Possible dependence of pressor and heart rate effects of N G ‐nitro‐ l ‐arginine on autonomic nerve activity

1 The effects of N G ‐nitro‐ l ‐arginine ( l ‐NNA) on mean arterial pressure (MAP) and heart rate (HR) were investigated in conscious rats. 2 Intravenous bolus cumulative doses of l ‐NNA (1–32 mg kg −1 ) dose‐dependently increased MAP. Both mecamylamine and phentolamine increased MAP responses to l ‐NNA, angiotensin II and methoxamine. Propranolol, reserpine, atropine and captopril did not affect MAP response to l ‐NNA. 3 A significant negative correlation of HR and MAP responses to l ‐NNA was obtained in control rats but not in rats pretreated with reserpine or mecamylamine. Significant negative correlations also occurred in the presence of atropine, propranolol, phentolamine or captopril. 4 A single i.v. bolus dose of l ‐NNA (32 mg kg −1 ) raised MAP to a peak value of 53 ± 3 mmHg and the effect lasted more than 2 h; the rise and recovery of MAP were accompanied by significant decrease and increase in HR, respectively. While both phentolamine and mecamylamine increased peak MAP response to l ‐NNA, mecamylamine abolished the biphasic HR response and phentolamine potentiated the bradycardic component of HR. 5 Blockade of the autonomic nervous and renin‐angiotensin systems did not attenuate the pressor effects of l ‐NNA. However, the biphasic HR response to l ‐NNA is mediated via modulation of autonomic nerve activities.