Chinese hamster ovary cells were transfected with both A1 adenosine receptor and muscarinic type 3 acetylcholine receptor cDNAs. The muscarinic receptor agonist carbachol stimulated phospholipase C activity, resulting in Ca2+ mobilization and arachidonate release. N6-Cyclopentyladenosine (CPA), an A1 receptor agonist, did not activate Ca(2+)-related signal transduction systems by itself but instead inhibited cAMP accumulation. In the presence of carbachol, however, the A1 receptor agonist enhanced muscarinic receptor agonist-induced phospholipase C/Ca2+ responses. In addition, the arachidonate release caused by Ca2+ ionophores or thapsigargin was also amplified by CPA, without a change in phospholipase C activity. Thus, CPA augments Ca(2+)-mediated phospholipase A2 activation in addition to and separate from its ability to amplify phospholipase C-mediated Ca2+ mobilization. Because the permissive actions of CPA on phospholipase C and phospholipase A2 activation were each reversed by pertussis toxin treatment, in a manner similar to that of the CPA-induced inhibition of cAMP accumulation, we conclude that a single species of A1 receptor expressed in Chinese hamster ovary cells can couple to multiple signal transduction systems stemming from phospholipase C stimulation, phospholipase A2-mediated and Ca(2+)-dependent arachidonate release, and inhibition of cAMP accumulation. A pertussis toxin-sensitive G protein (or proteins) mediates the permissive actions of the A1 receptor in the stimulation of phospholipase C- and phospholipase A2-mediated arachidonate release.