The sympathetic nervous system in hypertension due to unilateral renal artery stenosis in man

Effects of chronic sympatho-inhibition on reflex control of renal blood flow and plasma renin activity in renovascular hypertension

BACKGROUND AND PURPOSE

We determined if chronic sympatho-inhibition with rilmenidine has functional significance for the kidney by altering responses of renal blood flow (RBF) and plasma renin activity (PRA) to stress and acute hypotension in rabbits with renovascular hypertension.

EXPERIMENTAL APPROACH

RBF to each kidney and renal sympathetic nerve activity (RSNA) to the left kidney were measured in rabbits in which a renal artery clip induced hypertension (2K1C) and in sham-operated rabbits. After 2 weeks, a subcutaneous minipump was implanted to deliver rilmenidine (2.5 mg.kg(-1).day(-1)) to 2K1C rabbits for 3 weeks.

KEY RESULTS

After 5 weeks of renal artery stenosis, mean arterial pressure (MAP) was 23% higher and PRA 3-fold greater than in sham-operated rabbits. Blood flow and renal vascular conductance in the stenosed kidney were lower (-75% and -80%) compared with sham, and higher in the non-clipped kidney (68% and 39%). Responses of RBF and PRA to hypotension were similar in 2K1C and sham rabbits. Airjet stress evoked a greater increase in MAP in 2K1C rabbits than sham controls. Chronic rilmenidine normalized MAP, reduced RSNA and PRA, and did not reduce RBF in the stenosed kidney. Responses of RBF (clipped and non-clipped kidney), RSNA and PRA to hypotension and airjet were little affected by rilmenidine.

CONCLUSIONS AND IMPLICATIONS

Our observations suggest that chronic sympatho-inhibition is an effective antihypertensive therapy in renovascular hypertension. It normalizes MAP and reduces basal PRA without compromising blood flow in the stenosed kidney or altering responses of MAP, haemodynamics and PRA to acute hypotension and stress.

Renovascular Hypertension: Current Concepts

Hypertension produced by renal artery occlusive disease is an important secondary form of hypertension. Clinicians commonly encounter forms of renal arterial disease of varying severity, many of which are of little hemodynamic significance when first detected. Experimental studies emphasize that transient activation of the renin-angiotensin-aldosterone system is necessary for initiation of renovascular hypertension. At some point, angiotensin II activates additional mechanisms responsible for sustained increased blood pressure including sodium retention, endothelial dysfunction, and vasoconstriction related to production of reactive oxygen species. Widespread application of agents that block the renin-angiotensin system, including angiotensin-converting enzyme inhibitors and angiotensin-receptor blockers, render many patients with unilateral renal arterial disease manageable primarily by medical means for many years. In the setting of high a priori likelihood of renovascular disease, recognizing the potential for disease progression during medical therapy and individually evaluating the risks and benefits of renal revascularization are important tasks. Recent prospective studies show limited, but real, benefit regarding blood pressure control for patients with atherosclerotic disease. Whether earlier renal revascularization offers benefits regarding improved morbidity and mortality from cardiovascular end point reduction is an important question to be addressed in multicenter, prospective, randomized trials. Our paradigm stresses the fact that patients with renovascular hypertension require intensive blood pressure control and cardiovascular risk factor intervention, both before and after revascularization. Hence, management of such patients requires close attention and periodic review regarding restenosis and progression of vascular disease.

Percutaneous intervention for renovascular disease: rationale and patient selection

PURPOSE OF REVIEW

The anatomic diagnosis of renovascular disease is increasing in frequency due to the advent of sophisticated non-invasive imaging modalities such as MRA (Magnetic Resonance Angiography) and renal angiography at the time of cardiac catheterization. Despite this fact, the investigation and appropriate management of renovascular disease has remained a controversial topic. This review addresses the clinical syndromes associated with renal artery stenosis (RAS) and the published data guiding appropriate patient selection for revascularization.

RECENT FINDINGS

There is a growing literature in support of renal revascularization as an aid in improving anti-hypertensive control, preserving renal function, and easing the management of congestive heart failure. Meanwhile, technological advances have allowed intervention in an expanding pool of eligible patients. One such technology is the atheroembolic protection device, which may soon allow renal salvage in patients with significant baseline renal impairment that were previously denied intervention for fear of worsening renal function.

SUMMARY

The data reviewed herein helps to identify patients that will benefit from renal revascularization. Large-scale, randomized data further defining the role of renal revascularization and optimal patient selection is still needed.

Current issues in the diagnosis and management of patients with renal artery stenosis: a cardiologic perspective

Renal artery stenosis most often is caused by atherosclerosis. Although patients with renal artery stenosis can be managed conservatively, renal revascularization may be indicated, particularly in patients with refractory hypertension on a multidrug regimen and patients with declining renal function. Duplex ultrasonography of the renal arteries and magnetic resonance angiography are currently the most efficient noninvasive methods for the evaluation of renal artery stenosis. Selective digital subtraction renal arteriography remains the gold standard for the definitive diagnosis. In selected patients undergoing coronary studies and angiography immediately after the coronary procedure can be efficient. Atherosclerotic renal artery lesions, which commonly affect the renal artery ostium, can be treated safely and effectively with balloon-expandable stents. Successful angioplasty commonly results in improved control of hypertension, but an overall benefit on renal function and/or patient survival has not been shown. Generally the risk/benefit ratio of renal artery stenting seems favorable, but further randomized studies are needed for evidence-based decision making. All patients with atherosclerotic renal artery stenosis should receive rigorous secondary prevention measures including platelet inhibitors, statins, and beta-blockers.

Role of the sympathetic nervous system in human renovascular hypertension

The findings in humans as to whether elevated sympathetic nerve activity contributes to renovascular hypertension have been less consistent compared with the results obtained in experimental models of renovascular hypertension. Collectively, there are several lines of evidence to support the view that sympathetic nerve activity is elevated in patients with renovascular hypertension. It is uncertain, however, whether this adrenergic overactivity is specific for renovascular hypertension per se, or the cause of severe hypertension with target organ damage. Central or peripheral stimulation of sympathetic nerve activity by angiotensin II, or stimulation of central sympathetic outflow via afferent renal nerves of ischemic kidneys, are possible mechanisms to explain the elevated sympathetic nerve activity in renovascular hypertension. Therapy that diminishes the activity of the sympathetic nervous system and the renin-angiotensin system seems rational and could perhaps also improve the poor prognosis for these patients.

Increased sympathetic nerve activity in renovascular hypertension

BACKGROUND

Increased sympathetic nerve activity may contribute to the progression of renovascular hypertension. Because previous results have been inconclusive, we investigated whether renovascular hypertensives show increased total and regional sympathetic nerve activity.

METHODS AND RESULTS

Sixty-five patients underwent renal angiography and measurements of plasma renin activity and angiotensin II in conjunction with estimation of sympathetic nerve activity by means of radiotracer dilution and intraneural recordings of muscle sympathetic nerve activity (MSNA). Age-matched healthy subjects (n=15) were examined for comparison. Total body norepinephrine (NE) spillover, an index of overall sympathetic nerve activity, was increased by 100% and MSNA by 60% in the hypertensive patients compared with healthy subjects (P<0.01 for both). A subgroup of 24 patients with well-defined renovascular hypertension (cured or improved hypertension after renal angioplasty) showed similar increases in total body NE spillover compared with the group at large. Patients with arterial plasma renin activity and angiotensin II levels above median had higher values for total body NE spillover than patients below median (P<0.01).

CONCLUSIONS

This study unequivocally demonstrates elevated sympathetic nerve activity in patients with renovascular hypertension. The adrenergic overactivity may contribute to the blood pressure elevation and perhaps also to the high cardiovascular mortality in renovascular hypertension.