MECHANISM OF ACTION OF THE INHIBITORY EFFECT OF NIFEDIPINE ON THE GROWTH OF CULTURED AORTIC-CELLS FROM SPONTANEOUSLY HYPERTENSIVE AND NORMOTENSIVE RATS

1 To gain insight into the parameters which control vascular structure , we investigated the mechanisms whereby nifedipine, and other dihydro pyridines, inhibit the growth of cultured fibroblasts isolated from th e adventitia of the aorta of spontaneously hypertensive (SHR) and norm otensive Wistar Kyoto (WKY) rats. 2 The effects of nifedipine on cell proliferation and on serum-induced DNA synthesis were determined by me asuring the cell number and the incorporation of [H-3]-thymidine, resp ectively. The mechanism of action of nifedipine was studied by adding the drug either to randomly growing cells or to quiescent, G(0)/G(1) a rrested and synchronized cells. The effects of varying the duration of drug treatment were also examined. 3 In randomly growing cultures nif edipine, like other dihydropyridines concentration-dependently inhibit ed cell proliferation; the rank order of effect (measured at a concent ration of 10 mu M) was nifedipine>nisoldipine>nitrendipine similar to nimodipine. 4 In G(0)/G(1) arrested cell cultures, nifedipine concentr ation-dependently inhibited serum-induced [H-3]-thymidine incorporatio n. In this respect it had similar effects in cell cultures from WKY an d SHR. In both SHR and WKY cultures, nifedipine delayed the transition from G(0)/G(1) to S phase, and inhibited serum-induced DNA synthesis possibly by acting on the early G(1) phase. 5 In cell cultures from bo th SHR and WKY, serum-induced DNA synthesis was similarly (similar to 40%) inhibited after a 1 day treatment with 10 mu M nifedipine. In con trast, after 5 days treatment with the drug, the inhibition of DNA syn thesis was similar to 65% and similar to 10% in SHR and WKY cultures, respectively. The inhibitory effects of nifedipine against proliferati on of fibroblasts were similar to 25% and 60%, respectively, after 1 a nd 5 days of treatment, and were similar in cells derived from SHR and WKY. This indicates that 5 days treatment with nifedipine inhibited t he proliferation of SHR and WKY fibroblasts by acting mostly on the ea rly G(1) phase and the M phase, respectively. 6 Irrespective of the du ration of treatment (1 or 5 days) with 10 mu M nifedipine, the inhibit ion of DNA synthesis could be abolished and partially reduced by Bay K 8644 (1 mu M) in WKY and SHR fibroblasts, respectively. In cell cultu res from both SHR and WKY the inhibitory effects of a short term and o f a long term treatment with nifedipine against cell proliferation wer e reduced and unaffected, respectively by Bay K 8644. 7 These results indicate that nifedipine inhibited cell proliferation and serum-induce d DNA synthesis by altering the cell cycle through different mechanism s in SHR and WKY fibroblasts. They also suggest the existence in aorti c fibroblasts of interactions between calcium channel blockers of the dihydropyridine series and the mitogenic signalling pathways of growth factors contained in serum.