Platelet-derived growth factor and angiotensin II cause increases in cytosolic free calcium by different mechanisms in vascular smooth muscle cells

Platelet-derived growth factor (PDGF) and angiotensin II (AII) are thought to mediate their biological effects in vascular smooth muscle cells (VSMCs) by causing alterations in cytosolic free calcium ( Ca2+]i). In this study we examine the pathways by which PDGF and AII alter Ca2+]i in VSMCs. Addition of PDGF resulted in a rapid, transient, concentration-dependent increase in Ca2+]i; this rise in Ca2+]i was blocked completely by preincubation of cells with ethylene glycol-bis (beta-aminoethyl ether) N,N,N',N'-tetraacetic acid (EGTA) or CoCl2, by the voltage-sensitive Ca2+-channel antagonists verapamil or nifedipine, by 12-O-tetradecanoylphorbol-13-acetate (TPA), or by pertussis toxin. AII also caused an increase in Ca2+]i; however, AII-stimulated alterations in Ca2+]i displayed different kinetics compared with those caused by PDGF. Pretreatment of cells with 8-(diethylamine)-octyl-3,4,5-trimethyoxybenzoate hydrochloride (TMB-8), almost totally inhibited AII-induced increases in Ca2+]i. EGTA or CoCl2 only slightly diminished AII-stimulated increases in Ca2+]i. Nifedipine, verapamil, TPA, and pertussis toxin pretreatment were without effect on AII-induced increases in Ca2+]i. PDGF and AII both stimulated increases in total inositol phosphate accumulation, although the one-half maximal concentration (ED50) for alterations in Ca2+]i and phosphoinisitide hydrolysis differed by a factor of 10 for PDGF (3 X 10(-10) M for Ca2+ vs. 2.5 X 10(-9) M for phosphoinositide hydrolysis), but they were essentially identical for AII (7.5 X 10(-9) M for Ca2+ vs. 5.0 X 10(-9) M for phosphoinositide hydrolysis). PDGF stimulated mitogenesis (as measured by 3H]-thymidine incorporation into DNA) in VSMCs with an ED50 similar to that for PDGF-induced alterations in phosphoinositide hydrolysis. PDGF-stimulated mitogenesis was blocked by pretreatment of cells with voltage-sensitive Ca2+ channel blockers, TPA, or pertussis toxin. These results suggest that PDGF and AII cause alterations in Ca2+]i in VSMCs by at least quantitatively distinct mechanisms. PDGF binding activates a pertussis-toxin-sensitive Ca2+ influx into cells via voltage-sensitive Ca2+ channels (blocked by EGTA, verapamil, and nifedipine), as well as stimulating phosphoinositide hydrolysis leading to release of Ca2+ from intracellular stores. AII-induced alterations in Ca2+]i are mainly the result of phosphoinositide hydrolysis and consequent entry of Ca2+ into the cytoplasm from intracellular stores. Our data also suggest that changes in Ca2+]i caused by PDGF are required for PDGF-stimulated mitogenesis.