Effect of vasoactive intestinal polypeptide (VIP) on cyclic AMP level and relaxation in rat isolated aorta.

The effects of vasoactive intestinal polypeptide (VIP) on cyclic nucleotide (cAMP and cGMP) levels and smooth muscle relaxation were investigated in rat isolated aorta and compared to those of the beta-adrenergic agonist isoprenaline. VIP increased the cAMP level of rat aorta in a concentration-dependent manner with an EC50 of 0.1 microM. VIP 1 microM maximally increased the cAMP level 7 fold, whereas the beta-adrenergic agonist isoprenaline 10 microM elevated the cAMP level only 2.5 fold. VIP 1 microM relaxed the precontracted rat aorta by only about 16% whereas isoprenaline 10 microM induced a relaxation of about 86%. VIP did not alter the cGMP level and its effect on cAMP content was not changed in the presence of indomethacin (5 microM). No quantitative correlation could thus be established between increases in total tissue levels of cAMP and the degree of relaxation in rat isolated aorta.