Effective systemic blockade of endogenous nitric oxide production results in moderate hypertension, reduced sympathetic activity and shortened bleeding time in healthy volunteers.

Abstract

Background Nitric oxide (NO) is of fundamental importance in the regulation of the circulatory system. NO is synthesised in the endothelium, as well as in many other sites, and has been shown to be a potent vasodilator. It is also an important inhibitor of platelet function. The endogenous production of NO from its precursor L-arginine can be reduced by intravenous infusion of L-arginine analogues, like NGmonomethyl-L-arginine (L-NMMA). Short term infusion of L-NMMA is known to reversibly inhibit endogenous NO production in humans. L-NMMA (0.6 mg/kg/min) has previously been given to volunteers as 5 min infusions (Haynes et al, J Hypertens. 19931, and resulted in vasoconstriction as reflected by changes in several haemodynamic parameters. The effects were maximal 5-10 min after the L-NMMA infusion. Patients with increased NO production, such as patients with sepsis, have been treated with LNMMA, (0.017 mg/kg/&), during longer periods of time (6 hours) to restore vascular tone (Petros et al. Cardiovasc Res. 19941, but long lasting infusion in healthy individuals have not been reported. We studied 60 minutes infusion of L-NMMA in healthy volunteers. We attempted to establish sustained mild systemic hypertension, as evidence of partial and stable blockade of endogenous NO production, enabling future intervention studies. We specifically examined hemodynamic responses, including the pulmonary circulation and markers of sympathetic nervous system activity (i.e. catecholamines in plasma). We also studied parameters that might reflect the degree of inhibition of endogenous NO production, such as NO concentrations in nasal air, bleeding time (i.e. primary haemostasis) and plasma concentrations of cyclic guanosinmonophosphate (&MI'). Methods Eight healthy male volunteers, ages ranged from 18 to 23, partiapated in the study. Young healthy males were specifically recruited for safety reasons. All subjects rested for 30 minutes in the supine position after catheter (pulmonary, arterial and ankle vein) placement to establish a stable baseline. Baseline measurements were performed. During the experiment 500 ml 0.9% NaCl solution was infused via the ankle catheter, and through this catheter LNMMA was given intravenously with an infusion pump. The dose of L-NMMA was 0.3 mg/kg/min during 30 min, followed by 0.15 mg/kg/min during 30 min (n=6), or 0.3 mg/kg/min for 60 min (n=2). Haemodynamic and forearm blood flow measurements in the fore arm were repeated during (5, 15,30 and 50 min) and after (+30, +60 and +90 min) LNMMA infusion. Bleeding time was measured during (15 and 50 min) the infusion. Concentration of NO in nasal air (eNO) were determined during (15, 30 and 50 min) and after (+30, +60 and +90 min) LNMMA infusion. Blood sampling (PaO,, Sa02 PVO,, SVO,, as well as cGMP and cathecolamines in plasma) was repeated during (5, 15, 30 and 50 min) and after (+30 and +60 min) L-NMMA. The study was approved by the Ethics Committee of the Karolinska Hospital and the Swedish Medical Products Agency. Results The lower dose (se above) of L-NMMA a p peared to be safe and well tolerated, with only minor complaints of lower back pain, described as tiredness. L-NMNU significantly elevated mean arterial pressure by 12%*3%, due to an increase in systemic vascular resistance. Cardiac output decreased by 23%*3%. Heart rate was stable during infusion and increased slightly after the infusion. Mean pulmonary arterial pressure was stable while pulmonary vascular resistance tended to increase. Surprisingly, forearm vascular resistance decreased slightly, and forearm blood flow increased during the infusion despite the decrease in cardiac output. Bleeding time was shortened by 31 %*4%. LNMMA infusion decreased eNO by 64%*2% and eNO remained decreased more