DILTIAZEM INHIBITS DNA-SYNTHESIS AND CA2-I AND PDGF IN VASCULAR SMOOTH-MUSCLE CELLS( UPTAKE INDUCED BY INSULIN, IGF)

Proliferation of vascular smooth muscle cells (VSMC) has been shown to play a key role in the atherosclerotic lesions. It has been demonstra ted that serum-derived peptidic growth factors, such as insulin, plate let-derived growth factor (PDGF), or epidermal growth factor (EGF), pr ovide mitogenic signals in VSMC and that the interplay of Ca2+ and oth er messengers is necessary for triggering proliferation. Since Ca2+ ch annel blockers act on the voltage-dependent Ca2+ channel to inhibit Ca 2+ influx, it is conceivable that they affect the proliferative action of growth factors. In this study we have evaluated the effects of dil tiazem, a 1,5-benzothiazepine-derived Ca2+ channel blocker, on [H-3]th ymidine incorporation into DNA stimulated by insulin, insulinlike grow th factor I (IGF-I), or PDGF in cultured VSMC from rat aorta. We have also investigated the effects of insulin, IGF-I, and PDGF on Ca2+ upta ke in VSMC. After exposure to insulin (10(-10) to 8 x 10(-6) M) or IGF -I (10(-10) to 10(-7) M) for 48 hours, VSMC incorporated [H-3]thymidin e to 200-280% of maximum (with insulin or IGF-I alone) compared to con trol. The effect of IGF-I was approximately 10-100 times more potent t han that of insulin. PDGF (0.5-15 ng/ml) also induced an increase in [ H-3]thymidine incorporation into DNA of VSMC. Additivity is observed b etween PDGF with insulin or IGF-I, but not between insulin and IGF-I. Sixty minute treatment with insulin (5 x 10(-8) to 10(-6) M), IGF-I (1 0(-8) to 10(-6) M), or PDGF (1.0-15.0 ng/ml) increased the unidirectio nal Ca-45(2+) uptake during a 5 minute period. The 30 minute Ca-45(2+) uptake of insulin (10(-7) to 10(-6) M)- or IGF-I (10(-8) to 10(-7) M) -treated cells was significantly greater than that of nontreated cells ; 3.5 ng/ml PDGF accelerated Ca-45(2+) uptake more than 10(-6) M insul in or 10(-7) M IGF-I. Diltiazem (10(-9) to 2 x 10(-5) M) inhibited not only the increase in [H-3]thymidine incorporation but also that of th e Ca-45(2+) uptake stimulated by insulin, IGF-I, or PDGF in a dose-dep endent manner. These data suggest that coordinate control of VSMC prol iferation by insulin or IGF-I and PDGF may play a cardinal role in the development of atherosclerosis and that a Ca2+ channel blocker may su ppress atheroma formation by inhibiting VSMC proliferation evoked by s erum-derived growth factors through a mechanism that attenuates the in crease of intracellular Ca2+ concentration. Furthermore, the effects o f insulin, IGF-I, and PDGF on proliferation of VSMC may be coupled to an increase of Ca2+ influx.