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 concentration of cAMP via binding to adenylyl cyclase-inhibitory or -stimulatory adenosine receptors. Although adenosine has been found to increase cAMP in vascular smooth muscle cells (VSMC), we found by the polymerase chain reaction of reverse-transcribed RNA and subsequently by Northern blot analysis that rat VSMC express high levels of an A3-subtype adenosine receptor cDNA which encodes an adenylyl cyclase-inhibitory adenosine receptor. The A3-specific agonist, N6-(3-iodobenzyl) adenosine-5'-N-mehylcarboxamide (IB-MECA) indeed decreases cAMP levels in VSMC 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 adenosine-induced increases in cAMP and of the proto-oncogene c-fos. Abrogating the expression of the A3 adenosine receptor also largely abolishes 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, compared to VSMC from neonatal rats. The expression of a functional A3 adenosine receptor was also confirmed in preparations of isolated aortas. Our findings thus indicate that: (a) the A3-type receptor is a functional inhibitory adenosine receptor in VSMC; and (b) the regulation of expression of the A3 receptor is critical in determining effects of adenosine on the steady-state concentration of cAMP.