adrenergic receptors appear to be similar to the alpha 2-subclass of adrenergic receptor described in other tissues. These observations suggest a role for the large number of alpha 2-adrenergic-binding sites found in albino rabbit iris-ciliary body by ligand binding assays.