Renal substance P-containing neurons and substance P receptors impaired in hypertension

Effects of intrarenal pelvic infusion of tumour necrosis factor-α and interleukin 1-β on reno-renal reflexes in anaesthetised rats

OBJECTIVE

Reno-renal reflexes are disturbed in cardiovascular and hypertensive conditions when elevated levels of pro-inflammatory mediators/cytokines are present within the kidney. We hypothesised that exogenously administered inflammatory cytokines tumour necrosis factor alpha (TNF-α) and interleukin (IL)-1β modulate the renal sympatho-excitatory response to chemical stimulation of renal pelvic sensory nerves.

METHODS

In anaesthetised rats, intrarenal pelvic infusions of vehicle [0.9% sodium chloride (NaCl)], TNF-α (500 and 1000 ng/kg) and IL-1β (1000 ng/kg) were maintained for 30 min before chemical activation of renal pelvic sensory receptors was performed using randomized intrarenal pelvic infusions of hypertonic NaCl, potassium chloride (KCl), bradykinin, adenosine and capsaicin.

RESULTS

The increase in renal sympathetic nerve activity (RSNA) in response to intrarenal pelvic hypertonic NaCl was enhanced during intrapelvic TNF-α (1000 ng/kg) and IL-1β infusions by almost 800% above vehicle with minimal changes in mean arterial pressure (MAP) and heart rate (HR). Similarly, the RSNA response to intrarenal pelvic adenosine in the presence of TNF-α (500 ng/kg), but not IL-1β, was almost 200% above vehicle but neither MAP nor HR were changed. There was a blunted sympatho-excitatory response to intrapelvic bradykinin in the presence of TNF-α (1000 ng/kg), but not IL-1β, by almost 80% below vehicle, again without effect on either MAP or HR.

CONCLUSION

The renal sympatho-excitatory response to renal pelvic chemoreceptor stimulation is modulated by exogenous TNF-α and IL-1β. This suggests that inflammatory mediators within the kidney can play a significant role in modulating the renal afferent nerve-mediated sympatho-excitatory response.

Myocardial infarction with a preserved ejection fraction-the impaired function of the cardio-renal baroreflex

Introduction: In experimental myocardial infarction with reduced ejection fraction causing overt congestive heart failure, the control of renal sympathetic nerve activity (RSNA) by the cardio-renal baroreflex was impaired. The afferent vagal nerve activity under these experimental conditions had a lower frequency at saturation than that in controls. Hence, by investigating respective first neurons in the nodose ganglion (NG), we wanted to test the hypothesis that after myocardial infarction with still-preserved ejection fraction, the cardiac afferent nerve pathway is also already impaired. Material and methods: A myocardial infarction was induced by coronary artery ligature. After 21 days, nodose ganglion neurons with cardiac afferents from rats with myocardial infarction were cultured. A current clamp was used to characterize neurons as "tonic," i.e., sustained action potential (AP) firing, or "phasic," i.e., <5 APs upon current injection. Cardiac ejection fraction was measured using echocardiography; RSNA was recorded to evaluate the sensitivity of the cardiopulmonary baroreflex. Renal and cardiac histology was studied for inflammation and fibrosis markers. Results: A total of 192 neurons were investigated. In rats, after myocardial infarction, the number of neurons with a tonic response pattern increased compared to that in the controls (infarction vs. control: 78.6% vs. 48.5%; z-test, *p < 0.05), with augmented production of APs (23.7 ± 2.86 vs. 15.5 ± 1.86 APs/600 ms; mean ± SEM, t-test, *p < 0.05). The baseline activity of RSNA was subtly increased, and its control by the cardiopulmonary baroreflex was impaired following myocardial infarction: the fibrosis marker collagen I augmented in the renal interstitium. Discussion: After myocardial infarction with still-preserved ejection fraction, a complex impairment of the afferent limb of the cardio-renal baroreflex caused dysregulation of renal sympathetic nerve activity with signs of renal fibrosis.

Intrarenal pelvic bradykinin-induced sympathoexcitatory reno-renal reflex is attenuated in rats exposed to chronic intermittent hypoxia

OBJECTIVE

In this study, we hypothesized that excitatory reno-renal reflex control of sympathetic outflow is enhanced in rats exposed to chronic intermittent hypoxia (CIH) with established hypertension.

METHODS

Under anaesthesia, renal sensory nerve endings in the renal pelvic wall were chemically activated using bradykinin (150, 400 and 700 μmol/l) and capsaicin (1.3 μmol/l), and cardiovascular parameters and renal sympathetic nerve activity (RSNA) were measured.

RESULTS

CIH-exposed rats were hypertensive with elevated basal heart rate and increased basal urine flow compared with sham. The intrarenal pelvic infusion of bradykinin was associated with contralateral increase in the RSNA and heart rate, without concomitant changes in blood pressure. This was associated with a drop in the glomerular filtration rate, which was significant during a 5 min period after termination of the infusion but without significant changes in urine flow and absolute sodium excretion. In response to intrarenal pelvic infusion of 700 μmol/l bradykinin, the increases in RSNA and heart rate were blunted in CIH-exposed rats compared with sham rats. Conversely, the intrarenal pelvic infusion of capsaicin evoked an equivalent sympathoexcitatory effect in CIH-exposed and sham rats. The blockade of bradykinin type 1 receptors (BK1R) suppressed the bradykinin-induced increase in RSNA by ∼33%, with a greater suppression obtained when bradykinin type 2 receptors (BK2R) and BK1R were contemporaneously blocked (∼66%).

CONCLUSION

Our findings reveal that the bradykinin-dependent excitatory reno-renal reflex does not contribute to CIH-induced sympathetic hyperactivity and hypertension. Rather, there is evidence that the excitatory reno-renal reflex is suppressed in CIH-exposed rats, which might relate to a downregulation of BK2R.

AT II Receptor Blockade and Renal Denervation: Different Interventions with Comparable Renal Effects?

BACKGROUND

Angiotensin II (Ang II) and the renal sympathetic nervous system exert a strong influence on renal sodium and water excretion. We tested the hypothesis that already low doses of an Ang II inhibitor (candesartan) will result in similar effects on tubular sodium and water reabsorption in congestive heart failure (CHF) as seen after renal denervation (DNX).

METHODS

Measurement of arterial blood pressure, heart rate (HR), renal sympathetic nerve activity (RSNA), glomerular filtration rate (GFR), renal plasma flow (RPF), urine volume, and urinary sodium. To assess neural control of volume homeostasis, 21 days after the induction of CHF via myocardial infarction rats underwent volume expansion (0.9% NaCL; 10% body weight) to decrease RSNA. CHF rat and controls with or without DNX or pretreated with the Ang II type-1 receptor antagonist candesartan (0.5 ug i.v.) were studied.

RESULTS

CHF rats excreted only 68 + 10.2% of the volume load (10% body weight) in 90 min. CHF rats pretreated with candesartan or after DNX excreted from 92 to 103% like controls. Decreases of RSNA induced by volume expansion were impaired in CHF rats but unaffected by candesartan pointing to an intrarenal drug effect. GFR and RPF were not significantly different in controls or CHF.

CONCLUSION

The prominent function of increased RSNA - retaining salt and water - could no longer be observed after renal Ang II receptor blockade in CHF rats.

Neurogenic tachykinin mechanisms in experimental nephritis of rats

We demonstrated earlier that renal afferent pathways combine very likely “classical” neural signal transduction to the central nervous system and a substance P (SP)–dependent mechanism to control sympathetic activity. SP content of afferent sensory neurons is known to mediate neurogenic inflammation upon release. We tested the hypothesis that alterations in SP-dependent mechanisms of renal innervation contribute to experimental nephritis. Nephritis was induced by OX-7 antibodies in rats, 6 days later instrumented for recording of blood pressure (BP), heart rate (HR), drug administration, and intrarenal administration (IRA) of the TRPV1 agonist capsaicin to stimulate afferent renal nerve pathways containing SP and electrodes for renal sympathetic nerve activity (RSNA). The presence of the SP receptor NK-1 on renal immune cells was assessed by FACS. IRA capsaicin decreased RSNA from 62.4 ± 5.1 to 21.6 ± 1.5 mV s (*p < 0.05) in controls, a response impaired in nephritis. Suppressed RSNA transiently but completely recovered after systemic administration of a neurokinin 1 (NK1-R) blocker. NK-1 receptors occurred mainly on CD11⁺ dendritic cells (DCs). An enhanced frequency of CD11c⁺NK1R⁺ cell, NK-1 receptor⁺ macrophages, and DCs was assessed in nephritis. Administration of the NK-1R antagonist aprepitant during nephritis reduced CD11c⁺NK1R⁺ cells, macrophage infiltration, renal expression of chemokines, and markers of sclerosis. Hence, SP promoted renal inflammation by weakening sympathoinhibitory mechanisms, while at the same time, substance SP released intrarenally from afferent nerve fibers aggravated immunological processes i.e. by the recruitment of DCs.

Substance P Improves Renal Ischemia Reperfusion Injury Through Modulating Immune Response

Substance P (SP), an injury-inducible messenger that mobilizes bone marrow stem cells and modulates the immune response, has been suggested as a novel target for therapeutic agents. We evaluated the role of SP as an immune cell modulator during the progression of renal ischemic/reperfusion injury (IRI). Unilateral IRI induced the transient expression of endogenous SP and the infiltration of CCR7⁺ M1 macrophages in injured kidneys. However, SP altered the intrarenal macrophage polarization from CCR7⁺ M1 macrophages to CD206⁺ M2 macrophages in injured kidneys. SP also modulated bone marrow-derived neutrophils and mesenchymal stromal cells after IRI. SP treatment for 4 weeks starting one week after unilateral IRI significantly preserved kidney size and length and normal tubular structures and alleviated necrotic tubules, inflammation, apoptosis, and tubulointerstitial fibrosis. The beneficial effects of SP were accompanied by attenuation of intrarenal recruitment of CD4, CD8, and CD20 cells and abnormal angiogenesis. The immunomodulatory effect of SP suggested that SP could be a promising therapeutic target for preventing the progression of acute kidney injury to chronic kidney disease.

Crosstalk between the nervous system and the kidney

Under physiological states, the nervous system and the kidneys communicate with each other to maintain normal body homeostasis. However, pathological states disrupt this interaction as seen in hypertension, and kidney damage can cause impaired renorenal reflex and sodium handling. In acute kidney injury (AKI) and chronic kidney disease (CKD), damaged kidneys can have a detrimental effect on the central nervous system. CKD is an independent risk factor for cerebrovascular disease and cognitive impairment, and many factors, including retention of uremic toxins and phosphate, have been proposed as CKD-specific factors responsible for structural and functional cerebral changes in patients with CKD. However, more studies are needed to determine the precise pathogenesis. Epidemiological studies have shown that AKI is associated with a subsequent risk for developing stroke and dementia. However, recent animal studies have shown that the renal nerve contributes to kidney inflammation and fibrosis, whereas activation of the cholinergic anti-inflammatory pathway, which involves the vagus nerve, the splenic nerve, and immune cells in the spleen, has a significant renoprotective effect. Therefore, elucidating mechanisms of communication between the nervous system and the kidney enables us not only to develop new strategies to ameliorate neurological conditions associated with kidney disease but also to design safe and effective clinical interventions for kidney disease, using the neural and neuroimmune control of kidney injury and disease.

Is Aberrant Reno-Renal Reflex Control of Blood Pressure a Contributor to Chronic Intermittent Hypoxia-Induced Hypertension?

Renal sensory nerves are important in the regulation of body fluid and electrolyte homeostasis, and blood pressure. Activation of renal mechanoreceptor afferents triggers a negative feedback reno-renal reflex that leads to the inhibition of sympathetic nervous outflow. Conversely, activation of renal chemoreceptor afferents elicits reflex sympathoexcitation. Dysregulation of reno-renal reflexes by suppression of the inhibitory reflex and/or activation of the excitatory reflex impairs blood pressure control, predisposing to hypertension. Obstructive sleep apnoea syndrome (OSAS) is causally related to hypertension. Renal denervation in patients with OSAS or in experimental models of chronic intermittent hypoxia (CIH), a cardinal feature of OSAS due to recurrent apnoeas (pauses in breathing), results in a decrease in circulating norepinephrine levels and attenuation of hypertension. The mechanism of the beneficial effect of renal denervation on blood pressure control in models of CIH and OSAS is not fully understood, since renal denervation interrupts renal afferent signaling to the brain and sympathetic efferent signals to the kidneys. Herein, we consider the currently proposed mechanisms involved in the development of hypertension in CIH disease models with a focus on oxidative and inflammatory mediators in the kidneys and their potential influence on renal afferent control of blood pressure, with wider consideration of the evidence available from a variety of hypertension models. We draw focus to the potential contribution of aberrant renal afferent signaling in the development, maintenance and progression of high blood pressure, which may have relevance to CIH-induced hypertension.

Role of renal sensory nerves in physiological and pathophysiological conditions

Whether activation of afferent renal nerves contributes to the regulation of arterial pressure and sodium balance has been long overlooked. In normotensive rats, activating renal mechanosensory nerves decrease efferent renal sympathetic nerve activity (ERSNA) and increase urinary sodium excretion, an inhibitory renorenal reflex. There is an interaction between efferent and afferent renal nerves, whereby increases in ERSNA increase afferent renal nerve activity (ARNA), leading to decreases in ERSNA by activation of the renorenal reflexes to maintain low ERSNA to minimize sodium retention. High-sodium diet enhances the responsiveness of the renal sensory nerves, while low dietary sodium reduces the responsiveness of the renal sensory nerves, thus producing physiologically appropriate responses to maintain sodium balance. Increased renal ANG II reduces the responsiveness of the renal sensory nerves in physiological and pathophysiological conditions, including hypertension, congestive heart failure, and ischemia-induced acute renal failure. Impairment of inhibitory renorenal reflexes in these pathological states would contribute to the hypertension and sodium retention. When the inhibitory renorenal reflexes are suppressed, excitatory reflexes may prevail. Renal denervation reduces arterial pressure in experimental hypertension and in treatment-resistant hypertensive patients. The fall in arterial pressure is associated with a fall in muscle sympathetic nerve activity, suggesting that increased ARNA contributes to increased arterial pressure in these patients. Although removal of both renal sympathetic and afferent renal sensory nerves most likely contributes to the arterial pressure reduction initially, additional mechanisms may be involved in long-term arterial pressure reduction since sympathetic and sensory nerves reinnervate renal tissue in a similar time-dependent fashion following renal denervation.

Renal nerves in blood pressure regulation

PURPOSE

This review highlights the physiological mechanisms underlying the neural regulation of the kidney, normally to maintain cardiovascular homeostasis, and in pathophysiological states of hypertension and renal disease. It is relevant because of the demonstration that bilateral renal denervation in different hypertensive groups causes a sustained reduction in blood pressure.

RECENT FINDINGS

There are patients groups in whom their hypertension is resistant to antihypertensive drugs or with renal diseases in which they are contraindicated. Recently, medical devices have been developed to manipulate the sympathetic nervous system, for example, implantation of carotid sinus nerve stimulating electrodes and ablation of the renal innervation. These approaches have been relatively successful but there remains a lack of understanding of the neural mechanisms impinging on the kidney that regulate long-term control of blood pressure.

SUMMARY

The observation that bilateral renal nerve ablation can reduce blood pressure represents an important therapeutic milestone. Nonetheless, questions arise as to the underlying mechanisms, the long-term consequences, whether there may be re-innervation over a number of years, or whether some unknown consequence to the denervation may arise. This may point to the development of novel compounds targeted to the innervation of the kidney.

Impaired responsiveness of renal sensory nerves in streptozotocin-treated rats and obese Zucker diabetic fatty rats: role of angiotensin

Increasing afferent renal nerve activity decreases efferent renal nerve activity and increases urinary sodium excretion. Activation of renal pelvic mechanosensory nerves is impaired in streptozotocin (STZ)-treated rats (model of type 1 diabetes). Decreased activation of renal sensory nerves would lead to increased efferent renal nerve activity, sodium retention, and hypertension. We examined whether the reduced activation of renal sensory nerves in STZ rats was due to increased renal angiotensin activity and whether activation of the renal sensory nerves was impaired in obese Zucker diabetic fatty (ZDF) rats (model of type 2 diabetes). In an isolated renal pelvic wall preparation from rats treated with STZ for 2 wk, PGE2failed to increase the release of substance P, from 5 ± 1 to 6 ± 1 pg/min. In pelvises from sham STZ rats, PGE2increased substance P release from 6 ± 1 to 13 ± 2 pg/min. Adding losartan to the incubation bath increased PGE2-mediated release of substance P in STZ rats, from 5 ± 1 to 10 ± 2 pg/min, but had no effect in sham STZ rats. In pelvises from obese ZDF rats (22–46 wk old), PGE2increased substance P release from 12.0 ± 1.2 to 18.3 ± 1.2 pg/min, which was less than that from lean ZDF rats (10.3 ± 1.6 to 22.5 ± 2.4 pg/min). Losartan had no effect on the PGE2-mediated substance P release in obese or lean ZDF rats. We conclude that the mechanisms involved in the decreased responsiveness of the renal sensory nerves in STZ rats involve activation of the renin angiotensin system in STZ but not in obese ZDF rats.

Transient receptor potential vanilloid type 1 channels act as mechanoreceptors and cause substance P release and sensory activation in rat kidneys

Stimulation of capsaicin receptors results in an increase in afferent renal nerve activity (ARNA), but it is unclear how capsaicin contributes to sensory activation intrarenally. Here, we studied the relationships between capsaicin receptor activation, substance P (SP) release, and the sensory response in the rat renal pelvis. Immunoblots showed that one of the capsaicin receptors, transient receptor potential vanilloid type 1 channel (TRPV1), was found in various renal tissues and was especially abundant in the renal pelvis, where most sensory nerve fibers originate. Interestingly, immunolabeling showed colocalization of TRPV1, SP, and the panneuronal marker PGP9.5 in the renal pelvis. Electrophysiological recordings showed that SP and capsaicin activated the same mechanosensitive ARNA in a single-unit preparation. Intrapelvic administration of capsaicin or a specific TRPV1 agonist, resiniferatoxin, resulted in a dose-dependent increase in multi-unit ARNA and SP release, and these effects were blocked by the TRVP1 blocker capsazepine. Inhibition of the SP receptor by L-703,606 largely prevented capsaicin- or resiniferatoxin-induced ARNA. Capsazepine also prevented intrapelvic pressure (IPP)-dependent ARNA activation and contralateral diuresis/natriuresis in the renorenal reflex at an IPP of 20 mmHg, but had no effect at an IPP of 50 mmHg. These data indicate that TRPV1, a low-pressure baroreceptor, is present in the renal pelvis and exclusively regulates neuropeptide release from primary renal afferent C-fibers in response to mechanostimulation.

Renal sympathetic nerve activity modulates afferent renal nerve activity by PGE2-dependent activation of α1- and α2-adrenoceptors on renal sensory nerve fibers

Increasing efferent renal sympathetic nerve activity (ERSNA) increases afferent renal nerve activity (ARNA). To test whether the ERSNA-induced increases in ARNA involved norepinephrine activating α-adrenoceptors on the renal sensory nerves, we examined the effects of renal pelvic administration of the α1- and α2-adrenoceptor antagonists prazosin and rauwolscine on the ARNA responses to reflex increases in ERSNA (placing the rat's tail in 49°C water) and renal pelvic perfusion with norepinephrine in anesthetized rats. Hot tail increased ERSNA and ARNA, 6,930 ± 900 and 4,870 ± 670%·s (area under the curve ARNA vs. time). Renal pelvic perfusion with norepinephrine increased ARNA 1,870 ± 210%·s. Immunohistochemical studies showed that the sympathetic and sensory nerves were closely related in the pelvic wall. Renal pelvic perfusion with prazosin blocked and rauwolscine enhanced the ARNA responses to reflex increases in ERSNA and norepinephrine. Studies in a denervated renal pelvic wall preparation showed that norepinephrine increased substance P release, from 8 ± 1 to 16 ± 1 pg/min, and PGE2 release, from 77 ± 11 to 161 ± 23 pg/min, suggesting a role for PGE2 in the norepinephrine-induced activation of renal sensory nerves. Prazosin and indomethacin reduced and rauwolscine enhanced the norepinephrine-induced increases in substance P and PGE2. PGE2 enhanced the norepinephrine-induced activation of renal sensory nerves by stimulation of EP4 receptors. Interaction between ERSNA and ARNA is modulated by norepinephrine, which increases and decreases the activation of the renal sensory nerves by stimulating α1- and α2-adrenoceptors, respectively, on the renal pelvic sensory nerve fibers. Norepinephrine-induced activation of the sensory nerves is dependent on renal pelvic synthesis/release of PGE2.

Expression and localization of NK(1)R, substance P and CGRP are altered in dorsal root ganglia neurons of spontaneously hypertensive rats (SHR)

The kidneys play a pivotal role in the pathogenesis of essential hypertension because of a primary defect in renal hemodynamics and/or tubule hydro-saline handling that results in the retention of fluid and electrolytes. Previous studies have shown that increasing the renal pelvic pressure increased ipsilateral afferent renal nerve activity (ARNA), the ipsilateral renal pelvic release of substance P (SP) and the contralateral urinary sodium excretion in Wistar--Kyoto rats (WKy). However, spontaneously hypertensive rats (SHR) present an impaired renorenal reflex activity associated, partly, with a peripheral defect at the level of the sensory receptors in the renal pelvis. Furthermore, the renal pelvic administration of SP failed to increase ARNA in most of SHR at concentrations that produced marked increases in WKy. Since we have assessed the expression and localization of NK(1) receptor (NK(1)R), SP and calcitonin gene-related peptide (CGRP) in different dorsal root ganglia (DRG) cell subtypes and renal pelvis of 7- and 14-week-old SHR. The results of this study show increased SP and CGRP expression in the dorsal ganglia root cells of SHR compared to WKy rats. Additionally, there was a progressive, significant, age-dependent, decrease in NK(1)R expression on the membrane surface in SHR DRG cells and in the renal pelvis. In conclusion, the results of the present study suggest that the impaired activation of renal sensory neurons in SHR may be related to changes in the expression of neuropeptides and/or to a decreased presence of NK(1)R in DRG cells. Such abnormalities could contribute to the enhanced sodium retention and elevation of blood pressure seen in SHR.

Activation of EP4 receptors contributes to prostaglandin E2-mediated stimulation of renal sensory nerves

Induction of cyclooxygenase-2 (COX-2) in the renal pelvic wall increases prostaglandin E(2) (PGE(2)) leading to stimulation of cAMP production, which results in substance P (SP) release and activation of renal mechanosensory nerves. The subtype of PGE receptors involved, EP2 and/or EP4, was studied by immunohistochemistry and renal pelvic administration of agonists and antagonists of EP2 and EP4 receptors. EP4 receptor-like immunoreactivity (LI) was colocalized with calcitonin gene-related peptide (CGRP)-LI in dorsal root ganglia (DRGs) at Th(9)-L(1) and in nerve terminals in the renal pelvic wall. Th(9)-L(1) DRG neurons also contained EP3 receptor-LI and COX-2-LI, each of which was colocalized with CGRP-LI in some neurons. No renal pelvic nerves contained EP3 receptor-LI and only very few nerves COX-2-LI. The EP1/EP2 receptor antagonist AH-6809 (20 microM) had no effect on SP release produced by PGE(2) (0.14 microM) from an isolated rat renal pelvic wall preparation. However, the EP4 receptor antagonist L-161,982 (10 microM) blocked the SP release produced by the EP2/EP4 receptor agonist butaprost (10 microM) 12 +/- 2 vs. 2 +/- 1 and PGE(2), 9 +/- 1 vs. 1 +/- 0 pg/min. The SP release by butaprost and PGE(2) was similarly blocked by the EP4 receptor antagonist AH-23848 (30 microM). In anesthetized rats, the afferent renal nerve activity (ARNA) responses to butaprost 700 +/- 100 and PGE(2).780 +/- 100%.s (area under the curve of ARNA vs. time) were unaffected by renal pelvic perfusion with AH-6809. However, 1 microM L-161,982 and 10 microM AH-23848 blocked the ARNA responses to butaprost by 94 +/- 5 and 78 +/- 10%, respectively, and to PGE(2) by 74 +/- 16 and 74 +/- 11%, respectively. L-161,982 also blocked the ARNA response to increasing renal pelvic pressure 10 mmHg, 85 +/- 5%. In conclusion, PGE(2) increases renal pelvic release of SP and ARNA by activating EP4 receptors on renal sensory nerve fibers.

Impaired substance P release from renal sensory nerves in SHR involves a pertussis toxin-sensitive mechanism

Stretching the renal pelvic wall activates renal mechanosensory nerves by a PGE2-mediated release of substance P via activation of the cAMP-PKA pathway. Renal pelvic ANG II modulates the responsiveness of renal sensory nerves by suppressing the PGE2-mediated activation of adenylyl cyclase via a pertussis toxin (PTX)-sensitive mechanism. In SHR, activation of renal mechanosensory nerves is impaired. This is due to suppressed release of substance P in response to increased pelvic pressure. The present study was performed to investigate whether the PGE2-mediated release of substance P was suppressed in SHR vs. WKY and, if so, whether the impaired PGE2-mediated release of substance P was due to ANG II activating a PTX-sensitive mechanism. In an isolated renal pelvic wall preparation, PGE2, 0.14 μM, increased substance P release from 9 ± 3 to 22 ± 3 pg/min ( P < 0.01) in Wistar-Kyoto rats (WKY), but had no effect in spontaneously hypertensive rats (SHR). A tenfold higher concentration of PGE2, 1.4 μM, was required to increase substance P release in SHR, from 7 ± 1 to 22 ± 3 pg/min ( P < 0.01). In SHR, treating renal pelvises with losartan enhanced the release of substance P produced by subthreshold concentration of PGE2, 0.3 μM, from 16 ± 2 to 26 ± 3 pg/min ( P < 0.01). Likewise, treating renal pelvises with PTX enhanced the PGE2-mediated release of substance P from 10 ± 1 to 33 ± 3 pg/min ( P < 0.01) in SHR. In WKY, neither losartan nor PTX had an effect on the release of substance P produced by subthreshold concentrations of PGE2, 0.03 μM. In conclusion, the impaired responsiveness of renal sensory nerves in SHR involves endogenous ANG II suppressing the PGE2-mediated release of substance P via a PTX-sensitive mechanism.

Temporal decrease in renal sensory responses in rats after chronic ligation of the bile duct

Renal responses to renal sensory receptor activation were examined in rats after 1 and 4 wk of common bile duct ligation (CBDL). Compared with sham-operated rats (Sham), urine and sodium excretion after acute saline loading was significantly reduced at both times after CBDL. The blunted excretory responses in CBDL rats, accompanied by less activation of afferent renal nerve activity (ARNA), were already apparent at 1 wk and became severe at 4 wk. The defect in ARNA activation in CBDL rats was further studied using specific stimuli to activate renal sensory receptors. Graded increases in intrapelvic pressure or renal pelvic perfusion of substance P (SP) elicited an increase in ARNA in Sham rats, these responses being temporally attenuated in CBDL rats. Despite no significant change in renal pelvic SP release, no renorenal reflex was demonstrable in 4-wk CBDL rats. Immunoblotting showed that expression of renal pelvic neurokinin 1 (NK-1) receptors was 32 and 47% lower in 1- and 4-wk CBDL rats, respectively, than in Sham rats, this decrease correlating well with plasma SP levels. The quantitative real-time RT-PCR showed similar levels of NK-1 receptor mRNA in the renal pelvis in the Sham and 4-wk CBDL groups. We conclude that impairment of renal excretory and sensory responses increases with the duration of cirrhosis. An impaired renorenal reflex in cirrhotic rats is involved in the defective activation of the renal sensory receptors could be due, in part, to the low expression of NK-1 receptors, which is dependent on the duration of CBDL. The decrease in NK-1 receptor protein levels is not due to a decrease in mRNA levels.

Electrophysiological effects of tachykinin agonists on sympathetic ganglia of spontaneously hypertensive rats

This study investigated the cellular basis for the enhanced ganglionic responsiveness to NK1 agonists in the spontaneously hypertensive rat (SHR) in comparison to their normotensive counterpart, the Wistar-Kyoto (WKY) rat. Rats for in vivo studies were anesthetized with pentobarbital and treated with chlorisondamine (10.5 micromol/kg). Extracellular recordings from the external carotid nerve showed a greater responsiveness of decentralized SHR superior cervical ganglia (SCG) to intravenous injection of SP (32 nmol/kg). Blood pressure and heart rate were increased in SHRs, whereas WKY rats responded with a decrease in blood pressure and only slight tachycardia. Membrane properties of SCG neurons, as shown by intracellular microelectrode recordings, were similar between strains. Picospritzer application of the NK1 agonist GR-73632 (100 microM, 1 s) evoked slow depolarization and increased neuron excitability. Spontaneous firing was evoked only in some neurons. Depolarization amplitudes were similar between strains; however, the NK1 agonist depolarized a greater number of neurons in hypertensive rats. In conclusion, SHRs are more responsive to ganglion stimulation by NK1 agonists due to a greater number of responsive cells within the SCG rather than an enhanced responsiveness of individual neurons.

Endogenous angiotensin modulates PGE(2)-mediated release of substance P from renal mechanosensory nerve fibers

Increasing renal pelvic pressure increases afferent renal nerve activity (ARNA) by a prostaglandin E2 (PGE2)-mediated release of substance P (SP) from renal pelvic sensory nerves. We examined whether the ARNA responses were modulated by high- and low-sodium diets. Increasing renal pelvic pressure resulted in greater ARNA responses in rats fed a high-sodium than in those fed a low-sodium diet. In rats fed a low-sodium diet, increasing renal pelvic pressure 2.5 and 7.5 mmHg increased ARNA 2 +/- 1 and 13 +/- 1% before and 12 +/- 1 and 22 +/- 2% during renal pelvic perfusion with 0.44 mM losartan. In rats fed a high-sodium diet, similar increases in renal pelvic pressure increased ARNA 10 +/- 1 and 23 +/- 3% before and 1 +/- 1 and 11 +/- 2% during pelvic perfusion with 15 nM ANG II. The PGE2-mediated release of SP from renal pelvic nerves in vitro was enhanced in rats fed a high-sodium diet and suppressed in rats fed a low-sodium diet. The PGE2 concentration required for SP release was 0.03, 0.14, and 3.5 microM in rats fed high-, normal-, and low-sodium diets. In rats fed a low-sodium diet, PGE2 increased renal pelvic SP release from 5 +/- 1 to 6 +/- 1 pg/min without and from 12 +/- 1 to 21 +/- 2 pg/min with losartan in the incubation bath. Losartan had no effect on SP release in rats fed normal- and high-sodium diets. ANG II modulates the responsiveness of renal pelvic mechanosensory nerves by inhibiting PGE2-mediated SP release from renal pelvic nerve fibers.

Activation of renal mechanosensitive neurons involves bradykinin, protein kinase C, PGE(2), and substance P

Increased renal pelvic pressure or bradykinin increases afferent renal nerve activity (ARNA) via PGE(2)-induced release of substance P. Protein kinase C (PKC) activation increases ARNA, and PKC inhibition blocks the ARNA response to bradykinin. We now examined whether bradykinin mediates the ARNA response to increased renal pelvic pressure by activating PKC. In anesthetized rats, the ARNA responses to increased renal pelvic pressure were blocked by renal pelvic perfusion with the bradykinin B(2)-receptor antagonist HOE 140 and the PKC inhibitor calphostin C by 76 +/- 8% (P < 0.02) and 81 +/- 5% (P < 0.01), respectively. Renal pelvic perfusion with 4beta-phorbol 12,13-dibutyrate (PDBu) to activate PKC increased ARNA 27 +/- 4% and renal pelvic release of PGE(2) from 500 +/- 59 to 1, 113 +/- 183 pg/min and substance P from 10 +/- 2 to 30 +/- 2 pg/min (all P < 0.01). Indomethacin abolished the increases in substance P release and ARNA. The PDBu-mediated increase in ARNA was also abolished by the substance P-receptor antagonist RP 67580. We conclude that bradykinin contributes to the activation of renal pelvic mechanosensitive neurons by activating PKC. PKC increases ARNA via a PGE(2)-induced release of substance P.

PGE2 increases substance P release from renal pelvic sensory nerves via activation of N-type calcium channels

Activation of renal pelvic sensory nerves by increased pelvic pressure results in a renal pelvic release of substance P that is dependent on intact prostaglandin synthesis. An isolated renal pelvic wall preparation was used to examine whether PGE2 increases the release of substance P from renal pelvic sensory nerves and by what mechanisms. The validity of the model was tested by examining whether 50 mM KCl increased substance P release from the pelvic wall. Fifty millimolar KCl produced an increase in substance P release, from 9.6 +/- 1.6 to 26.8 +/- 4.0 pg/min, P < 0.01, that was blocked by the L-type calcium blocker verapamil (10 microM). PGE2 (0.14 microM) increased the release of substance P from the pelvic wall from 8.9 +/- 0.9 to 20.6 +/- 3.3 pg/min, P < 0.01. PGE2 failed to increase substance P release in a calcium-free medium. The PGE2-induced substance P release was blocked by the N-type calcium blocker omega-conotoxin (0.1 microM) but was unaffected by verapamil. In conclusion, PGE2 increases the release of substance P from renal pelvic sensory nerves by a calcium-dependent mechanism that requires influx of calcium via N-type calcium channels.