DIHYDROPYRIDINE CA2+ CHANNEL ANTAGONISTS INHIBIT THE SALVAGE PATHWAY FOR DNA-SYNTHESIS IN HUMAN VASCULAR SMOOTH-MUSCLE CELLS

We examined the mechanisms by which Ca2+ channel antagonists inhibit the growth of smooth muscle cells by determining their effect on epidermal growth factor (EGF)-stimulated (i) induction of the early signalling gene, c-fos, (ii) incorporation of [H-3]thymidine into cells as a measure of DNA synthesis, and (iii) increase in cell number. Verapamil, diltiazem, and the dihydropyridines felodipine, MDL 72892 A-15 (MDL) and nisoldipine had no effect on EGF-stimulated c-fos mRNA induction. Furthermore, only small inhibitory effects were observed on EGF-stimulated increases in cell number; felodipine, MDL, and nisoldipine at 0.3 muM inhibited EGF-stimulated cell proliferation by 9, 11, and 15%, respectively. In contrast, the dihydropyridine Ca2+ channel antagonists were found to be potent inhibitors of [H-3]thymidine incorporation suggesting that they inhibit DNA synthesis. However, further examination revealed that the potent effects of dihydropyridine Ca2+ channel antagonists on [H-3] thymidine incorporation were due not to an effect on incorporation of [H-3]thymidine into DNA, but to a marked inhibitory effect on the cellular uptake of [H-3]thymidine. Thus, we conclude that the small antiproliferative effects of the dihydropyridine antagonists are predominantly due to their ability to inhibit the activity of the salvage pathway for thymidylate synthesis in human vascular smooth muscle cells.