Regulation by acetylcholine of Ca2+ current in rabbit atrioventricular node cells

Effects of acetylcholine (ACh) on L-type Ca2+ current (I-Ca) were examined in isolated atrioventricular (AV) node culls exhibiting spontaneous contractions and pacemaker current (I-f). ACh at a saturating concentration of 10 mu M reduced basal I-Ca by 48 +/- 6%. The ACh effect was abolished by dialysis with 8-bromoadenosine 3',5'-cyclic monophosphate (8-BrcAMP), an adenosine 3',5'-cyclic monophosphate (cAMP)-dependent protein kinase inhibitor, ol guanosine-5'-O-(2-thiodiphosphate). Dialysis with guanosine 3',5'-cyclic monophosphate (cGMP) or N-G-manomethyl-L-arginine (L-NMMA) and application of the cGMP-dependent protein kinase inhibitor KT-5823 (1 mu M) did not affect ACh inhibition of I-Ca. Nitric oxide donor 3-morpholinosydnonimine (100 mu M) and type III phosphodiesterase (PDE) inhibitor trequinsin (10 nM) enhanced basal I-Ca by 10-20%, whereas type IV PDE inhibitor Ro-20-1724 (30 mu M) together with trequinsin caused a large I-Ca stimulation comparable to that by 3-isobutyl-1-methylxanthine (IBMX). These findings indicate that ACh inhibits basal I-Ca primarily by suppressing cAMP synthesis asci that these cells have a potent type III and IV PDE activity to determine the basal cAMP concentration. When I-Ca was stimulated by IBMX (100 mu M), the inhibitory effect of ACh was slightly reduced by L-NMMA, cGMP, and methylene blue but not by KT-5823 or Ro-20-1724. ACh hardly inhibited, or even enhanced, IBMX-stimulated I-Ca when forskolin (3 mu M) was coapplied or the IBMX concentration was increased to 500 mu M. These findings suggest that cAMP is degraded in the presence of 100 mu M IBMX to some extent. Type II PDE, for which IBMX has a relatively high inhibitor constant. sec?ms to contribute partially to the cAMP degradation.