Role of cGMP-inhibited phosphodiesterase and sarcoplasmic calcium in mediating the increase in basal heart rate with nitric oxide donors

Nitric oxide (NO) donors increase heart rate (HR) through a guanylyl cyclase-dependent stimulation of the pacemaker current I-f, without affecting basal ICa-L. The activity of I-f is known to be enhanced by cyclic nucleotides and by an increase in cytosolic Ca2+. We examined the role of cGMP-dependent signaling pathways and intracelluar Ca2+ stores in mediating the positive chronotropic effect of NO donors. In isolated guinea pig atria, the increase in HR in response to 1-100 mu mol/l 3-morpholinosydnonimine (SIN-1: with superoxide dismutase, n=6) or diethylamine-NO (DEA-NO, n=8) was significantly attenuated by blockers of the cGMP-inhibited phosphodiesterase (PDE3; trequinsin, milrinone or Ro-13-6438, n=22), In addition, the rate response to DEA-NO or sodium nitroprusside (SNP) was significantly reduced following inhibition of PKA (KT5720 or H-89, n=15) but not PKG (KT5728 or Rp-8-pCPT-cGMPs, n=16). Suppression of sarcoplasmic (SR) Ca2+ release by pretreatment of isolated atria with ryanodine or cyclopiazonic acid (2 mu mol/l and 60 mu mol/l, n=16) significantly reduced the chronotropic response to 1-100 mu mol/l SIN-1 or DEA-NO. Moreover, in isolated guinea pig sinoatrial node cells 5 mu mol/l SNP significantly increased diastolic and peak Ca2+ fluorescence (+13 +/- 1% and +28 +/- 1%, n=6. P<0.05). Our findings are consistent with a functionally significant role of cAMP/PKA signaling (via cGMP inhibition of PDE3) and SR Ca2+ in mediating the positive chronotropic effect of NO donors. (C) 2000 Academic Press.