Reply to the Letter to the

address completely the issue of guanylyl cyclase and soluble or particulate guanylyl cyclase, imposed direcmuscarinic inhibition. Moreover, we consider that recent tionality on the response to ligands. Such considerations investigations have strengthened rather than diminished the also apply to stimulation of L-type calcium current (I ) Ca(L) importance of cGMP signaling, per se and in connection by cyclic AMP [2]. Importantly, the muscarinic agonist with ACh action. For example, CCh increased carcarbachol (CCh) and atrial natriuretic peptide (ANP) diomyocyte cGMP in the presence of NOS inhibitors inhibited IBMX-stimulated I by an LY83583-sensitive Ca(L) indicating a NOS-independent path to regulate cGMP process, presumably particulate guanylyl cyclase. Carbachsynthesis [10]. ol continued to inhibit isoproterenol-stimulated I in the Ca(L) Several pathways promote cGMP synthesis preand presence of LY83583 implying that suppression of postjunctionally yet cGMP-inhibited PDE (PDE 3) is the adenylyl cyclase activity, not a cGMP-mediated action, only cGMP effector identified in cardiac cholinergic nerves accounted for muscarinic inhibition under this condition. [6,7]. Postjunctionally, different cGMP effectors exist However, LY83583 prevented ANP from inhibiting isowithin the same cell. In cardiac myocytes derived from proterenol-stimulated I . A soluble guanylyl cyclase Ca(L) embryonic stem cells, ACh inhibited isoproterenol-stimuinhibitor, ODQ, had no effect against either CCh or ANP lated I by activating cGMP-stimulated PDE (PDE 2) Ca(L) but prevented inhibition by the NO donor, SIN-1 (but see in early-stage cells and by inhibiting adenylyl cyclase in [3]). This underscores two features. First, cGMP is an late-stage cells [11]. In human atrial cells, cGMP exerted important component of ANP action. Second, cGMP origin opposite effects on I by acting on PDE 3 and, at Ca(L) and concentration bear heavily on the outcome of experihigher concentrations, through PDE 2 [12]. Further, cGMP ments with CCh. Targeted disruption of cGMP-dependent may arise from muscarinic stimulation of soluble guanylyl protein kinase I not only illustrated its role in cardioinhibicyclase to increase I via PDE 3 [13] or activation of Ca(L) tion by cGMP but also excluded its participation in particulate guanylyl cyclase [1] to inhibit it via cGMPmuscarinic inhibition of forskolin-stimulated contractions dependent protein kinase [14]. These observations illus[4]. This result is consistent with the dual pathway trate the complexity of this second messenger’s actions in hypothesis for muscarinic inhibition [1] because forskolin modulating cardiac activity. The manifold patterns for activates adenylyl cyclase as does isoproterenol. synthesis and targeting of cGMP actions may well serve Whether and how cGMP serves as a messenger for the the homeostatic function of the autonomic nervous system cardiac actions of the vagus transmitter acetylcholine and of hormones like ANP. (ACh) has attracted critical attention since first reported