Effects of vasoactive intestinal polypeptide (VIP) and cyclic-AMP on the isolated sphincter pupillae muscles of the albino rabbit.

Iris sphincter muscle strips were dissected from the albino rabbit eye pretreated with 0.5% topical indomethacin and incubated in a Krebs-Ringer solution. Vasoactive intestinal polypeptide (VIP) was added to the incubation medium, and the effects on the tension of the sphincter pupillae muscles and the cyclic AMP (c-AMP) level in the muscles were correlated. The c-AMP level in the muscles was determined by a radioimmunoassay method. VIP induced a relaxation of the sphincter muscles and a positive correlation was found between the VIP effects and the c-AMP levels. The ED50 was calculated to be 3.72 X 10(-9)M. A significant increase in the c-AMP level occurred prior to the onset of muscle relaxation after VIP treatment (10(-7)M), and a peak level of c-AMP was reached when the relaxation was almost completed. The sphincter muscles were treated with a c-AMP phosphodiesterase inhibitor, 1-methyl-3-isobutylxanthine (MIX) (10(-5)M), 10 minutes prior to the experiment. This pretreatment enhanced the relaxation of the muscles induced by VIP.