NO-cGMP pathway increases the hyperpolarisation-activated current, If, 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, I(CaL), and the hyperpolarisation activated current, I(f). We tested the hypothesis that following adrenergic stimulation, the NO-cGMP pathway stimulates phosphodiesterase 2 (PDE2) to reduce cAMP dependent stimulation of I(f) and heart rate (HR). METHODS: In the presence of norepinephrine (NE, 1 microM), 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 microM, 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 1H-(1,2,4)-oxadiazolo-(4,3-a)-quinoxalin-1-one (ODQ, 10 microM, n=5) abolished the increase in HR to SNP (100 microM) as did the I(f) blockers caesium chloride (2 mM, n=7) and 4-(N-ethyl-N-phenylamino)-1,2-dimethyl-6-(methylamino)-pyrimidinium chloride (ZD7288, 1 microM, n=7). Addition of SNP (10 microM) also transiently increased I(f) in SAN cells (n=5). After inhibition of PDE2 with erythro-9-(2-hydroxy-3-nonyl)-adenine (EHNA, 10 microM, 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 I(f) dependent, and is limited in magnitude and duration by stimulation of PDE2.