NITRIC-OXIDE SYNTHASE ACTIVITY IN GUINEA-PIG VENTRICULAR MYOCYTES IS NOT INVOLVED IN MUSCARINIC INHIBITION OF CAMP-REGULATED ION CHANNELS

It has recently been demonstrated that NO plays an obligatory role in muscarinic inhibition of beta-adrenergically stimulated ion channels i n cardiac sinoatrial node cells (J Gen Physiol. 1995;106:45-65). We lo oked for evidence that NO might play a similar role in ventricular cel ls by using histochemical staining for NO synthase (NOS) activity and whole-cell patch-clamp recording of cAMP-regulated Cl- currents. Myocy tes isolated from guinea pig hearts stained positively for NADPH-diaph orase activity suggesting that these cells do express NOS. Acetylcholi ne (ACh) inhibition of the R(-)isoproterenol bitartrate (Iso)-activate d Cl- current was also reversed by the cGMP-lowering agents LY-83583 a nd methylene blue, consistent with the idea that NO activation of guan ylate cyclase may contribute to muscarinic responses. However, LY-8358 3 and methylene blue activated the Cl- current in the presence of subt hreshold concentrations of Iso alone, suggesting that their effects ma y not be due to antagonism of an NO/cGMP-dependent response. Furthermo re, ACh inhibition of Iso-activated Cl- currents could not be mimicked by the NO donors sodium nitroprusside, 3-morpholinosydnoni mine, and spermine-NO. Similarly, ACh inhibition of the Iso-activated Cl- curren t could not be blocked by the NOS inhibitor N-G-monomethyl-L-arginine. These results indicate that even though ventricular myocytes possess NOS activity, NO production does not play an important role in muscari nic inhibition of beta-adrenergically regulated Cl- channels in these cells.