Inhibition of platelet and vascular smooth muscle function by cyclic nucleotides: Synergism between activators of adenylyl and guanylyl cyclases

Abstract

<p>In platelets, the activators of guanylyl cyclase used were sodium nitroprusside (SNP) and SIN-1 (the active metabolite of molsidomine). PGE$\sb1,$ a functional analogue of prostacyclin (PGI$\sb2),$ and adenosine were the activators of platelet adenylyl cyclase studied. Changes in cyclic ($\sp3$H) nucleotides were measured in platelets prelabelled with ($\sp3$H) guanine and ($\sp3$H) adenine. Incubation of labelled platelets with SNP or SIN-1 caused inhibitions of platelet function which were associated with large dose-dependent increases in ($\sp3$H) cGMP and 1.4 to 3.0-fold increases in ($\sp3$H) cAMP. However, addition of either SNP or SIN-1 with PGE$\sb1$ or adenosine, at concentrations of the latter that had little effect alone, caused much larger increases in ($\sp3$H) cAMP and greatly enhanced the inhibition of platelet function. Experiments with an adenylyl cyclase inhibitor, 2$\sp\prime,$5$\sp\prime$-dideoxyadenosine (DDA), suggested that these synergistic interactions depend on an enhanced accumulation of cAMP. Hemoglobin inhibited all nitrovasodilator-mediated effects. Cilostamide, a selective inhibitor of platelet cGMP-inhibited cAMP phosphodiesterase (cGI-PDE), had effects identical to those of SNP, suggesting that the actions of the latter depend on inhibition of the same enzyme. Also, the results obtained with M&B 22,948, a cGMP-selective PDE inhibitor, strongly suggest that the increases in platelet ($\sp3$H) cAMP caused by nitrovasodilators, in the presence or absence of activators of adenylyl cyclase, are mediated by the inhibition of cAMP breakdown of cGMP. To determine if a similar synergism is seen in vascular smooth muscle (VSM), the interactions between isoproterenol and either nitrovasodilators or cAMP-PDE inhibitors were studied. A fast and sensitive prelabelling method was established for the measurement of cyclic nucleotides in cultured vascular smooth muscle cells (VSMC) from rat aorta. Using this method, agonist-induced changes in the cyclic nucleotide levels of two different aortic VSMC lines were studied. (Abstract shortened by UMI.)</p>