Differential effects of beta-adrenoreceptor antagonists on central and peripheral blood pressure at rest and during exercise.

BACKGROUND: Differential effects of beta-adrenoreceptor antagonists (beta-ARB) on central and peripheral blood pressure may relate to change in heart rate and/or vasodilator tone and thus be exaggerated during exercise. AIMS: To examine acute effects of selective and nonselective beta-ARB on central and peripheral blood pressure, cardiac output and peripheral vascular resistance during exercise. METHODS: Healthy volunteers (n= 20, 18 men, 19-54 years) received propranolol 80 mg, bisoprolol 20 mg, and placebo 1 h before bicycle ergometry (50, 75 and 100 W each for 3 min) in a randomized, cross-over study. Cardiac output was determined by pulmonary uptake of soluble and inert gas tracers (InnoCor, Innovision). Central systolic blood pressure (SBP) was determined from the late systolic shoulder of the digital artery pressure waveform (Finometer, Finopres). RESULTS: At rest, both beta-ARB reduced brachial but not central SBP (compared with placebo). During exercise, beta-ARB reduced brachial SBP (reductions of 19.9 +/- 4.3 mmHg and 23.2 +/- 2.7 mmHg for propranolol and bisoprolol, respectively, at 100 W, each P < 0.0001) but not central SBP. The difference between peripheral and central SBP was reduced, relative to that during placebo, by 21.5 mmHg (95% confidence interval 8.8, 34.1) and 26.4 mmHg (18.1, 34.8) for propranolol and bisoprolol, respectively, at 100 W (each P < 0.01). There was no significant effect of beta-ARB on diastolic blood pressure or peripheral vascular resistance. CONCLUSIONS: Despite reducing brachial blood pressure, acute beta-adrenoreceptor blockade in man at rest and during exercise does not reduce central blood pressure.

Neuronal nitric oxide synthase regulates basal microvascular tone in humans in vivo.

BACKGROUND: Nitric oxide (NO) has a pivotal role in the regulation of vascular tone and blood flow, with dysfunctional release contributing to disease pathophysiology. These effects have been attributed to NO production by the endothelial NO synthase (eNOS); however, recent evidence suggests that a neuronal NO synthase (nNOS) may also be expressed in arterial vessels. METHODS AND RESULTS: We undertook a first-in-humans investigation of the role of nNOS in the local regulation of vascular blood flow in healthy subjects. Brachial artery infusion of the nNOS-specific inhibitor S-methyl-L-thiocitrulline (SMTC, 0.025 micromol/min to 0.2 micromol/min) caused a dose-dependent reduction in basal flow, with a 30.1+/-3.8% decrease at the highest dose (n=10; mean+/-SE; P<0.01). The effect of SMTC was abolished by coinfusion of the NO synthase substrate L-arginine but was unaffected by D-arginine. A similar reduction in basal flow with the nonselective NO synthase inhibitor N(G)-monomethyl-L-arginine (L-NMMA; 37.4+/-3.1%, n=10) required a 20-fold higher dose of 4 micromol/min. At doses that produced comparable reductions in basal flow, only L-NMMA (4 micromol/min) and not SMTC (0.2 micromol/min) inhibited acetylcholine-induced vasodilation; however, both SMTC and L-NMMA inhibited the forearm vasodilator response to mental stress. CONCLUSIONS: Basal forearm blood flow in humans is regulated by nNOS-derived NO, in contrast to the acetylcholine-stimulated increase in blood flow, which, as shown previously, is mediated primarily by eNOS. These data indicate that vascular nNOS has a distinct local role in the physiological regulation of human microvascular tone in vivo.

Elevation of plasma homocysteine by methionine loading increases the diastolic blood pressure response to exercise.

OBJECTIVE: Exercise blood pressure is elevated in conditions associated with endothelial dysfunction. To determine whether this is a causal association, we examined the effects of endothelial dysfunction induced by methionine loading, on exercise blood pressure. DESIGN: Healthy subjects (18-63 years) participated in randomized, double-blind, cross-over studies receiving methionine and placebo on separate occasions before exercise. METHODS: In study 1 (n = 32), subjects received placebo and methionine 100 mg/kg 4 h before exercise and, in study 2 (n = 11), placebo and methionine 200 mg/kg (given as 100 mg 20 and 12 h before exercise). A further study confirmed that methionine 100 mg/kg impaired brachial artery flow-mediated dilation. Blood pressure was measured by mercury sphygmomanometry at rest and during low workload bicycle ergometry. RESULTS: Plasma homocysteine increased after methionine compared to placebo (22.5 +/- 1.2 versus 7.7 +/- 0.7 mumol/l in study 1, and 50.2 +/- 7.6 versus 12.8 +/- 0.7 mumol/l in study 2, means +/- SE, each P < 0.0001). In both studies resting systolic and diastolic blood pressures and the increases in systolic blood pressure during exercise were similar after methionine and placebo. By contrast, exercise diastolic blood pressure responses (measured as the change from resting values) increased more after methionine than after placebo. In study 1, diastolic blood pressure responses increased relative to placebo by 3.5 +/- 1.0, 3.7 +/- 1.0 and 2.4 +/- 1.0 mmHg at 50, 75 and 100 W, respectively (P < 0.01). In study 2, diastolic blood pressure responses increased by 4.9 +/- 1.0, 5.4 +/- 2.1 and 6.1 +/- 1.1 mmHg at 50, 75 and 100 W, respectively (P < 0.001). CONCLUSION: Endothelial dysfunction produced by methionine loading is associated with an increased exercise diastolic blood pressure response.

Impaired neuronal nitric oxide synthase-mediated vasodilator responses to mental stress in essential hypertension.

Neuronal NO synthase (nNOS) regulates blood flow in resistance vasculature at rest and during mental stress. To investigate whether nNOS signaling is dysfunctional in essential hypertension, forearm blood flow responses to mental stress were examined in 88 subjects: 48 with essential hypertension (42±14 years; blood pressure, 141±17/85±15 mm Hg; mean±SD) and 40 normotensive controls (38±14 years; 117±13/74±9 mm Hg). A subsample of 34 subjects (17 hypertensive) participated in a single blind 2-phase crossover study, in which placebo or sildenafil 50 mg PO was administered before an intrabrachial artery infusion of the selective nNOS inhibitor S-methyl-l-thiocitrulline (SMTC, 0.05, 0.1, and 0.2 µmol/min) at rest and during mental stress. In a further subsample (n=21) with an impaired blood flow response to mental stress, responses were measured in the presence and absence of the α-adrenergic antagonist phentolamine. The blood flow response to mental stress was impaired in hypertensive compared with normotensive subjects (37±7% versus 70±8% increase over baseline; P<0.001). SMTC blunted responses to mental stress in normotensive but not in hypertensive subjects (reduction of 40±11% versus 3.0±14%, respectively, P=0.01, between groups). Sildenafil reduced the blood flow response to stress in normotensive subjects from 89±14% to 43±14% (P<0.03) but had no significant effect in hypertensive subjects. Phentolamine augmented impaired blood flow responses to mental stress from 39±8% to 67±13% (P<0.02). Essential hypertension is associated with impaired mental stress-induced nNOS-mediated vasodilator responses; this may relate to increased sympathetic outflow in hypertension. nNOS dysfunction may impair vascular homeostasis in essential hypertension and contribute to stress-induced cardiovascular events.

Acute haemodynamic effects of lipolysis-induced increase of free fatty acids in healthy men.

Circulating free fatty acids (FFA) are elevated in subjects with insulin resistance and Type II diabetes, and increase during myocardial ischaemia, but their haemodynamic effects are incompletely understood. During an investigation of the effects of FFA on endothelial function, we administered lipid emulsion (150 mg x min(-1) of soybean oil) with heparin (0.2 unit x kg(-1) x min(-1)) intravenously to eight healthy men for 2 h. This increased circulating FFA to 3.1+/-0.5 mmol/l. Forearm blood flow was measured by venous occlusion plethysmography during brachial artery infusions of saline, acetylcholine and nitroprusside before, and at 1 and 2 h. Lipid/heparin infusion had no significant effect on vasodilation to nitroprusside but progressively increased responses to acetylcholine (from 6.3+/-2.0 during 30 microg x min(-1) before-lipid infusion to 7.9+/-1.3 at 1 h and 12.2+/-1.1 ml x min(-1) x 100 ml(-1) at 2 h, P<0.001). Basal flow increased from 2.7+/-0.7 to 4.7+/-0.8 ml x min(-1) x 100 ml(-1) from 0 to 2 h. We performed a second study to clarify this effect on basal blood flow. Healthy men (n=8) received, on separate occasions, 4 h intravenous infusions of lipid emulsion with heparin and, as a control, saline with heparin. Lipid with heparin increased mean arterial blood pressure (maximum increment 8.2+/-2.7 mm Hg, P<0.01 compared with saline/heparin control) and forearm blood flow (from 1.7+/-0.2 to 2.9+/-0.3 ml x min(-1) x 100 ml(-1), P<0.01) without a significant effect on heart rate, and reduced calculated forearm vascular resistance (from 49.1+/-5.4 to 31.3+/-3.9 arbitrary units, P<0.01). In conclusion, acute elevation of FFA in healthy men increases arterial blood pressure and reduces vascular resistance. These haemodynamic changes could be clinically relevant.

Cardiovascular risk factors and endothelial dysfunction.

Endothelial dysfunction is a feature of atherosclerosis and is associated with CHD (coronary heart disease) risk factors. This study aimed to determine the relationship between the degree of endothelial dysfunction and calculated cardiovascular risk. Endothelial function, as determined by the ACh/NP (acetycholine/sodium nitroprusside response) ratio on brachial plethysmography, was compared with cardiovascular risk as calculated from the Framingham, PROCAM (Prospective Cardiovascular Munster) and MRFIT (Multiple Risk Factor Intervention Trial) algorithms in 246 (187 male) patients, including 44 (22%) with established CHD. Endothelial dysfunction correlated with the total number of risk factors (r2=0.22; P=0.002) and was related to LDL (low-density lipoprotein)-cholesterol in men and triacylglycerols (triglycerides) in women. The ACh/NP ratio correlated with the occurrence of diabetes, CHD and the LDL-cholesterol concentration (r2=0.58; P<0.001). Endothelial dysfunction was associated with presence of CHD on receiver-operating characteristic plot analysis (area=0.706+/-0.04; P=0.001). There was no correlation between ACh/NP ratio and CHD risk calculated with the Framingham algorithm in men, although both ACh and NP response correlated separately with risk in women. The endothelial ACh/NP ratio correlated with absolute risk in the PROCAM algorithm (r2=0.41; P<0.005). Intermediate results were obtained with MRFIT. Individual risk factors make different contributions to endothelial dysfunction compared with their role in risk calculators. The stronger relationship of endothelial dysfunction with PROCAM risk reflects the contribution of male sex, LDL-cholesterol and triacylglycerols to risk calculated by this algorithm.