Role of nitric oxide in the control of the heart rate within the nucleus ambiguous of rats

The aim of this study was to determine whether NO plays a role in the control of heart rate (HR) within the nucleus ambiguus (NA). Experiments were performed in 29 male Wistar rats anaesthetized with urethane. Microinjections of the NO-donor sodium nitroprusside (SNP; 5 mmol) as well as of L-arginine (L-arg; 50 mmol) into functionally identified cardioinhibitory sites within the NA significantly decreased HR (-57.7 ± 8.4 and -53.8 ± 3.2 bpm, respectively), whereas the NO-synthase inhibitor Nω-nitro-L-arginine methyl ester (L-NAME) significantly increased HR (+40 ± 2.7 bpm). Bilateral vagotomy and i.v. injection of atropine (0.5 mg/kg) always abolished the HR decrease induced by SNP and L-arg, whereas propranolol did not affect the HR responses. These results demonstrated that NO mechanisms within the NA play a role in the parasympathetic control of the HR. (C) 2000 Lippincott Williams and Wilkins.