EFFECTS OF CYCLIC-GMP ELEVATION ON ISOPRENALINE-INDUCED INCREASE IN CYCLIC-AMP AND RELAXATION IN RAT AORTIC SMOOTH-MUSCLE - ROLE OF PHOSPHODIESTERASE-3

1 In rat aortic rings precontracted with phenylephrine, the beta-adren oceptor agonist isoprenaline (10 nM to 30 mu M) produces greater relax ant effects in preparations with endothelium than in endothelium-denud ed preparations. The aim of this study was to determine the mechanisms involved in this effect and in particular investigate the possibility of a synergistic action between adenosine 3':5'-cyclic monophosphate (cyclic AMP) and guanosine 3':5'-cyclic monophosphate (cyclic GMP). 2 Isoprenaline-induced relaxation of rat aortic rings precontracted with phenylephrine was greatly reduced by the nitric oxide (NO) synthase i nhibitor N-omega-nitro-L-arginine methyl ester (L-NAME, 300 mu M) or t he soluble guanylate cyclase inhibitors methylene blue (10 mu M) or 1H -[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (ODQ, 10 mu M) but unaffecte d by indomethacin (10 mu M), a cyclooxygenase inhibitor. Similarly, in intact rings, the concentration-response curve of forskolin (10 nM to 1 mu M) was shifted to the right upon endothelium removal or treatmen t with methylene blue. 3 In endothelium-denuded rat aortic rings, isop renaline-induced relaxation was potentiated by the guanylate cyclase a ctivators atrial natriuretic factor (ANF, 1 to 10 nM) and sodium nitro prusside (SNP, 1 to 10 nM), and to a greater extent in the presence of the cyclic GMP-specific phosphodiesterase (PDE 5) inhibitor, 1,3dimet hyl-6-(2-propoxy-5-methane sulphonylamidophenyl) pyrazolo [3,4-d] pyri midin-4-(5H)-one (DMPPO 30 nM). Relaxation induced by isoprenaline was also potentiated by the cyclic GMP-inhibited PDE (PDE 3) inhibitor ci lostamide (100 nM). 4 Intracellular cyclic nucleotide levels were meas ured either in rat cultured aortic smooth muscle cells or in de-endoth elialized aortic rings. In both types of preparation, isoprenaline (5 nM and 10 mu M) increased cyclic AMP levels and this effect was potent iated by cilostamide (10 mu M), by rolipram, a cyclic AMP-specific PDE (PDE 4) inhibitor (10 mu M) and by cyclic GMP-elevating agents (50 nM ANF or 30 nM SNP plus 100 nM DMPPO). In isoprenaline-stimulated condi tions, the increase in cyclic AMP induced by rolipram was further pote ntiated by cilostamide and by cyclic GMP-elevating agents. Cilostamide and cyclic GMP-elevating agents did not potentiate each other, sugges ting a similar mechanism of action. 5 We conclude that in vascular smo oth muscle (VSM) cells an increase in cyclic GMP levels may inhibit PD E 3 and, thereby, cyclic AMP catabolism. Under physiological condition s of constitutive NO release, and to a greater extent in the presence of the PDE 5 inhibitor DMPPO, cyclic GMP should act synergistically wi th adenylate cyclase activators to relax VSM.