LONG-TERM REGULATION OF NA+-H+ EXCHANGE IN VASCULAR SMOOTH-MUSCLE CELLS - ROLE OF PROTEIN-KINASE-C

Regulation of intracellular pH (pH(i) plays an important role in vascular smooth muscle cell (VSMC) contractile tone and growth. We have shown that pH(i) in proliferating VSMC is more alkaline (7.25) than in growth-arrested cells (7.10). To study the Na+-H+ exchanger in the growth-dependent regulation of VSMC pH(i), ethylisopropylamiloride (EIPA)-sensitive Na+ influx was measured. Exposure of growth-arrested VSMC to 10% serum initially increased Na+ influx (145% of baseline at 30 min), which then decreased (52% of baseline at 24 h). Serum-induced alterations in the kinetic properties of the Na+-H+ exchanger were studied by analysis of its external Na+ binding site properties. Exposure of growth-arrested VSMC to 10% serum for 24 h increased the K(m) for external Na+ from 54 to 380 mM, with a change in the V(max) from 155 to 199 nmol Na+.mg protein-1.min-1. The change in K(m) was due to activation of protein kinase C (PKC). Phorbol 12,13-dibutyrate caused a 48% decrease in EIPA-sensitive influx, the inactive 4-alpha-phorbol 12,13-didecanoate had no effect, and the PKC inhibitor sphingosine reversed the effect. Therefore, the Na+-H+ exchanger in VSMC is regulated in a growth-dependent manner via PKC.