Role of cyclic AMP- and cyclic GMP-phosphodiesterases in the control of cyclic nucleotide levels and smooth muscle tone in rat isolated aorta: A study with selective inhibitors

Abstract

Three isoforms of cyclic nucleotide phosphodiesterase (PDE) have been recently isolated from aortic tissue and two of them specifically hydrolyzed adenosine 3′,5′-cyclic monophosphate (cAMP) and guanosine 3′:5′-cyclic monophosphate (cGMP), respectively (Lugnier et al., Biochem. Pharmac. 35, 1743, 1986). The role of these forms in controlling cyclic nucleotide levels and smooth muscle tone was investigated by the use of PDE inhibitors. The effects of selective inhibitors of the two forms specifically hydrolyzing cAMP or cGMP (cAMP-PDE and cGMP-PDE, respectively) were compared to those of non-selective inhibitors of the three aortic PDE forms, including the calmodulin-sensitive one (CaM-PDE). Relaxation responses and accumulation of tissue cAMP and cGMP induced by these drugs were studied in precontracted rat isolated aorta, and compared to the effects of isoprenaline and forskolin (stimulants of adenylate cyclase) or sodium nitroprusside (SNP) and sodium azide (stimulants of guanylate cyclase). The eight PDE inhibitors tested all relaxed aorta with potencies that correlated with their potencies as inhibitors of cAMP-PDE, but not of cGMP-PDE. At a concentration producing half-maximal relaxation, all PDE inhibitors induced a moderate but significant accumulation of cAMP, which was comparable to the accumulation of cAMP elicited by half-maximally relaxing concentrations of adenylate cyclase stimulating agents. At this concentration, some PDE inhibitors (M&B 22,948, dipyridamole and to a lesser extent, trequinsin) also induced a significant increase in cGMP levels, of the same order of magnitude as that caused by agents stimulating guanylate cyclase. However, the cGMP-increasing effect of these inhibitors was dissociated from their relaxing effect. In particular, the relaxing concentrations of M&B 22,948 (a selective inhibitor of cGMP-PDE) were clearly higher than the cGMP-increasing concentrations of the compound. At a concentration at which they elicited 10% relaxation by themselves, the selective cAMP-PDE inhibitor, rolipram, as well as the mixed inhibitor of cAMP- and cGMP-PDE, AAL 05 (a cilostamide analogue) enhanced both the cAMP-increasing and the relaxing effect of isoprenaline. Under the same conditions, no clear enhancement of the relaxation induced by SNP was observed. Only M&B 22,948 showed a slight potentiating effect on SNP-induced relaxation, but this effect was limited to low concentrations of SNP (< 10 nM). Conversely, M&B 22,948 inhibited the relaxation elicited by higher concentrations of SNP (⩾ 0.1 μM), and some of the PDE inhibitors completely displaced the concentration-relaxation curve of SNP to the right. One of them, cilostamide, also inhibited the cGMP-increasing effect of SNP, but the other PDE inhibitors did not. The results strongly suggest that inhibition of cAMP-PDE produces relaxation of rat aorta, as a consequence of cAMP accumulation. They do not provide evidence that inhibition of aortic cGMP-PDE can lead by itself to relaxation via cGMP accumulation in this tissue.