Dual Effect of Nitric Oxide on the Hyperpolarization-activated Inward Current (If) in Sino-atrial Node Cells of the Rabbit

Using the whole-cell voltage-clamp technique, we have investigated the effect of nitric oxide (NO) donor (sodium nitroprusside, SNP) on hyperpolarization-activated inward current,If, in isolated rabbit sinoatrial node (SAN) cells.Ifin the basal state increased when NO was applied but decreased whenIfwas pre-stimulated by isoproterenol (ISO) or by adding cAMP to the pipette solution. Both the stimulatory and the inhibitory effects of NO were abolished by guanylyl cyclase inhibitor, methylene blue (MB), suggesting that the effect of NO is mediated by cGMP. The inhibitory effect of NO was abolished whenIfwas pre-stimulated by 3-isobutyl-1-methylxanthine (IBMX), which is a phosphodiesterase (PDE) inhibitor, or by adding 8Br-cAMP (which is resistant to PDE) to the pipette solution. An analogue of cGMP, 8Br-cGMP, which is a potent stimulator of cGMP-dependent protein kinase (PKG) but has little effect on PDE, did not inhibitIfwhenIfwas pre-stimulated by ISO. In its basal state,Ifwas still increased by 8Br-cGMP, and this effect was not prevented by the pretreatment with H-7, PKG inhibitor. The effect of acetylcholine (ACh) was not identical to that of NOIfdecreased when pre-stimulated not only by ISO, but also by IBMX. The above results suggest that via cGMP, NO exerts a dual effect onIf: the inhibitory effect is mediated by cGMP-stimulated PDE, and the stimulatory effect may be attributable to direct binding of cGMP toIfchannels.