Mechanism for muscarinic inhibition of I is determined by the path Ca(L) for elevating cyclic AMP in cardiac myocytes

Objective: Does carbachol (CCh) require NO/cGMP for inhibition of L-type calcium current (I ) when either adenylyl cyclase Ca(L) activation or phosphodiesterase suppression is used to raise cAMP? Methods: The effects of the NO donor SIN-1 (3-morpholinosydnonimine), CCh and atrial natriuretic peptide (ANP) were evaluated when I had been stimulated by isoproterenol (ISO) or Ca(L) 3-isobutyl-1-methylxanthine (IBMX) in guinea pig isolated ventricular myocytes (358C). Results: Carbachol, SIN-1 or ANP did not affect basal I ; each inhibited IBMX-stimulated I . Dialyzed (30–100 mM) ODQ (1H-(1,2,4)oxadiazolo(4,3-a)quinoxalin-1-one), a Ca(L) Ca(L) soluble guanylyl cyclase (sGC) inactivator, blocked inhibition of IBMX-stimulated I by SIN-1 (10 mM) but not by CCh (1–100 mM) Ca(L) or ANP (100 nM). Dialysis with 3 mM LY83583 (6-anilino-5,8-quinolinedione), a particulate (pGC) and sGC inactivator, opposed muscarinic-, ANPand SIN-1-induced inhibition of IBMX-stimulated I . Thus CCh can increase cGMP synthesis via pGC. Even with Ca(L) 100 mM [LY83583] , CCh inhibited ISO-stimulated I , an effect referable to suppression of adenylyl cyclase activity. However, 3 pip Ca(L) mM [LY83583] prevented inhibition of ISO-stimulated I by ANP. [LY83583] did not affect inhibition by 8 bromo-cGMP (100 pip Ca(L) pip mM) of ISOor IBMX-stimulated I . The observations indicate that: (1) myocytes have ODQ-sensitive sGC activated by NO and Ca(L) LY8353-sensitive pGC activated by ANP, (2) CCh does not inhibit I via NO, (3) the mechanism for muscarinic inhibition depends Ca(L) upon the cAMP-elevating agent and (4) LY83583 distinguishes between two pathways for muscarinic inhibition. Conclusion: The nature of the stimulant pathway that increases cAMP determines intracellular transduction of muscarinic inhibition. This hypothesis accords with distinct cyclic nucleotide compartments for the differential expression of muscarinic inhibition of I .  2001 Elsevier Science B.V. Ca(L) All rights reserved.