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Background: NO donors exert a positive chronotropic effect by stimulating the pacemaker current I f in sino-atrial node cells via a mechanism requiring guanylyl cyclase (GC) and cGMP, and involving mobilization of sarcoplasmic (SR) Ca 2+ . Cyclic GMP can directly stimulate If but it is ∼10-fold less potent than the 'physiological' I f -gating nucleotide, cAMP. In ventricular myocytes, NO can increase cAMP levels through a cGMP-dependent inhibition of PDE3. Aim: To elucidate the role of PDE3, PKA and PKG in mediating the NO-I f -dependent increase in HR. Methods and Results: In isolated guinea-pig atria with basal HR (B/L) of 174±3 bpm (n=69), we evaluated the HR response to increasing concentrations (from 0.1 to 100μmol/L) of the NO donors DEA-NO (n=8) or SIN-1 (+SOD 100 U/mL, n=6) or to 10 μmol/L SNP (n=9), (1) alone, and (2) after pretreatment (ca. 40min) with inhibitors of (i) PDE3 (milrinone, trequinsin or Ro-13-6482, n=22) or (ii) PKA (KT5720 or H-89, n=15) or (iii) PKG (KT5823 or Rp-8-pCPT-cGMPs, n=16). Whereas PKG inhibition had no effect on the magnitude of the increase in HR with DEA-NO (see Fig; bars show the peak increase in HR, mean± SEM; =p < 0.05 vs. B/L, †=p < 0.05 vs. the HR response DEA-NO alone), inhibition of PDE3 or PKA markedly attenuated the HR response to DEA-NO (and, similarly, to SIN-1 or SNP; not shown). Conclusion: cGMP-PDE3-cAMP signalling plays a significant part in eliciting the 'direct' positive chronotropic response to NO donors. Since PKA is known to stimulate SR Ca 2+ release by phosphorylating ryanodine receptors, the present findings provide an important link between the activation of GC with NO donors and the NO-dependent mobilization of SR Ca 2+ in SA node which we reported previously. (Graph Presented).


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