Effect of losartan on muscle sympathetic activity and baroreceptor function in systemic hypertension

The Role of Angiotensin Receptor Blockers in the Personalized Management of Diabetic Neuropathy

Neuropathy is a frequent complication of diabetes mellitus (DM) and is associated with the increased risk ofamputation and vascular events. Tight glycemic control is an important component inthe prevention of diabetic neuropathy. However, accumulating data suggest that angiotensin receptor blockers (ARBs) might also be useful in this setting. We discuss the findings of both experimental and clinical studies that evaluated the effects of ARBs on indices of diabetic neuropathy. We also review the implicated mechanisms of the neuroprotective actions of these agents. Overall, it appears that ARBs might be a helpful tool for preventing and delaying the progression of diabetic neuropathy, but more data are needed to clarify their role in the management of this overlooked complication of DM.

Effects of losartan and allopurinol on cardiorespiratory regulation in obstructive sleep apnoea

NEW FINDINGS

What is the central question of this study? In sleep apnoea, a putative link between intermittent hypoxia and hypertension is the generation of oxygen radicals by angiotensin II and xanthine oxidase within the chemoreflex arc and vasculature. We tested whether chemoreflex control of sympathetic outflow, hypoxic vasodilatation and blood pressure are altered by angiotensin blockade (losartan) and/or xanthine oxidase inhibition (allopurinol). What is the main finding and its importance? Both drugs lowered blood pressure without altering sympathetic outflow, reducing chemoreflex sensitivity or enhancing hypoxic vasodilatation. Losartan and allopurinol are effective therapies for achieving blood pressure control in sleep apnoea.

ABSTRACT

Chemoreflex sensitization produced by chronic intermittent hypoxia in rats is attenuated by angiotensin II type 1 receptor (AT₁ R) blockade. Both AT₁ R blockade and xanthine oxidase inhibition ameliorate chronic intermittent hypoxia-induced endothelial dysfunction. We hypothesized that treatment with losartan and allopurinol would reduce chemoreflex sensitivity and improve hypoxic vasodilatation in patients with obstructive sleep apnoea. Eighty-six hypertensive patients with apnoea-hypopnoea index ≥25 events h^-1 and no other cardiovascular, pulmonary, renal or metabolic disease were randomly assigned to receive allopurinol, losartan or placebo for 6 weeks. Treatment with other medications and/or continuous positive airway pressure remained unchanged. Tests of chemoreflex sensitivity and hypoxic vasodilatation were performed during wakefulness before and after treatment. Ventilation (pneumotachography), muscle sympathetic nerve activity (microneurography), heart rate (electrocardiography), arterial oxygen saturation (pulse oximetry), blood pressure (sphygmomanometry), forearm blood flow (venous occlusion plethysmography) and cerebral flow velocity (transcranial Doppler ultrasound) were measured during eupnoeic breathing and graded reductions in inspired O₂ tension. Losartan and allopurinol lowered arterial pressure measured during eupnoeic breathing and exposure to acute hypoxia. Neither drug altered the slopes of ventilatory, sympathetic or cardiovascular responses to acute hypoxia. We conclude that losartan and allopurinol are viable pharmacotherapeutic adjuncts for achieving blood pressure control in hypertensive obstructive sleep apnoea patients, even those who are adequately treated with continuous positive airway pressure.

Neuroendocrine and Inflammatory Responses to Losartan and Continuous Positive Airway Pressure in Patients with Hypertension and Obstructive Sleep Apnea. A Randomized Controlled Trial

RATIONALE

Blood pressure reduction in response to antihypertensive agents is less for patients with obstructive sleep apnea (OSA). Increased sympathetic and inflammatory activity, as well as alterations in the renin-angiotensin-aldosterone system, may play a role in this context.

OBJECTIVES

To address the cardiovascular mechanisms involved in response to an angiotensin II receptor antagonist, losartan, and continuous positive airway pressure (CPAP) as add-on treatment for hypertension and OSA.

METHODS

Newly diagnosed hypertensive patients with or without OSA (allocated in a 2:1 ratio for OSA vs. no OSA) were treated with losartan 50 mg daily during a 6-week two-center, open-label, prospective, case-control, parallel-design study. In the second 6-week, sex-stratified, open-label, randomized, parallel-design study, all subjects with OSA continued to receive losartan and were randomly assigned to either CPAP as add-on therapy or to no CPAP (1:1 ratio for CPAP vs. no CPAP). Study subjects without OSA were followed in parallel while they continued to take losartan. Blood samples were collected at baseline, after 6 weeks, and after 12 weeks for analysis of renin, aldosterone, noradrenaline, adrenaline, and inflammatory markers.

MEASUREMENTS AND MAIN RESULTS

Fifty-four patients with OSA and 35 without OSA were included in the first 6-week study. Losartan significantly increased renin levels and reduced aldosterone levels in the group without OSA. There was no significant decrease in aldosterone levels among patients with OSA. Add-on CPAP treatment tended to lower aldosterone levels, but reductions were more pronounced in measures of sympathetic activity. No significant changes in inflammatory markers were observed following treatment with losartan and CPAP.

CONCLUSIONS

Hypertensive patients with OSA responded to losartan treatment with smaller reductions in aldosterone compared with hypertensive patients without OSA. Sympathetic system activity seemed to respond primarily to add-on CPAP treatment in patients with newly discovered hypertension and OSA. Clinical trial registered with www.clinicaltrials.gov (NCT00701428).

Muscle sympathetic nerve activity and volume-regulating factors in healthy pregnant and nonpregnant women

Healthy, normotensive human pregnancies are associated with striking increases in both plasma volume and vascular sympathetic nerve activity (SNA). In nonpregnant humans, volume-regulatory factors including plasma osmolality, vasopressin, and the renin-angiotensin-aldosterone system have important modulatory effects on control of sympathetic outflow. We hypothesized that pregnancy would be associated with changes in the relationships between SNA (measured as muscle SNA) and volume-regulating factors, including plasma osmolality, plasma renin activity, and arginine vasopressin (AVP). We studied 46 healthy, normotensive young women (23 pregnant and 23 nonpregnant). We measured SNA, arterial pressure, plasma osmolality, plasma renin activity, AVP, and other volume-regulatory factors in resting, semirecumbent posture. Pregnant women had significantly higher resting SNA (38 ± 12 vs. 23 ± 6 bursts/min in nonpregnant women), lower osmolality, and higher plasma renin activity and aldosterone (all P < 0.05). Group mean values for AVP were not different between groups [4.64 ± 2.57 (nonpregnant) vs. 5.17 ± 2.03 (pregnant), P > 0.05]. However, regression analysis detected a significant relationship between individual values for SNA and AVP in pregnant (r = 0.71, P < 0.05) but not nonpregnant women (r = 0.04). No relationships were found for other variables. These data suggest that the link between AVP release and resting SNA becomes stronger in pregnancy, which may contribute importantly to blood pressure regulation in healthy women during pregnancy.NEW & NOTEWORTHY Sympathetic nerve activity and blood volume are both elevated during pregnancy, but blood pressure is usually normal. Here, we identified a relationship between vasopressin and sympathetic nerve activity in pregnant but not nonpregnant women. This may provide mechanistic insights into blood pressure regulation in normal pregnancy and in pregnancy-related hypertension.

Renin-angiotensin and sympathetic nervous system contribution to high blood pressure in Schlager mice

OBJECTIVE

Schlager hypertensive (BPH/2J) mice have been suggested to have high blood pressure (BP) due to an overactive sympathetic nervous system (SNS), but the contribution of the renin-angiotensin system (RAS) is unclear. In the present study, we examined the cardiovascular effects of chronically blocking the RAS in BPH/2J mice.

METHODS

Schlager normotensive (BPN/3J, n = 6) and BPH/2J mice (n = 8) received the angiotensin AT 1A-receptor antagonist losartan (150 mg/kg per day) in drinking water for 2 weeks. Pre-implanted telemetry devices were used to record mean arterial pressure (MAP), heart rate (HR) and locomotor activity.

RESULTS

MAP was reduced by losartan treatment in BPN/3J (-23 mmHg, P < 0.01) and in BPH/2J mice (-25 mmHg, P < 0.001), whereas HR was increased. Losartan had little effect on initial pressor responses to feeding or to stress, but did attenuate the sustained pressor response to cage-switch stress. During the active period, the hypotension to sympathetic blockade with pentolinium was greater in BPH/2J than BPN/3J (suggesting neurogenic hypertension), but was not affected by losartan. During the inactive period, a greater depressor response to pentolinium was observed in losartan-treated animals.

CONCLUSION

The hypotensive actions of losartan suggest that although the RAS provides an important contribution to BP, it contributes little, if at all, to the hypertension-induced or the greater stress-induced pressor responses in Schlager mice. The effects of pentolinium suggest that the SNS is mainly responsible for hypertension in BPH/2J mice. However, the RAS inhibits sympathetic vasomotor tone during inactivity and prolongs sympathetic activation during periods of adverse stress, indicating an important sympatho-modulatory role.

Is kidney ischemia the central mechanism in parallel activation of the renin and sympathetic system?

In chronic kidney disease simultaneous activation of the renin-angiotensin and sympathetic systems occurs. Kidney ischemia seems to play a key role in the pathogenesis. This review firstly summarizes experimental and clinical evidence in chronic kidney disease supporting this idea and addresses the possibility that this mechanism is also relevant in some other disease conditions.

Baroreceptor reflex is suppressed in rats that develop hyperalgesia behavior after nerve injury

The baroreceptor reflex buffers autonomic changes by decreasing sympathetic activity and increasing vagal activity in response to blood pressure elevations, and by the reverse actions when the blood pressure falls. Because of the many bidirectional interactions of pain and autonomic function, we investigated the effect of painful nerve injury by spinal nerve ligation (SNL) on heart rate (HR), blood pressure (BP) and their regulation by the baroreceptor reflex. Rats receiving SNL were separated into either a hyperalgesic group that developed sustained lifting, shaking and grooming of the foot after plantar punctate nociceptive stimulation by pin touch or a group of animals that failed to show this hyperalgesic behavior after SNL. SNL produced no effect on resting BP recorded telemetrically in unrestrained rats compared to control rats receiving either skin incision or sham SNL. However, two tests of baroreceptor gain showed depression only in animals that developed sustained hyperalgesia after SNL. The animals that failed to develop hyperalgesia after SNL were found to have elevations in HR both before and for the first 4 days after SNL, and HR variability analysis gave indications of decreased vagal control of resting HR and elevated sympatho-vagal balance at these same time intervals. In human patients, other research has shown that blunted baroreceptor reflex sensitivity predicts poor outcome during conditions such as hypertension, congestive heart failure, myocardial infarction, and stroke. If baroreceptor reflex suppression is also found in human subjects during chronic neuropathic pain, this may adversely affect survival.

Whole body norepinephrine kinetics in ANG II-salt hypertension in the rat

The purpose of this study was to investigate total body norepinephrine (NE) kinetics as an index of global sympathetic nervous system (SNS) outflow in a rat model of chronic ANG II-salt hypertension. Male Sprague-Dawley rats fed a 0.4% (normal salt, NS) or 2% (HS) NaCl diet were instrumented with arterial and venous catheters. After 5 days of recovery and a 3-day control period, ANG II (150 ng.kg(-1).min(-1)) was given subcutaneously by minipump for 14 days. Plasma NE levels and total body NE spillover and clearance were determined on control day 3 and ANG II infusion days 7 and 14 using radioisotope dilution principles. To perform this analysis, 3H-NE and NE were measured in arterial plasma after a 90-min infusion of tracer amounts of 3H-NE. Mean arterial pressure (MAP) was similar during the control period in NS and HS rats; however, MAP increased to a higher level in HS rats. During the control period, plasma NE tended to be lower in rats on HS, whereas NE clearance tended to be higher in HS rats. As a result NE spillover was similar in NS and HS rats during the control period. In NS rats, plasma NE, NE spillover, and NE clearance were unchanged by ANG II. In contrast, in rats on the HS diet, plasma NE and NE spillover increased during ANG II infusion, whereas NE clearance was unchanged. In conclusion, a HS diet alone or ANG II infusion in animals fed NS do not affect global sympathetic outflow. However, the additional hypertensive response to ANG II in animals fed HS is accompanied by SNS activation.

Angiotensin II-based hypertension and the sympathetic nervous system: the role of dose and increased dietary salt in rabbits

There is accumulating evidence that angiotensin II may exert its hypertensive effect through increasing sympathetic drive. However, this action may be dependent on the dose of angiotensin II as well as salt intake. We determined the effect of different doses of angiotensin II and different levels of salt intake on neurogenic pressor activity. We also examined the effect of renal denervation. New Zealand White rabbits were instrumented to continuously measure arterial pressure. The depressor response to the ganglionic blocker pentolinium tartrate (5 mg kg(-1)) was used to assess pressor sympathetic drive on days 0, 7 and 21 of a 20 or 50 ng kg(-1) min(-1) continuous i.v. angiotensin II infusion. A 50 ng kg(-1) min(-1) infusion caused an immediate increase in pressure (23 +/- 5 mmHg), whereas a 20 ng kg(-1) min(-1) infusion caused a slow increase in pressure, peaking by day 12 (17 +/- 4 mmHg). The ganglionic blockade profiles indicated sympathoinhibition in the 50 ng kg(-1) min(-1) group by day 7 and sympathoinhibition in the 20 ng kg(-1) min(-1) group at day 21, corresponding to the development of hypertension. Animals receiving increased dietary salt (0.9% NaCl in drinking water), however, showed a similar slow increase in pressure with 20 ng kg(-1) min(-1) angiotensin II (16 +/- 5 mmHg) but no sympathoinhibition at day 21. Bilateral renal denervation delayed the onset but not the extent of hypertension in this group. We conclude that different doses of angiotensin II produce distinct profiles of hypertension and associated changes in pressor sympathetic drive and that increased dietary salt intake disrupts the normal sympathoinhibitory response to angiotensin II-based hypertension.

Nitric oxide in hypertension

Hypertension is a major risk factor for cardiovascular disease, and reduction of elevated blood pressure significantly reduces the risk of cardiovascular events. Endothelial dysfunction, which is characterized by impairment of nitric oxide (NO) bioavailability, is an important risk factor for both hypertension and cardiovascular disease and may represent a major link between the conditions. Evidence suggests that NO plays a major role in regulating blood pressure and that impaired NO bioactivity is an important component of hypertension. Mice with disruption of the gene for endothelial NO synthase have elevated blood pressure levels compared with control animals, suggesting a genetic component to the link between impaired NO bioactivity and hypertension. Clinical studies have shown that patients with hypertension have a blunted arterial vasodilatory response to infusion of endothelium-dependent vasodilators and that inhibition of NO raises blood pressure. Impaired NO bioactivity is also implicated in arterial stiffness, a major mechanism of systolic hypertension. Clarification of the mechanisms of impaired NO bioactivity in hypertension could have important implications for the treatment of hypertension.

Chronic low-dose angiotensin II infusion increases venomotor tone by neurogenic mechanisms

The purpose of this study was to identify changes in venomotor tone in the chronic low-dose angiotensin II (Ang II) model of hypertension and to establish the contribution of sympathetic nerve activation to these venomotor tone changes. Male Sprague-Dawley rats were acclimatized to a 0.4% or 2.0% NaCl diet for 7 days and then catheterized to allow chronic and repeated measures of arterial pressure, central venous pressure, and mean circulatory filling pressure (MCFP), an index of venous smooth muscle tone, in conscious undisturbed rats. After 4 days of recovery and a 3-day control period, an Ang II or physiological saline-filled osmotic minipump was implanted subcutaneously to deliver Ang II (150 ng/kg per minute) or vehicle control for 14 days. MCFP was measured in duplicate before and after acute ganglionic blockade with hexamethonium (30 mg/kg i.v.) on control day 2 and Ang II infusion on days 1, 3, 7, and 14. Blood volume was also measured on these days and was unchanged for the duration of the study in all of the groups. Arterial pressure was increased for the duration of Ang II infusion in rats on both 0.4% and 2% NaCl diets, but the increase was significantly greater in the 2% NaCl group and completely abolished by hexamethonium. MCFP was significantly increased for the entire Ang II infusion period only in rats fed 2% NaCl, and this increase was completely abolished by hexamethonium. We conclude that the combination of chronic low-dose Ang II infusion and high dietary salt intake engages the sympathetic nervous system to increase venomotor tone.

Angiotensin-sympathetic system interactions in cardiovascular and metabolic disease

BACKGROUND

Blood pressure, as well as blood volume homeostasis, depends to a large extent on humoral influences stemming from the renin-angiotensin axis and the sympathetic nervous system. Evidence has been provided that a large part of this homeostatic modulation is effected by the complex interactions between the two systems.

OBJECTIVES

The present review will focus on three major issues. First it will examine the physiological and pathophysiological relevance of angiotensin-sympathetic crosstalk discussing possible sites, mechanisms and effects of the interaction. It will then address the clinical relevance of these inter-relationships by reviewing data collected in cardiovascular and non-cardiovascular diseases. Finally, the influences of angiotensin II on adrenergic function will be examined as possible targets of cardiovascular drug treatment.

CONCLUSIONS

By interrupting the influences of angiotensin II on sympathetic function, therapeutic interventions aimed at blocking the renin-angiotensin system exert favourable effects on the haemodynamic, metabolic and renal profile. This has important implications for the treatment of hypertension, congestive heart failure, renal insufficiency and metabolic syndrome.