Role of cyclic nucleotide phosphodiesterase isoenzymes in contractile responses of denuded rat aorta related to various Ca2+ sources

We have examined the cyclic nucleotide phosphodiesterase isoforms (PDE) inv olved in the contractile response of rat aorta to different agonists and di fferent experimental procedures for use in functional studies. The inhibito ry effect of AAL 05 on the different PDEs isolated from bovine aortic smoot h muscle was examined. Compound AAL 05 appeared to be a selective PDE3 inhi bitor. We analyzed the ability of the non-selective inhibitor IBMX (3-isobu tyl-1-methylxanthine) and the isoenzyme selective inhibitors nimodipine (ty pe1), AAL 05 (6-(N-methyl-N-cyclohexyl butyl carboxamide) quinolin-2-one) a nd SK&F 94120 (5-(4-acetamidophenyl) pyrazin-2-(1H)-one; type3), rolipram ( type4) and zaprinast (type5) to affect the contractile responses of denuded rat aortic rings to KCl (80 mM) and noradrenaline (NA, 1 I-LM) in the pres ence or absence of extracellular Ca2+. Rolipram (10-100 muM) and zaprinast (1-100 muM) failed to relax the aortic strips, but IBMX (0.1-30 muM), nimod ipine (1 fM-10 muM), AAL 05 (0.01-100 muM) and SK&F 94120 (0.1-100 muM) pro duced a concentration-dependent relaxation or inhibition of contractile res ponses to the different agonists, but the pIC(50) obtained for each inhibit or was different depending on the experimental procedure. Except for nimodi pine (a Ca2+ channel blocker), all the PDE inhibitors showed the following rank of potency: pIC(50) on NA-induced contractions in Ca2+-free medium > p IC(50) on NA-induced contractions in Ca2+-containing solution > pIC(50) on depolarizing solution-induced contraction. This ranking apparently depends on the differences in the Ca2+ sources. We obtained a good correlation betw een the pK(i) of PDE3 inhibitors in biochemical studies and the pIC(50) on NA-induced contraction in Ca2+-free medium. In conclusion, PDE1 and PDE3 is oenzymes play an important role as modulators of rat aortic smooth muscle c ontractility regardless of the experimental procedure used. Since intracell ular mechanisms are more dependent on PDE activity, experimental procedures performed in absence of extracellular calcium are the most suitable for an alyzing the modulatory role of PDE inhibitors.