CYCLIC-AMP ADENOSINE PATHWAY INDUCES NITRIC-OXIDE SYNTHESIS IN AORTICSMOOTH-MUSCLE CELLS

The main purpose of this investigation was to evaluate whether the cyc lic AMP-adenosine pathway, ie, the conversion of cAMP to AMP and, henc e, to adenosine, is involved in the regulation of nitric oxide (NO) sy nthesis by vascular smooth muscle cells (SMCs). Treatment of confluent monolayers of SMCs with adenosine, 2-chloroadenosine (stable analog o f adenosine), and agents that elevate endogenous (SMC-derived) adenosi ne (EHNA and iodotubericidin) increased nitrite/nitrate (stable metabo lites of NO) levels in the medium and enhanced the conversion of H-3-L -arginine to H-3-L-citrulline by cytosolic extracts obtained from the pretreated SMCs. The stimulatory effects of adenosine were not mimicke d by low (1 to 100 nmol/L) concentrations of CGS31680, an A(2A) recept or agonist, or CPA, a selective A(1) receptor agonist. The stimulatory effects of 2-chloroadenosine and EHNA plus iodotubericidin were signi ficantly inhibited by KF17837, a selective A(2) receptor antagonist, a nd by DPSPX, an A(1)/A(2) receptor antagonist, but not by DPCPX, a sel ective A(1) receptor antagonist. DDA (adenylyl cyclase inhibitor) and Rp-cyclic AMP (protein kinase A inhibitor) did not block the effects o f adenosine on NO synthesis. Incubation of SMCs with exogenous cyclic AMP, at concentrations previously shown to elevate levels of adenosine in the medium, also increased nitrite/nitrate levels and H-3-L-citrul line formation, and the effects of cyclic AMP on NO synthesis were blo cked by DPSPX and KF17837, but not by DPCPX. These findings provide ev idence that exogenous and SMC-derived adenosine induce NO synthesis vi a A(2B) receptors linked to a pathway not involving adenylyl cyclase/p rotein kinase A. Moreover, extracellular cyclic AMP induces NO synthes is via conversion to adenosine and activation of A(2B) adenosine recep tors. The cyclic AMP-adenosine pathway may be importantly involved in the vascular production of NO.