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 (All) 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 All alter [Ca^{2 +}]_i in VSMCs. Addition of PDGF resulted in a rapid, transient, concentration‐dependent increase in [Ca²⁺]_i; this rise in [Ca^{2 +}]_i was blocked completely by preincubation of cells with ethylene glycol‐bis (β‐aminoethyl ether) N,N,N′,N′‐tetraacetic acid (EGTA) or CoCl₂, by the voltage‐sensitive Ca^{2 +}‐channel antagonists verapamil or nifedipine, by 12‐O‐tetradecanoylphorbol‐13‐acetate (TPA), or by pertussis toxin. All also caused an increase in [Ca^{2 +}]_i; however, All‐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 All‐induced increases in [Ca²⁺]_i. EGTA or CoCl₂ only slightly diminished All‐stimulated increases in [Ca²⁺]_i. Nifedipine, verapamil, TPA, and pertussis toxin pretreatment were without effect on All‐induced increases in [Ca²⁺]_i. PDGF and All 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 × 10⁻¹⁰ M for Ca²⁺ vs. 2.5 × 10⁻⁹ M for phosphoinositide hydrolysis), but they were essentially identical for All (7.5 × 10⁻⁹ M for Ca²⁺ vs. 5.0 × 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 Ca^{2 +} channel blockers, TPA, or pertussis toxin. These results suggest that PDGF and All 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. All‐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 [Ca^{2 +}]_i caused by PDGF are required for PDGF‐stimulated mitogenesis.