Pharmacological profile of adenosine A₂ receptor in PC12 cells

Abstract

The PC12 cell line, a clone isolated from a pheochromocytoma tumor of rat adrenal medulla, was shown to exclusively contain stimulatory adenosine (A₂) receptors linked to adenylate cyclase (AC). AC was stimulated 6–7 fold by several agonists with a rank order of potency of 5′-N-Ethyl carboxamidoadenosine (NECA) > 2-Chloroadenosine (2-CADO) > (R)-N⁶-Phenylisopropyladenosine (R-(−)-PIA) > N⁶-Cyclopentyladenosine (CPA) > N⁶-Cyclohexyladenosine (CHA) > S-(+)-PIA. AC activity was antagonized by a variety of adenosine receptor antagonists with a potency order of 1,3,-Dipropyl-8-(2-amino-4-chlorophenyl)xanthine (PACPX) > 1,3,-Diethyl-8-phenylxanthine (DPX) > 8-Phenyltheophylline > 3-Isobutyl-1-methylxanthine (IBMX) > 8-(p-sulfophenyl)theophylline (PST) > 7-(β-chloroethyl)theophylline > theophylline = enprofylline = caffeine. Under conditions known to favour receptor-mediated N_i-coupled inhibition of AC, R-(−)-PIA failed to inhibit both basal and forskolin stimulated AC activity in PC12 cells, confirming the absence of an A₁ mediated response. On the other hand, adenosine agonists inhibited AC activity in rat cortical membranes with a rank order of potency of CPA > R-(−)-PIA > CHA > NECA > S-(+)-PIA > 2-CADO. These findings suggest that PC12 cells are functionally deficient in an A₁ receptor linked AC response but are efficiently coupled to A₂ stimulatory receptors. The cells should prove useful for further study of A₂ adenosine receptors and to establish selectivity profiles of compounds acting at both A₁ and A₂ receptors.