Two opposite signal transducing mechanisms regulate a G-protein-coupled guanylyl cyclase

Membrane-bound guanylyl cyclase (GC) is regulated by muscarinic receptors (mAChRs), Carbamylcholine (CC) induces a "dual" biological response on GC activity, Thus, an activation is observed at 0.1 nM and a maximal response at 1 nM CC, However, at higher agonist concentration (>100 nM), there is an agonist-dependent inhibition of GC, This CC dual response is affected by 4-DAMP and HDD (M-3 antagonists), which produce a right-shift of the CC curve; the maximal CC dose response with 4-DAMP is more potent than that with HDD, Moreover, AFDX-DS (an M-2 antagonist) increases basal activity and decreases the agonist-dependent inhibition, Neither the CC response nor the CC maximal dose responses are affected by pirenzepine (PZ, M-1 antagonist), The agonist-dependent stimulation of GC activity is inhibited by 4-DAMP showing a -log IC50 = 8.4 +/- 0.4, while AFDX116 DS poorly inhibits such activity with a -log IC50 = 5.0 +/- 0.2, The agonist-independent (basal) GC activity also was inhibited by 4-DAMP, in a dose-dependent manner, with an IC50 = 8.5 +/- 0.2, Nonetheless, other muscarinic antagonists (PZ and HDD) were not able to inhibit this basal GC, Pertussis toxin treatment produces a complete blockade of the agonist-dependent inhibition of GC with a full expression of the agonist-dependent activation of membrane-bound GC, These results indicate that membrane-bound GC is regulated by muscarinic agents through two opposite signaling pathways; one involves the activation of GC via an M-3 mAchR coupled to a PTX-insensitive G protein, while the GC inhibition is mediated through a PTX-sensitive G(i/o) protein possibly coupled to an M-2 mAChR. (C) 1998 Academic Press.