Loss of expression of the beta subunit of soluble guanylyl cyclase prevents nitric oxide-mediated inhibition of DNA synthesis in smooth muscle cells of old rats

We compared the effects of NO donors and cGMP analogues on the growth of ao rtic smooth muscle cells (SMCs) derived from newborn, adult (aged 3 months) , and old (aged 2 years) rats. We found that the NO donor S-nitroso-N-acety lpenicillamine failed to block DNA synthesis in SMCs from old rats but was effective in SMCs from newborn and adult rats. However, cGMP analogues were inhibitory in all 3 SMC types. We demonstrated that in SMCs from old rats, NO was unable to increase the concentration of intracellular cGMP, suggest ing that either cGMP synthesis was defective or cGMP degradation was enhanc ed. Western blot analysis revealed that SMCs from old rats do not express t he beta subunit of soluble guanylyl cyclase. To confirm the importance of t his observation in vivo, we balloon-injured the carotid arteries of adult a nd old rats. Whereas soluble guanylyl cyclase was expressed at the same lev el in the media of injured vessels and uninjured vessels of both groups, it s expression in the intimas of old rats was reduced by 70% compared with in timas from adult animals. Furthermore, N-omega-nitro-L-arginine, an inhibit or of NO synthesis, enhanced the intimal thickening in injured vessels in a dult rats but not in old rats. We conclude that the loss of NO responsivene ss in aged rats is due to the lack of the beta subunit of soluble guanylyl cyclase, and we speculate that this defect contributes to the enhanced inti mal thickening in response to injury in old animals.