Ⅰ Electrophysiological Effects Of Nitric Oxide On Spontaneous Activity Of Rabbit Atrioventricular Node Cells

Objective: Nitric oxide (NO), derived from endothelial cell of blood vessel, exists in many tissues and organs in body. As an important messanger, it regulates various physiological and pathophysiological processes in the cardiovascular system. It causes vasodilation, inhibits platelet aggregation, reduces blood pressure, and dually modulates cardiac contractility. The present study was undertaken to investigate the effects of NO on spontaneous activity of rabbit atrioventricular (AV) node cells and the underlying mechanism.Methods: Rabbit of either sex was stunned by heavy blow on the head and the heart was removed and placed in cold (0~4℃) oxygenated (95% O2 and 5% CO2) Krebs-Henseleit (K-H) solution. The preparation of AV node was isolated carefully and pinned down on a thin silicon disc, perfused with K-H solution (4 ml/min) at 36.0 ± 0.5℃. The action potential was recorded using intracellular microelectrode technique. Drugs were administered and the changes of AP were observed.Results: (1) AV node cells had a mean rate of spontaneous activity (RSF) of 195 ± 3.5 beats/min. The velocity of diastolic (phase 4) depolarization (VDD) was 63 ± 6.4 mV/s. Most cells exhibited relatively slow action potential upstroke and the maximal rate of depolarization (Vmax) was 14.6 ± 0.7 V/s. The amplitude of action potential (APA) was 98 ± 1.7 mV, and maximal diastolic potential (MDP) was -75 ± 1.5 mV. Duration of 90% repolarization (APD90) was 110 ± 5.0 ms. (2) NO donors sodium nitroprusside (SNP, 10-6~10-3 mol/L) and 3-morpholinosydnonimine (SIN-1, 10-4, 10-3 mol/L) decreased APA, Vmax, VDD, and RSF in a concentration-dependent manner. SNP (10-6 mol/L) had no effects on AV node, while SNP (10-5 mol/L) decreased Vmax to 13.1 ± 0.7 V/s, VDD to 55 ± 6.4 mV/s, and RSF to 185 ± 5.0 beats/min. SNP (10-4 mol/L) decreased APA, Vmax, VDD, and RSF to 94 ± 1.2 mV, 11.3 ± 1.3 V/s, 41 ± 5.8 mV/s, 173 ± 3.5 beats/min, respectively. SNP (10-3 mol/L) decreased APA, Vmax, VDD, and RSF to 92 ± 1.4 mV, 10.0 ± 1.1 V/s, 36 ± 5.0 mV/s, 155 ± 4.5 beats/min, respectively. (3) Pretreatment with either L-type calcium channel agonist Bay K8644 (0.25 μmol/L) or elevation of Ca2+ concentration (5 mmol/L) in superfusate antagonized the effects of SNP (10-4 mol/L) on AV node cells. (4) Perfusion with Ca2+-free K-H solution completely abolished the effects of SNP(10-4 mol/L) on AV node cell. (5) Application of methylene blue (50 μmol/L), a guanylyl cyclase inhibitor, failed to abolish the inhibitory effects of SNP (10-4 mol/L).Conclusion: All these results suggest that NO exert a negative effect on spontaneous activity of AV node cells in rabbit. These effects are likely due to reduction in calcium influx via a cGMP-independent mechanism.