AN A3-SUBTYPE ADENOSINE RECEPTOR IS HIGHLY EXPRESSED IN RAT VASCULAR SMOOTH-MUSCLE CELLS - ITS ROLE IN ATTENUATING ADENOSINE-INDUCED INCREASE IN CAMP

Adenosine analogs are known to induce changes in the steady-state conc entration of cAMP via binding to adenylyl cyclase-inhibitory or -stimu latory adenosine receptors. Although adenosine has been found to incre ase cAMP in vascular smooth muscle cells (VSMC), we found by the polym erase chain reaction of reverse-transcribed RNA and subsequently by No rthern blot analysis that rat VSMC express high levels of an A3-subtyp e adenosine receptor cDNA which encodes an adenylyl cyclase-inhibitory adenosine receptor. The A3-specific agonist, N-6-(3-iodobenzyl) adeno sine-5'-N-mehylcarboxamide (IB-MECA) indeed decreases cAMP levels in V SMC cultured in the presence of forskolin. Antisense oligomers to the A3 adenosine receptor significantly reduce the level of this receptor in VSMC and potentiate endogenous adenosine-or 5'-N-ethylcarboxamido a denosine-induced increases in cAMP and of the proto-oncogene c-fos. Ab rogating the expression of the A3 adenosine receptor also largely abol ishes IB-MECA-induced inhibition of adenylyl cyclase. The level of A3 adenosine receptor mRNA and the extent of changes in cAMP in response to IB-MECA were lower in cultures of VSMC derived from adult rats, com pared to VSMC from neonatal rats. The expression of a functional A3 ad enosine receptor was also confirmed in preparations of isolated aortas . Our findings thus indicate that: (a) the A3-type receptor is a funct ional inhibitory adenosine receptor in VSMC; and (b) the regulation of expression of the A3 receptor is critical in determining effects of a denosine on the steady-state concentration of cAMP. (C) 1997 Academic Press.