Fatty acids, not insulin, modulate alpha1-adrenergic reactivity in dorsal hand veins

Bradykinin or acetylcholine as vasodilators to test endothelial venous function in healthy subjects

INTRODUCTION

The evaluation of endothelial function has been performed in the arterial bed, but recently evaluation within the venous system has also been explored. Endothelial function studies employ different drugs that act as endothelium-dependent vasodilatory response inductors.

OBJECTIVES

The aim of this study is to compare the endothelium-dependent venous vasodilator response mediated by either acetylcholine or bradykinin in healthy volunteers.

METHODS AND RESULTS

Changes in vein diameter after phenylephrine-induced venoconstriction were measured to compare venodilation induced by acetylcholine or bradykinin (linear variable differential transformer dorsal hand vein technique). We studied 23 healthy volunteers; 31% were male, and the subject had a mean age of 33 +/- 8 years and a mean body mass index of 23 +/- 2 kg/m(2). The maximum endothelium-dependent venodilation was similar for both drugs (p = 0.13), as well as the mean responses for each dose of both drugs (r = 0.96). The maximum responses to acetylcholine and bradykinin also had good agreement.

CONCLUSION

There were no differences between acetylcholine and bradykinin as venodilators in this endothelial venous function investigation.

Reduced venous endothelial responsiveness after oral lipid overload in healthy volunteers

The aim of this study was to investigate endothelial venous function, inflammatory markers, and systemic oxidative stress after an oral lipid overload (OLO). We studied 18 healthy adults (9 men; age, 29.2 +/- 0.9 years; body mass index, 22.3 +/- 0.4 kg/m(2)). Blood samples were collected in the fasting state and 3, 4, and 5 hour after the OLO (1000 kcal, 58% fat) for metabolic variables, oxidative stress, inflammatory markers, adiponectin, and resistin. Changes in vein diameter to phenylephrine, acetylcholine, and sodium nitroprusside (dorsal hand vein technique) were measured before and after the OLO. Oral lipid overload increased triglycerides (61 +/- 6 vs 134 +/- 17 mg/dL, P < .001), insulin (7.2 +/- 0.8 vs 10.7 +/- 1.3 muU/mL, P < .05), and resistin (5.38 +/- 0.5 vs 6.81 +/- 0.7 ng/mL, P < .05) and reduced antioxidant capacity (plasma total antioxidant capacity: 186.7 +/- 56 vs 161.8 +/- 50 U Trolox per microliter plasma, P < .01), vascular reactivity (171.3 +/- 85 vs 894.4 +/- 301 ng/mL, P < .001), and maximum acetylcholine venodilation (105.9% +/- 9% vs 61.0% +/- 7%, P < .05). No changes were observed for sodium nitroprusside. Post-OLO triglycerides were positively correlated with phenylephrine dose (rho = 0.38, P < .05) and resistin (rho = 0.43, P < .01) and negatively correlated with the maximum acetylcholine venodilation (rho = -0.36, P < .05). In conclusion, an OLO impaired venoconstriction responsiveness in healthy subjects, probably because of a reduction in the antioxidant capacity.

Mechanism of lipid enhancement of α1-adrenoceptor pressor sensitivity in hypertension

OBJECTIVE AND DESIGN

Plasma lipids enhance alpha1-adrenoceptor pressor sensitivity, impair baroreflex function, and correlate with increased blood pressure. This clinical study was designed to determine whether the enhanced alpha1-pressor sensitivity induced by acute hyperlipidemia is primarily mediated by increased vascular alpha1 responsiveness, reduced baroreflex sensitivity (BRS) or both.

METHOD

Regional alpha1-adrenoceptor vasoreactivity was measured using a graded brachial artery infusion of the alpha1 agonist, phenylephrine, in seven subjects with stage 1 hypertension. Forearm blood flow was estimated from venous occlusion plethysmography. The phenylephrine dose-forearm blood flow response curve was used to determine alpha1-vascular reactivity (slope of the dose-response curve) and sensitivity, EC50 (phenylephrine dose inducing 50% maximal response). BRS (ms/mmHg) was measured as the slope of the progressive rise in systolic blood pressure and the resultant lengthening in the subsequent R-R interval after systemic intravenous boluses of phenylephrine. Subsequently, plasma lipids were raised with a 1-h systemic co-infusion of intralipid and heparin, after which measurements of regional vasoreactivity and BRS were repeated.

RESULTS

Mean arterial pressure was 109 +/- 4 versus 110 +/- 3 (P = NS), vasoreactivity was -0.71 +/- 0.10 versus -0.82 +/- 0.10 (P = NS) and log EC50 was 1.47 +/- 0.29 versus 1.52 +/- 0.34 nmol/l (P = NS) before and after raising non-esterified fatty acids, respectively. In contrast, mean BRS was acutely reduced from 8.2 +/- 2.1 to 6.2 +/- 1.8 ms/mmHg (P = 0.02) after the lipid infusion.

CONCLUSIONS

These findings suggest that in hypertensive patients, the primary mechanism for short-term alpha1-pressor hypersensitivity in response to hyperlipidemia is via the acute impairment of BRS.

Update on the cardiometabolic syndrome

An array of metabolic, hemodynamic, and renal abnormalities constitutes the cardiometabolic syndrome. A hallmark of this syndrome is visceral obesity and associated insulin resistance/hyperinsulinemia. The syndrome is also associated with essential hypertension, abnormalities in the circadian rhythm of blood pressure and heart rate, the diabetic dyslipidemic syndrome, hypercoagulability, hyperuricemia, increased cardiovascular inflammation, and microalbuminuria, all of which contribute to an increased risk of cardiovascular disease morbidity and mortality. This article reviews current knowledge about the interrelationship of the various factors that make up the cardiometabolic syndrome and its implications for individuals with and without diabetes mellitus.

Elevated plasma fatty acid concentrations stimulate the cardiac autonomic nervous system in healthy subjects

BACKGROUND

Fatty acids have been shown to stimulate the sympathetic nervous system in rats. Power spectral analysis of heart rate variability (HRV) is a safe and useful tool with which to evaluate cardiac autonomic nervous system (ANS) activity. Whether changes in plasma fatty acid concentrations affect the sympathetic nervous system or HRV in humans is unknown.

OBJECTIVE

We investigated the possible changes in HRV after a significant increase in plasma fatty acid concentration.

DESIGN

Subjects were randomly assigned to receive an infusion of lipid emulsion (10% triacylglycerol emulsion for 180 min) + heparin (a bolus of 200 U followed by 0.2 U*min(-)(1)*kg body wt(-)(1); n = 20) or 0.9% NaCl (for 180 min; n = 10).

RESULTS

Lipid emulsion + heparin infusion was associated with a rise in plasma epinephrine and norepinephrine concentrations. The rise in plasma fatty acid concentration was associated with a significant decline in the RR interval (P: < 0.03) and in total power (P: < 0.03). Analysis of the different components of HRV showed that lipid emulsion + heparin infusion stimulated low- frequency (LF) components (P: < 0.03 at the second hour and P: < 0. 01 at the third hour) and inhibited high-frequency (HF) components (P: < 0.03 at the second and third hours). Consequently, the LF-HF ratio was significantly stimulated (P: < 0.03 at the second hour and P: < 0.01 at the third hour). Such results persisted, although attenuated, when the study was repeated in association with a propranolol infusion (n = 8).

CONCLUSION

Elevated plasma fatty acid concentrations may stimulate cardiac autonomic nervous system activity.

Inhibitory effect of insulin on bradykinin-induced venodilation

BACKGROUND

Excessive insulin is one of the risk factors of hypertension and arteriosclerosis despite its vasodilative properties shown in recent studies. Although many vasoactive substances contribute and interact with each other in the development of hypertension, the interactions between insulin and other vasoactive substances have yet to be elucidated.

OBJECTIVE

To assess the effect of insulin on the action of bradykinin.

METHODS

The vasodilating effect of bradykinin was evaluated, with or without coadministration of insulin, in human dorsal hand veins of healthy volunteers. In cultured porcine aortic endothelial cells, the bradykinin-induced increase of intracellular calcium was also investigated before and after insulin administration.

RESULTS

Insulin significantly attenuated bradykinin-induced increase in intracellular calcium and venodilation in cultured endothelial cells and human dorsal hand veins, respectively.

CONCLUSIONS

These findings suggest that insulin attenuates the bradykinin-induced calcium elevation in endothelial cells and may decrease the production of vasodilative substances from endothelial cells, resulting in the reduction of vasodilation. This effect may contribute to the development of hypertension in patients with hyperinsulinaemia.

Insulin-induced endothelin release and vasoreactivity in hypertriglyceridemic and hypertensive rats

Insulin-elicited endothelin release in hypertriglyceridemic, hypertensive, hyperinsulinemic (HTG) rats was shown. Weanling male Wistar rats were given 30% sucrose in their drinking water for 20-24 wk. In vitro contractions of aorta and femoral arteries were elicited with 40 mM KCl. Endothelin release induced with KCl plus 50 microU/ml insulin resulted in increases in contractile responses: 41 +/- 5.9 and 57 +/- 6% for control and 65.5 +/- 6 and 95 +/- 9% for HTG aortas and femoral arteries, respectively. The endothelin ET(B)-receptor blocker BQ-788 decreased responses to KCl + insulin by 39 +/- 8 and 53 +/- 5% in control and 48 +/- 13 and 79 +/- 3.5% in HTG aortas and femoral arteries, respectively. The ET(A)-receptor antagonist PD-151242 inhibited these responses by 12 +/- 10 and 1 +/- 9% in control and by 51.5 +/- 9 and 58.5 +/- 1% in HTG aortas and femoral arteries, respectively. These results suggest that endothelin may contribute to the hypertension in this model.

The sympathetic nervous system and baroreflexes in hypertension and hypotension

Blood pressure and blood volume are closely regulated by the interrelated actions of the sympathetic nervous system (SNS) and the renin-angiotensin-aldosterone system (RAAS). Reflex vasoconstriction caused by parallel SNS and RAAS activation is modulated by two interactive negative feedback systems called baroreflex. The aortic-carotid baroreflex systems respond to momentary changes in systolic blood pressure, adjusting the degree of SNS-dependent peripheral vasoconstriction and cardiac output to allow maintenance of a relatively constant perfusion pressure. Cardiopulmonary baroreflexes respond to momentary changes in cardiac filling, adjusting the degree of peripheral venoconstriction and venous return to maintain cardiac preload and stroke volume. Under normal conditions, each baroreflex system exhibits a degree of tonic negative feedback so that it can alter SNS output immediately, providing counterregulatory increases or decreases in pressure or volume to maintain homeostasis. The SNS is inappropriately active in obesity and hypertension and plays a causal or permissive role in all forms of chronic hypertension. If the negative feedback control exerted by the baroreflexes over the SNS and renin-angiotensin-aldosterone system (RAAS) were perfect, chronic hypertension would not occur. Activity of the baroreflexes, however, is chronically altered by maladaptive changes such as cardiac and vascular fibrosis and hypertrophy. Long-term increases in SNS and RAAS activity also exert ongoing deleterious effects on the heart and vasculature by directly facilitating further cardiac hypertrophy and arterial stiffening. These effects appear to contribute to a vicious cycle of chronic hypertension and target organ damage. Other syndromes of abnormal blood pressure (BP) control, including orthostatic hypotension and baroreflex failure are examples of abnormal baroreflex activity and SNS control.