In the present study we set out to define, using discriminatory agonists and antagonists, the adenosine receptors modulating sympathetic neurotransmission in the rat kidney. Isolated kidneys from male Wistar rats were perfused with modified Krebs-Henseleit buffer solution at constant flow. The neuronal noradrenaline stores were labeled with 3H-noradrenaline and the renal nerves stimulated electrically (2 Hz, 3 msec, 9 mA, during 20 sec at intervals of 6 min). 3H overflow was taken as an index of 3H-noradrenaline release. The A, receptor selective agonists N6-cyclopentyladenosine (CPA), N6-cyclohexyladenosine (CHA), and N6-[R(-)-l-phenyl-2-propyl] adenosine (R-PIA), and the mixed A,/A2, receptor agonists 5'-N-ethylcarboxamidoadenosine (NECA) and 2-chloroadenosine (CADO) inhibited evoked 3H outflow concentration-dependently. The selective A , , receptor agonist 2-[p-(2-carboxyethyl)phenylethylamino]-5'-N-ethylcarboxamidoadenosine (CGS 21680), at concentrations selective for AH receptors, failed to modify 3H outflow, whereas at higher concentrations it induced inhibition. The rank order of potency of agonists, CPA > CHA = R-PIA > NECA > CADO >> CGS 21680, is typical for an interaction with the A, receptor. 1,3-Dipropyl-8-cyclopentylxanthine (DPCPX), at concentrations selective for blockade of A, receptors, blocked concentration-dependently the inhibitory effects of CPA and NECA; no evidence of an increase in outflow was seen with NECA in the presence of DPCPX. The selective A , , receptor antagonist 9-chloro-2-(2-furany1)[1,2,4]triazol[l,5-c] quinazoline-5-amine (CGS 15943) did not influence the agonist effects at concentrations interacting selectively with A ,
, receptors but antagonized them concentration-dependently at higher, non-selective concentrations. Taken together, our data establish the presence of inhibitory adenosine A, receptors on the terminal sympathetic neurons of rat kidney. No evidence was obtained for the presence of functional AM receptors in this preparation. o 1996 WiIey-Liss, Inc.