Hypertrophy and hyperplasia cause differing effects on vascular smooth muscle cell Na+/H+ exchange and intracellular pH

Mitogens and vasoconstrictors stimulate many of the same early intracellular signals (e.g. phospholipase C and protein kinase C activation) in vascular smooth muscle cells (VSMC). Despite these shared signals, angiotensin II is not mitogenic for cultured VSMC. The nonmitogenic effect of angiotensin II suggests that other intracellular signals associated with growth should differ between mitogens and vasoconstrictors. Because of the importance of intracellular pH (pHi) in growth, we compared the effects of 10% calf serum, 10 ng/ml platelet-derived growth factor, and 100 nM angiotensin II on pHi and Na+/H+ exchange. All agonists stimulated a rapid (less than 1 min) rise in pHi mediated by Na+/H+ exchange. However, exposure of growth-arrested VSMC to these agonists for 24 h caused significant differences in pHi: 7.18 (10% serum), 7.16 (platelet-derived growth factor), 6.99 (angiotensin II), and 7.08 (0.4% serum). Na+/H+ exchange activity was measured in acid-loaded cells by the ethyl isopropyl amiloride-sensitive influx of Na+ and efflux of H+. Both techniques showed that exposure to 10% serum caused approximately 45% decrease in Na+/H+ exchange activity without significant change in angiotensin II-treated cells. Thus, although the rapid changes in pHi and Na+/H+ exchange function are the same for angiotensin II and mitogens, the long term effects differ. The data suggest that differences in pHi regulatory mechanisms are important in determining whether an agonist causes VSMC hypertrophy or hyperplasia.