ON THE HIGH-AFFINITY OF 8-CYCLOHEXYLCAFFEINE FOR THE PRESYNAPTIC INHIBITORY ADENOSINE RECEPTOR PRESENT IN RAT MOTOR-NERVE TERMINALS

Rat neuromuscular junction was used to study the characteristics of pr esynaptic A(1) adenosine receptors. We investigated the ability of the 8-substituted caffeine, 8-cyclohexylcaffeine (CHC), as well as of 1,3 ,8-substituted xanthines, 1,3-dipropyl-8-p-sulfophenylxanthine (DPSPX) and 8-p-sulfophenyl-1-isoamyl-3-isobutylxanthine (SPIIBX) to antagoni ze the inhibitory effect of 2-chloroadenosine on the amplitude of nerv e-evoked twitches of the rat phrenic-hemidiaphragm, and we compared th e affinity of these xanthines with that of 1,3-dipropyl-8-cyclopenthyl xanthine (DPCPX). CHC, DPSPX and SPIIBX in a near parallel manner shif ted to the right the log concentration-response curve for the inhibito ry effect of 2-chloroadenosine on nerve-evoked twitch amplitude. Linea r Schild plots with slopes near to unity were obtained for all these x anthines. The order of potency of the xanthines was DPCPX (K-i=0.53 nM ) >DPSPX (38 nM) approximate to CHC (41 nM) >SPIIBX (404 nM). The affi nities of DPSPX and SPIIBX for the A(1) receptor at the rat neuromuscu lar junction are in agreement with the affinities described for A(1) r eceptors at brain membranes. The now reported affinity of CHC for the presynaptic A(1) receptor is 683 times higher than that obtained in bi nding studies in rat brain membranes, and is only 49 times higher than that obtained in functional assays (adenylate cyclase activity) in no n-neuronal preparations (rat fat cells).