CYCLIC AMP-MEDIATED REGULATION OF VASCULAR SMOOTH-MUSCLE CELL CYCLIC-AMP PHOSPHODIESTERASE ACTIVITY

1 Rat cultured aortic vascular smooth muscle cells (VSMC) express both cyclic GMP-inhibited cyclic AMP phosphodiesterase (PDE3) and Ro 20-17 24-inhibited cyclic AMP phosphodiesterase (PDE4) activities. By utiliz ing either cilostamide, a PDE3-selective inhibitor, or Ro 20-1724, a P DE4-selective inhibitor, PDE3 and PDE4 activities were shown to accoun t for 15% and 55% of total VSMC cyclic AMP phosphodiesterase (PDE) act ivity. 2 Treatment of VSMC with either forskolin or 8-bromo-cyclic AMP caused significant concentration- and time-dependent increases in tot al cellular cyclic AMP PDE activity. Using cilostamide or Ro 20-1724, we demonstrated that both PDE3 and PDE4 activities were increased foll owing forskolin or 8-bromo-cyclic AMP treatment, with a relatively lar ger effect observed on PDE3 activity. The increase in cyclic AMP PDE a ctivity induced by forskolin or 8-bromo-cyclic AMP was inhibited by ac tinomycin D or cycloheximide, demonstrating that new mRNA synthesis an d protein synthesis were required. An analogue of forskolin which does not activate adenylyl cyclase (1,9-dideoxyforskolin) or an analogue o f cyclic GMP (8-bromo-cyclic GMP) did not affect total cyclic AMP PDE activity. 3 Incubation of VSMC with 8-bromo-cyclic AMP for 16 h caused a marked rightward shift in the concentration-response curves for bot h isoprenaline-and forskolin-mediated activation of adenylyl cyclase. A role for up-regulated cyclic AMP PDE activity in this reduced potenc y is supported by our observation that cyclic AMP PDE inhibitors (IBMX , cilostamide or Ro 20-1724) partially normalized the effects of isopr enaline or forskolin in treated cells to those in untreated cells. 4 W e conclude that VSMC cyclic AMP PDE activity is increased following lo ng-term elevation of cyclic AMP and that increases in PDE3 and PDE4 ac tivities account for more than 70% of this effect. Furthermore, we con clude that increases in cyclic AMP PDE activity contribute to the redu ced potency of isoprenaline or forskolin in treated VSMC. These result s have implications for long-term use of cyclic AMP PDE inhibitors as therapeutic agents.