NO-cGMP pathway increases the hyperpolarisation-activated current, I(f), and heart rate during adrenergic stimulation.

Objectives: The role of the nitric oxide (NO)-cGMP pathway in the autonomic modulation of cardiac pacemaking is controversial and may involve an interplay between the L-type calcium current, ICaL, and the hyperpolarisation activated current, If. We tested the hypothesis that following adrenergic stimulation, the NO-cGMP pathway stimulates phosphodiesterase 2 (PDE2) to reduce cAMP dependent stimulation of If and heart rate (HR). Methods: In the presence of norepinephrine (NE, 1 μM), the effects of the NO donor sodium nitroprusside (SNP) were evaluated in sinoatrial node (SAN)/atria preparations and isolated SAN cells from adult guinea pigs. Results: Contrary to our hypothesis, SNP (10 and 100 μM, n =5) or the membrane permeable cGMP analogue, 8Br-cGMP (0.5 mM, n =6) transiently increased HR by 5±1, 12±1 and 12±2 beats/min, respectively. The guanylyl cyclase inhibitor 1 H -(1,2,4)-oxadiazolo-(4,3-a)-quinoxalin-1-one (ODQ, 10 μM, n =5) abolished the increase in HR to SNP (100 μM) as did the If blockers caesium chloride (2 mM, n =7) and 4-( N -ethyl- N -phenylamino)-1,2-dimethyl-6-(methylamino)-pyrimidinium chloride (ZD7288, 1 μM, n =7). Addition of SNP (10 μM) also transiently increased If in SAN cells ( n =5). After inhibition of PDE2 with erythro-9-(2-hydroxy-3-nonyl)-adenine (EHNA, 10 μM, n =5), the increase in HR to SNP in the presence of NE was significantly augmented and maintained. RT-PCR analysis confirmed the presence of PDE2 in addition to cGMP inhibited PDE3 mRNA in central SAN tissue. Conclusions: These results suggest that during adrenergic stimulation, activation of the NO-cGMP pathway does not decrease HR, but has a transient stimulatory effect that is If dependent, and is limited in magnitude and duration by stimulation of PDE2.