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

OBJECTIVE

Does carbachol (CCh) require NO/cGMP for inhibition of L-type calcium current (I(Ca(L))) when either adenylyl cyclase activation or phosphodiesterase suppression is used to raise cAMP?

METHODS

The effects of the NO donor SIN-1 (3-morpholino-sydnonimine), CCh and atrial natriuretic peptide (ANP) were evaluated when I(Ca(L)) had been stimulated by isoproterenol (ISO) or 3-isobutyl-1-methylxanthine (IBMX) in guinea pig isolated ventricular myocytes (35 degrees C).

RESULTS

Carbachol, SIN-1 or ANP did not affect basal I(Ca(L)); each inhibited IBMX-stimulated I(Ca(L)). Dialyzed (30-100 microM) ODQ (1H-(1,2,4)oxadiazolo(4,3-a)quinoxalin-1-one), a soluble guanylyl cyclase (sGC) inactivator, blocked inhibition of IBMX-stimulated I(Ca(L)) by SIN-1 (10 microM) but not by CCh (1-100 microM) or ANP (100 nM). Dialysis with 3 microM LY83583 (6-anilino-5,8-quinolinedione), a particulate (pGC) and sGC inactivator, opposed muscarinic-, ANP- and SIN-1-induced inhibition of IBMX-stimulated I(Ca(L)). Thus CCh can increase cGMP synthesis via pGC. Even with 100 microM [LY83583](pip), CCh inhibited ISO-stimulated I(Ca(L)), an effect referable to suppression of adenylyl cyclase activity. However, 3 microM [LY83583](pip) prevented inhibition of ISO-stimulated I(Ca(L)) by ANP. [LY83583](pip) did not affect inhibition by 8 bromo-cGMP (100 microM) of ISO- or IBMX-stimulated I(Ca(L)). The observations indicate that: (1) myocytes have ODQ-sensitive sGC activated by NO and LY8353-sensitive pGC activated by ANP, (2) CCh does not inhibit I(Ca(L)) via NO, (3) the mechanism for muscarinic inhibition depends 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(Ca(L)).