Gerald F. DiBona

Neural Control of Renal Function

The renal nerves are the communication link between the central nervous system and the kidney. In response to multiple peripheral and central inputs, efferent renal sympathetic nerve activity is altered so as to convey information to the major structural and functional components of the kidney, the vessels, glomeruli, and tubules, each of which is innervated. At the level of each of these individual components, information transfer occurs via interaction of the neurotransmitter released at the sympathetic nerve terminal-neuroeffector junction with specific postjunctional receptors coupled to defined intracellular signaling and effector systems. In response to normal physiological stimuli, changes in efferent renal sympathetic nerve activity contribute importantly to homeostatic regulation of renal blood flow, glomerular filtration rate, renal tubular epithelial cell solute and water transport, and hormonal release. Afferent input from sensory receptors located in the kidney participates in this reflex control system via renorenal reflexes that enable total renal function to be self-regulated and balanced between the two kidneys. In pathophysiological conditions, abnormal regulation of efferent renal sympathetic nerve activity contributes significantly to the associated abnormalities of renal function which, in turn, are of importance in the pathogenesis of the disease.

Neural control of renal function.

The kidney is innervated with efferent sympathetic nerve fibers that directly contact the vasculature, the renal tubules, and the juxtaglomerular granular cells. Via specific adrenoceptors, increased efferent renal sympathetic nerve activity decreases renal blood flow and glomerular filtration rate, increases renal tubular sodium and water reabsorption, and increases renin release. Decreased efferent renal sympathetic nerve activity produces opposite functional responses. This integrated system contributes importantly to homeostatic regulation of sodium and water balance under physiological conditions and to pathological alterations in sodium and water balance in disease. The kidney contains afferent sensory nerve fibers that are located primarily in the renal pelvic wall where they sense stretch. Stretch activation of these afferent sensory nerve fibers elicits an inhibitory renorenal reflex response wherein the contralateral kidney exhibits a compensatory natriuresis and diuresis due to diminished efferent renal sympathetic nerve activity. The renorenal reflex coordinates the excretory function of the two kidneys so as to facilitate homeostatic regulation of sodium and water balance. There is a negative feedback loop in which efferent renal sympathetic nerve activity facilitates increases in afferent renal nerve activity that in turn inhibit efferent renal sympathetic nerve activity so as to avoid excess renal sodium retention. In states of renal disease or injury, there is activation of afferent sensory nerve fibers that are excitatory, leading to increased peripheral sympathetic nerve activity, vasoconstriction, and increased arterial pressure. Proof of principle studies in essential hypertensive patients demonstrate that renal denervation produces sustained decreases in arterial pressure.

Translational medicine: the antihypertensive effect of renal denervation

Translational medicine is concerned with the translation of research discoveries into clinical applications for the prevention, diagnosis, and treatment of human diseases. Here we briefly review the research concerning the role of the renal sympathetic nerves (efferent and afferent) in the control of renal function, with particular reference to hypertension. The accumulated evidence is compelling for a primary role of the renal innervation in the pathogenesis of hypertension. These research discoveries led to the development of a catheter-based procedure for renal denervation in human subjects. A proof-of-principle study in patients with hypertension resistant to conventional therapy has demonstrated that the procedure is safe and produces renal denervation with sustained lowering of arterial pressure.

Neural Control of the Kidney: Past, Present, and Future

This article provides a chronological perspective on the development of knowledge concerning the neural control of renal function and is divided into three parts: the past, the present, and the future.

Nervous Kidney: Interaction Between Renal Sympathetic Nerves and the Renin-Angiotensin System in the Control of Renal Function

Increases in renal sympathetic nerve activity regulate the functions of the nephron, the vasculature, and the renin-containing juxtaglomerular granular cells. Because increased activity of the renin-angiotensin system can also influence nephron and vascular function, it is important to understand the interactions between the renal sympathetic nerves and the renin-angiotensin system in the control of renal function. These interactions can be intrarenal, for example, the direct (by specific innervation) and indirect (by angiotensin II) contributions of increased renal sympathetic nerve activity to the regulation of renal function. The effects of increased renal sympathetic nerve activity on renal function are attenuated when the activity of the renin-angiotensin system is suppressed or antagonized with ACE inhibitors or angiotensin II-type AT(1)-receptor antagonists. The effects of intrarenal administration of angiotensin II are attenuated after renal denervation. These interactions can also be extrarenal, for example, in the central nervous system, wherein renal sympathetic nerve activity and its arterial baroreflex control are modulated by changes in activity of the renin-angiotensin system. In addition to the circumventricular organs, whose permeable blood-brain barrier permits interactions with circulating angiotensin II, there are interactions at sites behind the blood-brain barrier that depend on the influence of local angiotensin II. The responses to central administration of angiotensin II-type AT(1)-receptor antagonists into the ventricular system or microinjected into the rostral ventrolateral medulla are modulated by changes in activity of the renin-angiotensin system produced by physiological changes in dietary sodium intake. Similar modulation is observed in pathophysiological models wherein activity of both the renin-angiotensin and sympathetic nervous systems is increased (eg, congestive heart failure). Thus, both renal and extrarenal sites of interaction between the renin-angiotensin system and renal sympathetic nerve activity are involved in influencing the neural control of renal function.

Sympathetic Nervous System and Hypertension

With the development and implementation of device-based therapeutic interventions to decrease renal and systemic nerve activity in patients with resistant hypertension, there has been an increase in research dealing with the role of the sympathetic nervous system in hypertension. These interventions have produced substantial decreases in blood pressure in patients wherein pharmacological treatments, including agents which inhibit the effects of the renin–angiotensin–aldosterone system, have failed serves to confirm and reassert the essential role of the sympathetic nervous system in hypertension. This review will encompass recent publications dealing with the sympathetic nervous system and hypertension, focusing on those recently published in Hypertension . ### Obesity-Related Hypertension Although there has been a debate as to whether sympathetic activation is a cause or consequence of obesity, the studies noted below support the view that it is the obesity that leads to sympathetic activation. The importance of this sympathetic activation for the development of the hypertension is supported by the finding that renal denervation prevents the development of obesity hypertension in the dog. Studies have now focused on the developmental phase of obesity hypertension regarding the renal sympathoexcitation. In rabbits fed high-fat diets, body weight, plasma insulin and leptin concentrations, mean arterial pressure, heart rate, and renal sympathetic nerve activity were all increased after 1 week.1 Mean arterial pressure and body weight continued to increase over 3 weeks of high-fat diet, whereas heart rate and renal sympathetic nerve activity did not change further. Arterial baroreflex control of renal sympathetic nerve activity was attenuated from the first week of the high-fat diet. Excitatory responses to air jet stress diminished over 3 weeks of high-fat diet. Resumption of normal diet normalized glucose, insulin, leptin, and heart rate, but body weight, visceral fat content, mean arterial pressure, and renal sympathetic nerve activity remained elevated. Increased renal sympathetic nerve activity …

Gene Transfer of Extracellular Superoxide Dismutase Reduces Arterial Pressure in Spontaneously Hypertensive Rats: Role of Heparin-Binding Domain

Abstract— Oxidative stress may contribute to hypertension. The goals of this study were to determine whether extracellular superoxide dismutase (ECSOD) reduces arterial pressure in spontaneously hypertensive rats (SHR) and whether its heparin-binding domain (HBD), which is responsible for cellular binding, is necessary for the function of ECSOD. Three days after intravenous injection of an adenoviral vector expressing human ECSOD (AdECSOD), mean arterial pressure (MAP) decreased from 165±4 mm Hg (mean±SE, n=7) to 124±3 mm Hg (n=7) in adult anesthetized SHR (P <0.01) but was not altered in normotensive Wistar-Kyoto rats. Cardiac output was not changed in SHR 3 days after AdECSOD. Gene transfer of ECSOD with deletion of the HBD (AdECSOD&Dgr;HBD) had no effect on SHR MAP, even though plasma SOD activity was greater after AdECSOD&Dgr;HBD than after AdECSOD. Immunohistochemistry revealed intense staining for ECSOD in blood vessels and kidneys after AdECSOD but not after AdECSOD&Dgr;HBD. Impaired relaxation of the carotid artery to acetylcholine in SHR was significantly improved after AdECSOD. Cumulative sodium balance in SHR was reduced by AdECSOD compared with AdECSOD&Dgr;HBD. Gene transfer of ECSOD also reduced MAP in conscious SHR, although the effect was not as profound as in anesthetized SHR. In summary, gene transfer of ECSOD, with a strict requirement for its HBD, reduces systemic vascular resistance and arterial pressure in a genetic model of hypertension. This reduction in arterial pressure may be mediated by vasomotor and/or renal mechanisms.

Renal adrenoceptor mediation of antinatriuretic and renin secretion responses to low frequency renal nerve stimulation in the dog.

We evaluated renal adrenoceptor mediation of the renin secretion and antinatriuretic responses to low frequency (1.0 Hz) electrical stimulation of the renal nerves in the dog using renal a-adrenoceptor blockade with phentolamine {α-i/α-i), prazosin (α,), yohimbine (α2), and rauwolscine (α2), and β-adrenoceptor blockade with d,β-propranolol Oβ1//β2) and atenolol (β,). In all animals studied, renal blood flow and glomerular filtration rate remained constant throughout the experiment. In 11 dogs, low frequency renal nerve stimulation decreased urinary sodium excretion (119 ± 13 to 86 ± 18 μEq/min) and increased renin secretion (79 ± 22 to 348 ± 73 μg/ min). Renal arterial infusion of phentolamine (2–10 μg/kg per min) prevented the antinatriuresis but did not change the response of renin secretion (96 ± 46 to 412 ± 93 μg/min). In six dogs, renal arterial infusion of prazosin (0.7 μg/kg per min) similarly blocked the antinatriuretic but not the renin secretion responses to low frequency renal nerve stimulation. Renal arterial infusion of either yohimbine or rauwolscine did not affect the antinatriuretic or renin secretion responses to low frequency renal nerve stimulation. Intrarenal /S]-adrenoceptor blockade with low dose atenolol (0.5 μg/kg per min, n = 9) had no effect on the antinatriuretic responses to low frequency renal nerve stimulation (—47 ± 12 vs. —37 ± 8 μEq/min) but significantly decreased the increment in renin secretion during low frequency renal nerve stimulation (636 ± 249 vs. 305 ± 157 μg/ min; P < 0.05). Renal arterial infusion of d,β-propranolol (0.5 μgAg per min, n = 4) or a high dose of atenolol (5.0 μg/kg per min, n = 8) abolished the renin secretion but not the antinatnuretic responses to low frequency renal nerve stimulation. These results demonstrate that: antinatriuresis during 1.0 Hz renal nerve stimulation (where renal blood flow and glomerular filtration rate are unchanged) is mediated by renal oα-adrenoceptors and not by α2- or β-adrenoceptors, that renin secretion elicited by low frequency renal nerve stimulation is mediated by renal βi-adrenoceptors and not by a-adrenoceptors, and that the renin secretion response to low frequency renal nerve stimulation is evoked by direct stimulation of juxtaglomerular granular cell β-adrenoceptors and not indirectly by stimulation of the macula densa receptor through decreased urinary sodium excretion.

Neural control of renal function: cardiovascular implications.

The innervation of the kidney serves to influence the function of its component parts, for example, the blood vessels, the nephron (glomerulus, tubule), and the juxtaglomerular apparatus. Alterations in efferent renal sympathetic nerve activity produce significant changes in renal blood flow, glomerular filtration rate, the reabsorption of water, sodium, and other ions, and the release of renin, prostaglandins, and other vasoactive substances. These functional effects contribute significantly to the renal regulation of total body sodium and fluid volumes with important implications for the control of arterial pressure. The renal nerves, both efferent and afferent, are known to be important contributors to the pathogenesis of hypertension. In addition, the efferent renal nerves participate in the mediation of the excessive renal sodium retention, which characterizes edema-forming states such as congestive heart failure. Thus, the renal nerves play an important role in overall cardiovascular homeostasis in both normal and pathological conditions.

Regulation of gene expression in the mammalian eye and its relevance to eye disease

We used expression quantitative trait locus mapping in the laboratory rat (Rattus norvegicus) to gain a broad perspective of gene regulation in the mammalian eye and to identify genetic variation relevant to human eye disease. Of >31,000 gene probes represented on an Affymetrix expression microarray, 18,976 exhibited sufficient signal for reliable analysis and at least 2-fold variation in expression among 120 F2 rats generated from an SR/JrHsd × SHRSP intercross. Genome-wide linkage analysis with 399 genetic markers revealed significant linkage with at least one marker for 1,300 probes (α = 0.001; estimated empirical false discovery rate = 2%). Both contiguous and noncontiguous loci were found to be important in regulating mammalian eye gene expression. We investigated one locus of each type in greater detail and identified putative transcription-altering variations in both cases. We found an inserted cREL binding sequence in the 5′ flanking sequence of the Abca4 gene associated with an increased expression level of that gene, and we found a mutation of the gene encoding thyroid hormone receptor β2 associated with a decreased expression level of the gene encoding short-wavelength sensitive opsin (Opn1sw). In addition to these positional studies, we performed a pairwise analysis of gene expression to identify genes that are regulated in a coordinated manner and used this approach to validate two previously undescribed genes involved in the human disease Bardet–Biedl syndrome. These data and analytical approaches can be used to facilitate the discovery of additional genes and regulatory elements involved in human eye disease.