Effect of renal denervation on release and content of renin in anesthetized dogs

The intrarenal blood pressure modulation system is differentially altered after renal denervation guided by different intensities of blood pressure responses

The aim of this study was to investigate alterations in the intrarenal blood pressure (BP) regulation system after renal denervation (RDN) guided by renal nerve stimulation (RNS). Twenty-one dogs were randomized to receive RDN at strong (SRA group, n = 7) or weak (WRA group, n = 7) BP-elevation response sites identified by RNS or underwent RNS only (RNS-control, RSC, n = 7). After 4 weeks of follow-up, renal sympathetic components, the main components of renin-angiotensin system (RAS) and the major transporters involved in sodium and water reabsorption were assessed by immunohistochemical analysis. Compared with RSC treatment, RDN therapy significantly reduced renal norepinephrine and tyrosine hydroxylase levels, decreased the renin content and inhibited the onsite generation of angiotensinogen. Moreover, the expression of exciting axis components, including angiotensin-converting enzyme (ACE), angiotensin II and angiotensin II type-1 receptor, was downregulated, while protective axis components for the cardiovascular system, including ACE2 and Mas receptors, were upregulated in both WRA and SRA groups. Moreover, RDN reduced the abundance of aquaporin-1 and aquaporin-2 in kidneys. Although RDN had a minimal effect on overall NKCC2 expression, its activation (p-NKCC2) and directional enrichment in the apical membrane (mNKCC2) were dramatically blunted. All these changes were more obvious in the SRA group than WRA group. Selective RDN guided by RNS effectively reduced systemic BP by affecting the renal neurohormone system, as well as the sodium and water transporter system, and these effects at sites with a strong BP response were more superior.

Potential Neuromodulation of the Cardio-Renal Syndrome

The cardio-renal syndrome (CRS) type 2 is defined as a progressive loss of renal function following a primary insult to the myocardium that may be either acute or chronic but is accompanied by a decline in myocardial pump performance. The treatment of patients with CRS is difficult, and the disease often progresses to end-stage renal disease that is refractory to conventional therapy. While a good deal of information is known concerning renal injury in the CRS, less is understood about how reflex control of renal sympathetic nerve activity affects this syndrome. In this review, we provide insight into the role of the renal nerves, both from the afferent or sensory side and from the efferent side, in mediating renal dysfunction in CRS. We discuss how interventions such as renal denervation and abrogation of systemic reflexes may be used to alleviate renal dysfunction in the setting of chronic heart failure. We specifically focus on a novel cardiac sensory reflex that is sensitized in heart failure and activates the sympathetic nervous system, especially outflow to the kidney. This so-called Cardiac Sympathetic Afferent Reflex (CSAR) can be ablated using the potent neurotoxin resinferitoxin due to the high expression of Transient Receptor Potential Vanilloid 1 (TRPV1) receptors. Following ablation of the CSAR, several markers of renal dysfunction are reversed in the post-myocardial infarction heart failure state. This review puts forth the novel idea of neuromodulation at the cardiac level in the treatment of CRS Type 2.

Adrenergic control of renin during dietary sodium deprivation in conscious dogs

These experiments evaluated the contribution of alpha- and beta-adrenergic stimulation to plasma renin activity (PRA) during early and long-term dietary sodium restriction, compared with normal sodium intake. Uninephrectomized conscious dogs with catheters in the aorta, vena cava, and remaining renal artery were studied during normal sodium diet (approximately 70 meq/day), after 2-3 days of low-sodium diet (5-7 meq/day), and after greater than or equal to 2 wk of low-sodium diet. Direct renal arterial (ira) infusion of phenoxybenzamine plus propranolol decreased PRA by similar proportions (39-48%) during all three states of dietary sodium intake. The PRA achieved after adrenergic blockade remained higher (P less than 0.05) during early and long-term sodium restriction than during normal sodium intake. The effect on PRA of ira infusion of propranolol alone was not different from that of phenoxybenzamine plus propranolol during normal or low-sodium diet, and the magnitude of decrease in PRA during low-sodium diet was the same whether propranolol (1 microgram.kg-1.min-1) was infused ira or intravenously. In summary, beta-adrenergic stimulation accounts for similar proportions of PRA during early and long-term dietary sodium restriction and during normal sodium intake. Renal alpha-adrenoceptors appear to play little or no role in control of PRA under these conditions.

Afferent renal nerve-dependent hypertension following acute renal artery stenosis in the conscious rat

Anatomical and electrophysiological evidence indicates that the kidneys contain both mechano- and chemoreceptor nerve endings. We conducted the present study to determine whether conditions of reduced renal blood flow elicit cardiovascular alterations that are dependent on afferent renal nerves. Removal of the renin-angiotensin system with the angiotensin I-converting enzyme inhibitor, captopril, and/or reduction in baroreflex gain by sinoaortic denervation, were combined in conscious rats with acute renal artery stenosis to prevent these systems from potentially obscuring any afferent renal nerve-dependent effects. One week after sinoaortic denervation or sham sinoaortic denervation, each rat was chronically instrumented with Doppler flow probes on the lower abdominal aorta and superior mesenteric and right renal arteries, as well as with intravascular catheters, and a perivascular balloon occluder on the right renal artery. After surgical recovery, sham sinoaortic-denervated animals responded to a 60-minute period of stenosis (50% reduction in renal blood flow) with increases in arterial pressure, regional resistance, and plasma renin activity. Captopril abolished the increases in arterial pressure, hindquarters, and left renal resistance, but both bradycardia and increased mesenteric resistance persisted, indicating that baroreflex activation might be buffering a non-renin-angiotensin system pro-hypertensive mechanism. In support of this, sinoaortic-denervated animals during captopril administration responded to stenosis with substantial increases in arterial pressure (25-30 mm Hg) and regional resistance (30-50%) that were unrelated to the renin-angiotensin system, but which were abolished after denervation of the stenotic kidney. The data suggest that acute reductions in renal blood flow activate an afferent renal nerve-dependent cardiovascular response that is strongly expressed under conditions of reduced gain of the renin-angiotensin and baroreflex systems. We speculate that this reflex may assume particular importance in chronic renal hypertension when baroreflexes become impaired and activation of the renin-angiotensin system is reduced.

Control of renin release: a review of experimental evidence and clinical implications

Present knowledge of the mechanisms regulating release of renin is reviewed with particular emphasis on neural factors. Evidence is given for a direct effect of renal innervation on beta adrenergic receptors in juxtaglomerular cells, and for the involvement of reflex release of renin in conditions such as tilting and acute salt depletion. Participation of neural and nonneural mechanisms of control is also shown to occur in other conditions, such as aortic constriction and hemorrhage. The view is held that neural sympathetic factors might explain some of the renin disturbances found in essential hypertension. First, in patients with high renin hypertension part of the hypertension is renin-dependent, and these pressor levels of renin seem to be neurally induced since they can commonly be suppressed by beta adrenoreceptor blocking agents. Second, the hypothesis is presented that patients with low renin hypertension, at least those who have no volume disturbance, have a blunted sympathetic control of renin release. Therefore a sufficiently precise test of sympathetic activity, and possibly of body fluid volumes, should be associated with renin profiles for a better understanding of the pathophysiology of arterial hypertension and as a better guide to therapeutic management. Indeed, most of the available antihypertensive drugs act on sympathetic activity, body fluid volume or renin, and this multifaceted profile would provide more rational guidelines for treatment.

Selective stimulation of renal nerves in the anesthetized dog. Effect on renin release during controlled changes in renal hemodynamics

The combined effects of low-frequency stimulation of the renal sympathetic nerves and reductions in renal arterial blood pressure on various hemodynamic parameters of the in situ kidney and on renal venous renin levels were determined in 17 dogs. Autoregulation of flow (measured using noncannulating electromagnetic probes) and glomerular filtration (measured using continuous extraction of 131 I-iothalamate) was followed before, during, and after nerve stimulation. Stimulation of isolated renal nerves (0.5 msec, 10-15 v, 0.1-3.0/sec) produced distinct changes in the release of renin with only minimal changes in the autoregulatory curves for glomerular filtration rate and renal plasma flow. Reducing arterial blood pressure from 150 to 50 mm Hg caused an increase in the release of renin from 4 to 35 ng/ml hour -1 . Stimulation of the renal nerves increased renin production at both pressures (from 4 to 10 ng/ml hour -1 at 150 mm Hg and from 35 to 50 ng/ml hour -1 at 50 mm Hg). In fact, renal nerve stimulation elicited increases in renin release at all pressures examined. In contrast, sodium excretion following nerve stimulation was depressed at comparable sodium loads in spite of minimal changes in renal hemodynamics. It appears that the effect of the renal sympathetic nerves on renin release is evident at all pressures; however, the sympathetic nerves are proportionately more important at pressures above 100 mm Hg and the renal artery pressure plays a greater role at pressures below 115-100 mm Hg.

Renin and acute circulatory renal failure in the rabbit

Plasma renin concentration (PRC) was measured in 25 rabbits before and 6, 24, or 72 hours after subcutaneous injection of glycerol. Renal failure and tubular necrosis developed in most animals and PRC rose sixfold to a maximum at 24 hours. Small insignificant changes of PRC were present at 6 and 72 hours. None of these changes was observed in a control group of nine animals killed 24 hours after an injection of saline. The amount of renin extractable from single superficial glomeruli and from renal cortical tissue was reduced after injection of glycerol. In a second study of 11 anesthetized rabbits, renal venous PRC increased on average from 151 to 1810 units/liter following a 4-hour period of renal artery occlusion. Arterial PRC did not change significantly during this time, but the kidneys showed histological changes of acute tubular necrosis. These experiments are compatible with the suggestion that renin is involved in the pathogenesis of acute circulatory renal failure.

Effects of catecholamines and renal nerve stimulation on renin release in the nonfiltering kidney

The mechanisms whereby catecholamines and renal nerve stimulation increase renin secretion were studied in dogs with nonfiltering kidneys. In six dogs, epinephrine was infused into the renal artery at a rate that decreased renal blood flow to half of the control value. Papaverine was then infused into the renal artery to block the decrease in renal blood flow produced by the catecholamine, and the epinephrine infusion was resumed while the papaverine infusion was continued. In this experiment, renin release increased during infusion of epinephrine alone, but no change occurred with epinephrine during papaverine infusion. The protocol for the experiment on six other dogs was similar except that the infused catecholamine was norepinephrine. In this experiment, norepinephrine increased renin release both prior to and during papaverine infusion. In seven dogs, the effect of electrical stimulation of the renal nerves on renin secretion was studied both before and during the infusion of papaverine into the renal artery; renin release increased strikingly both before and during papaverine infusion. It is suggested that epinephrine increased renin secretion in the nonfiltering kidney by an action on the renal arterioles. In contrast, norepinephrine and renal nerve stimulation apparently increased renin secretion in the nonfiltering kidney by a direct effect on the juxtaglomerular cells. These data provide evidence for specific mechanisms of action of epinephrine, norepinephrine, and the renal nerves in renin release.

Intrarenal distribution of blood flow and cortico-medullary sodium gradient after unilateral splanchnicotomy in the dog

1. Intrarenal distribution of blood flow and cortico-medullary sodium gradients have been studied in anaesthetized dogs, previously subjected to unilateral splanchnicotomy. Experiments were carried out on hydropoenic, normal, isotonic and hypotonic volume expanded animals.2. Polyuria and natriuresis were present under all experimental conditions after denervation.3. Denervation resulted in an increase in medullary blood flow, particularly in hydropoenic and normal dogs. In isotonic and hypotonic volume expansion, the increases in the medullary circulation were smaller despite a greater increase in urine flow and sodium excretion of the denervated side.4. Under no experimental condition did the cortico-medullary sodium gradient of denervated kidneys differ significantly from that of the intact kidneys.5. It is concluded that denervation polyuria might be attributed in part to an increase in medullary blood flow; the natriuresis, however, could not be accounted for by these circulatory changes. Other possibilities are discussed.

Renin and acute renal failure: studies in man

Plasma renin concentration was increased, usually appreciably, in 22 out of 25 patients with acute renal failure, the average value being 226 units/litre (mean for normal subjects 8.2 units/1.). The highest renin values were found in the first 10 days of the disease; lower and sometimes normal values were found subsequently. Unequivocal acute tubular necrosis was present in only two of the eight cases examined post mortem.These findings are compatible with Goormaghtigh's proposal that an excess of renin and angiotensin may act within the kidney to produce acute renal failure.

Renin release in patients with benign essential hypertension

In 16 of 28 patients with benign essential hypertension, reduction of mean arterial pressure to 75 to 120 mm Hg by infusion of sodium nitroprusside caused significant increase in renin release estimated by the renal-systemic difference of renin activity and renal plasma flow (RPF). The threshold at which renin release increased was shifted to a range that was intermediate between that previously reported for normotensive subjects and that for patients with renovascular hypertension, but the average magnitude of renin release was almost comparable to that in normotensive subjects. In the other 12 patients, no significant renin release occurred during reduction in pressure of comparable degree.

The difference in renin release was difficult to explain by the difference in severity of hypertensive disease or in sodium excretion, but increased renin release was usually associated with a decrease in RBF and a rise in renal vascular resistance, while insignificant release was usually associated with an insignificant change in RBF and a decrease in resistance. The average known duration of hypertension was significantly greater in the unresponsive patients. It is suggested that renal sympathetic nerve activity may be a factor in the variations in renin release in patients with essential hypertension.