Activation of renal afferent pathways following furosemide treatment. II. Effect Of angiotensin blockade

Etiology and Management of Edema: A Review

The development of peripheral edema can often pose a significant diagnostic and therapeutic challenge for practitioners due to its association with a wide variety of underlying disorders ranging in severity. Updates to the original Starling's principle have provided new mechanistic insights into edema formation. Additionally, contemporary data highlighting the role of hypochloremia in the development of diuretic resistance provide a possible new therapeutic target. This article reviews the pathophysiology of edema formation and discusses implications for treatment.

Renal Nerve Deafferentation Attenuates the Fall in GFR during Intravenous Infusion of Furosemide in Anesthetized Rats

INTRODUCTION

Furosemide reduces the glomerular filtration rate (GFR) and increases the renal vascular resistance (RVR) despite inhibiting tubuloglomerular feedback but increases proximal tubule pressure, renin release, and renal nerve activity.

OBJECTIVE

This study tested the hypothesis that the fall in GFR with furosemide is due to volume depletion or activation of angiotensin type 1 (AT1) receptors or renal nerves.

METHODS

Furosemide was infused for 60 min at 1.0 mg·kg-1·h-1 in groups of 5-8 anesthetized rats. Additional groups received intravenous volume replacement to prevent fluid and Na+ losses or volume replacement plus losartan or plus sham denervation or plus renal denervation or renal nerve deafferentation.

RESULTS

At 60 min of infusion, furosemide alone reduced the GFR (-37 ± 4%; p < 0.01). This fall was not prevented by volume replacement or pretreatment with losartan, although losartan moderated the increase in RVR with furosemide (+44 ± 3 vs. +82 ± 7%; p < 0.01). Whereas the GFR fell after furosemide in rats after sham procedure (-31 ± 2%), it was not changed significantly after prior renal deafferentation. Proximal tubule pressure increased significantly but returned towards baseline over 60 min of furosemide, while urine output remained elevated, and GFR and renal blood flow depressed.

CONCLUSIONS

The fall in GFR over 60 min of furosemide infusion is independent of volume depletion or activation of AT1 receptors but is largely dependent on renal afferent nerves.

Upregulation of hypothalamic arginine vasopressin by peripherally administered furosemide in transgenic rats expressing arginine vasopressin-enhanced green fluorescent protein

Furosemide, which is used worldwide as a diuretic agent, inhibits sodium reabsorption in the Henle's loop, resulting in diuresis and natriuresis. Arginine vasopressin (AVP) is synthesized in the supraoptic nucleus (SON), paraventricular nucleus (PVN), and suprachiasmatic nucleus (SCN) of the hypothalamus. The synthesis AVP in the magnocellular neurons of SON and PVN physiologically regulated by plasma osmolality and blood volume and contributed water homeostasis by increasing water reabsorption in the collecting duct. Central AVP dynamics after peripheral administration of furosemide remain unclear. Here, we studied the effects of intraperitoneal (i.p.) administration of furosemide (20 mg/kg) on hypothalamic AVP by using transgenic rats expressing AVP-enhanced green fluorescent protein (eGFP) under the AVP promoter. The i.p. administration of furosemide did not affect plasma osmolality in the present study; however, eGFP in the SON and magnocellular divisions of the PVN (mPVN) were significantly increased after furosemide administration compared to the control. Immunohistochemical analysis revealed Fos-like immunoreactivity (IR) in eGFP-positive neurons in the SON and mPVN 90 min after i.p. administration of furosemide, and AVP heteronuclear (hn) RNA and eGFP mRNA levels were significantly increased. These furosemide-induced changes were not observed in the suprachiasmatic AVP neurons. Furthermore, furosemide induced a remarkable increase in Fos-IR in the organum vasculosum laminae terminals (OVLT), median preoptic nucleus (MnPO), subfornical organ (SFO), locus coeruleus (LC), nucleus of the solitary tract (NTS), and rostral ventrolateral medulla (RVLM) after i.p. administration of furosemide. In conclusion, we were able to visualize and quantitatively evaluate AVP-eGFP synthesis and neuronal activations after peripheral administration of furosemide, using the AVP-eGFP transgenic rats. The results of this study may provide new insights into the elucidation of physiological mechanisms underlying body fluid homeostasis induced by furosemide. This article is protected by copyright. All rights reserved.

CNS sites activated by renal pelvic epithelial sodium channels (ENaCs) in response to hypertonic saline in awake rats

In some patients, renal nerve denervation has been reported to be an effective treatment for essential hypertension. Considerable evidence suggests that afferent renal nerves (ARN) and sodium balance play important roles in the development and maintenance of high blood pressure. ARN are sensitive to sodium concentrations in the renal pelvis. To better understand the role of ARN, we infused isotonic or hypertonic NaCl (308 or 500mOsm) into the left renal pelvis of conscious rats for two 2hours while recording arterial pressure and heart rate. Subsequently, brain tissue was analyzed for immunohistochemical detection of the protein Fos, a marker for neuronal activation. Fos-immunoreactive neurons were identified in numerous sites in the forebrain and brainstem. These areas included the nucleus tractus solitarius (NTS), the lateral parabrachial nucleus, the paraventricular nucleus of the hypothalamus (PVH) and the supraoptic nucleus (SON). The most effective stimulus was 500mOsm NaCl. Activation of these sites was attenuated or prevented by administration of benzamil (1μM) or amiloride (10μM) into the renal pelvis concomitantly with hypertonic saline. In anesthetized rats, infusion of hypertonic saline but not isotonic saline into the renal pelvis elevated ARN activity and this increase was attenuated by simultaneous infusion of benzamil or amiloride. We propose that renal pelvic epithelial sodium channels (ENaCs) play a role in activation of ARN and, via central visceral afferent circuits, this system modulates fluid volume and peripheral blood pressure. These pathways may contribute to the development of hypertension.

Angiotensin II-induced expression of brain-derived neurotrophic factor in human and rat adrenocortical cells

Angiotensin II (Ang II) is a major regulator of steroidogenesis in adrenocortical cells, and is also an effective inducer of cytokine and growth factor synthesis in several cell types. In microarray analysis of H295R human adrenocortical cells, the mRNA of brain-derived neurotrophic factor (BDNF), a neurotrophin widely expressed in the nervous system, was one of the most up-regulated genes by Ang II. The aim of the present study was the analysis of the Ang II-induced BDNF expression and BDNF-induced effects in adrenocortical cells. Real-time PCR studies have shown that BDNF is expressed in H295R and rat adrenal glomerulosa cells. In H295R cells, the kinetics of Ang II-induced BDNF expression was faster than that of aldosterone synthase (CYP11B2). Inhibition of calmodulin kinase by KN93 did not significantly affect the Ang II-induced stimulation of BDNF expression, suggesting that it occurs by a different mechanism from the CYP11B2-response. Ang II also caused candesartan-sensitive, type-1 Ang II receptor-mediated stimulation of BDNF gene expression in primary rat glomerulosa cells. In rat adrenal cortex, BDNF protein was localized to the subcapsular region. Ang II increased BDNF protein levels both in human and rat cells, and BDNF secretion of H295R cells. Ang II also increased type-1 Ang II receptor-mediated BDNF expression in vivo in furosemide-treated rats. In rat glomerulosa cells, BDNF induced tropomyosin-related kinase B receptor-mediated stimulation of EGR1 and TrkB expression. These data demonstrate that Ang II stimulates BDNF expression in human and rat adrenocortical cells, and BDNF may have a local regulatory function in adrenal glomerulosa cells.

Role of the serotoninergic system in the sodium appetite control

The present article reviews the role of the serotoninergic system in the regulation of the sodium appetite. Data from the peripheral and icv administration of serotoninergic (5-HTergic) agents showed the participation of 5-HT2/3 receptors in the modulation of sodium appetite. These observations were extended with the studies carried out after brain serotonin depletion, lesions of DRN and during blockade of 5-HT2A/2C receptors in lateral parabrachial nucleus (LPBN). Brain serotonin depletion and lesions of DRN increased the sodium appetite response, in basal conditions, after sodium depletion and hypovolemia or after beta-adrenergic stimulation as well. These observations raised the hypothesis that the suppression of ascending pathways from the DRN, possibly, 5-HTergic fibers, modifies the angiotensinergic or sodium sensing mechanisms of the subfornical organ involved in the control of the sodium appetite. 5-HTergic blockade in LPBN induced to similar results, particularly those regarded to the natriorexigenic response evoked by volume depletion or increase of the hypertonic saline ingestion induced by brain angiotensinergic stimulation. In conclusion, many evidences lead to acceptation of an integrated participation resulting of an interaction, between DRN and LPBN, for the sodium appetite control.

Temporal relationship between renal cyst development, hypertension and cardiac hypertrophy in a new rat model of autosomal recessive polycystic kidney disease

BACKGROUND/METHODS

We have examined the hypothesis that cyst formation is key in the pathogenesis of cardiovascular disease in a Lewis polycystic kidney (LPK) model of autosomal-recessive polycystic kidney disease (ARPKD), by determining the relationship between cyst development and indices of renal function and cardiovascular disease.

RESULTS

In the LPK (n = 35), cysts appear at week 3 (1.1 +/- 0.1 mm) increasing to week 24 (2.8 +/- 2 mm). Immunostaining for nephron-specific segments indicate cysts develop predominantly from the collecting duct. Cyst formation preceded hypertension (160 +/- 22 vs. Lewis control 105 +/- 20 mm Hg systolic blood pressure (BP), n = 12) at week 6, elevated creatinine (109 +/- 63 vs. 59 +/- 6 micromol/l, n = 16) and cardiac mass (0.7 vs. 0.4% bodyweight, n = 15) at week 12, and left ventricular hypertrophy (2,898 +/- 207 vs. 1,808 +/- 192 mum, n = 14) at week 24 (all p < or = 0.05). Plasma-renin activity and angiotensin II were reduced in 10- to 12-week LPK (2.2 +/- 2.9 vs. Lewis 11.9 +/- 4.9 ng/ml/h, and 25.0 +/- 19.1 vs. 94.9 +/- 64.4 pg/ml, respectively, n = 26, p < or = 0.05). Ganglionic blockade (hexamethonium 3.3 mg/kg) significantly reduced mean BP in the LPK (52 vs. Lewis 4%, n = 9, p < or = 0.05).

CONCLUSION

Cyst formation is a key event in the genesis of hypertension while the sympathetic nervous system is important in the maintenance of hypertension in this model of ARPKD.

Pathogenesis of hypertension in renal failure: role of the sympathetic nervous system and renal afferents

1. The kidney receives a dense innervation of sympathetic and sensory fibres and can be both a target of sympathetic activity and a source of signals that drive sympathetic tone. In the normal state, interactions between the kidney and sympathetic nervous system (SNS) serve to maintain blood pressure and glomerular filtration rate within tightly controlled levels. In renal failure, a defect in renal sodium excretory function leads to an abnormal pressure natriuresis relationship and activation of the renin-angiotensin-aldosterone system, contributing to the development of hypertension and progression of kidney disease. 2. Evidence now strongly indicates a role for the SNS in the pathogenesis of hypertension in renal failure. Hypertension occurs commonly and early in renal disease and is paralleled by increases in SNS activity, as indicated by increased muscle sympathetic nerve activity and circulating catecholamines. This appears to be driven by the diseased kidneys, because nephrectomy or denervation has been shown to correct blood pressure and SNS activity in human and animal studies. 3. Afferent signals from the kidney, detected by chemoreceptors and mechanoreceptors, feed directly into central nuclei of the SNS, including the hypothalamus and circumventricular organs, in addition to the stimulus provided by circulating and brain-derived angiotensin II. Therefore, the pathogenesis of hypertension in renal failure is complex and arises from the interaction of haemodynamic and neuroendocrine factors. 4. Increased SNS activity has significant implications with regard to increased risk of cardiovascular disease and is an important consideration in the treatment of renal failure.

Dipsogenic stimulation in ibotenic DRN-lesioned rats induces concomitant sodium appetite

The main purpose of this study was to investigate whether dipsogenic stimuli influences the sodium appetite of rats with ibotenic acid lesion of the dorsal raphe nucleus (IBO-DRN). Compared to control, rats microinjected with phosphate buffer (PB-DRN), the ingestion of 0.3M NaCl was enhanced in IBO-DRN at 21 and 35 days after DRN lesion under a protocol of fluids and food deprivation. Despite of similar dipsogenic response observed both in IBO-DRN and PB-DRN treated with isoproterenol (ISO, 300 microg/kg, sc), the 0.3M NaCl intake was again significantly enhanced in IBO-DRN at 21 and 35 days post-lesion. Finally, treatment with polyethylene glycol (PEG, MW=20,000, 20%, w/v, 16.7 ml/kg, sc) induced higher dipsogenic response in IBO-DRN than PB-DRN at 21 day after lesion. In addition, IBO-DRN also expressed higher sodium appetite than PB-DRN, concomitantly with a drinking response. These results suggest that ibotenic lesion of DRN promote an increase of the brain angiotensinergic response, possibly settled within the subfornical organ, through paradigms which increase circulating ANG II levels. The current paper supports the hypothesis that the ibotenic lesion of DRN suppresses a serotonergic component implicated on the modulation of the sodium appetite and, therefore, furthering homeostatic restoration of extracellular fluid volume.

Diuretic Effects of Subcutaneous Furosemide in Human Volunteers: A Randomized Pilot Study

BACKGROUND

Furosemide is usually administered by the oral or intravenous route to cardiac patients with hypertension and heart failure, as well as edema. Occasionally, furosemide cannot be administered by these routes.

OBJECTIVE

To evaluate the diuretic/natriuretic efficacy of subcutaneously administered furosemide in healthy volunteers.

METHODS

This single-center, double-blind, placebo-controlled, randomized, crossover pilot study compared the diuretic effect of subcutaneously administered furosemide and NaCl 0.9% (placebo) in 12 healthy volunteers. The study was conducted over 5 days. Baseline values were determined on day 1. On days 3 and 5, each volunteer was injected with either furosemide 20 mg (2 mL) or 2 mL of placebo subcutaneously. A washout period occurred on day 4. Fluid and dietary intake were controlled on all 3 data collection days. Primary outcome measures consisted of urine volume voided, urine sodium concentration, onset time and volume of initial urine output, and number of voids during 8 hours of urine collection.

RESULTS

All outcomes demonstrated statistically significant differences when treatment and placebo groups were compared (p < 0.05). Adverse effects most commonly reported by the participants were minor and included transient burning and stinging at the injection site.

CONCLUSIONS

This study demonstrates that subcutaneously administered furosemide is a viable alternative when the oral or intravenous route of administration is not desirable or possible in humans. However, the results of this study need to be corroborated in various patient populations.

Induction of c-fos in forebrain circumventricular organs after renal artery stenosis

Experiments were done in the anaesthetized rat to determine the effect of activation of renal receptors following renal arterial occlusion (RAO) on the induction of c-fos in neurons of the lamina terminalis in the forebrain. Following RAO, fos labeled neurons were found in both the subfornical organ (SFO) and the organum vasculosum of the lamina terminalis (OVLT). Transection of the renal nerves ipsilateral to RAO reduced ( approximately 61%) the number of fos labeled neurons in the SFO and prevented the fos labeling in the OVLT. Similarly, administration of the angiotensin II converting enzyme inhibitor enalapril maleate prior to RAO also reduced ( approximately 27%) the number of fos labeled neurons in the SFO to RAO. However, the number of fos labeled neurons was not altered in the OVLT. The number of fos labeled neurons in the SFO of the intact animals after RAO was found to be greater than the algebraic sum of the number of fos labeled neurons in the renal nerve transected and enalapril treated animals. These results suggest that neurons in the SFO are activated by at least two different mechanisms following renal artery occlusion; those involving the activation of afferent renal nerves and those due to changes in circulating levels of angiotensin II. In addition, afferent renal nerve inputs combined with the effect of increased circulating levels of angiotensin II produce a greater activation of the SFO than either input alone. On the other hand, the OVLT appears to be selectively activated by afferent renal nerve inputs following RAO. Taken together, these data suggest that neural inputs from the kidney may play an important role in controlling body fluid balance and arterial pressure (AP) by influencing the activity of forebrain circumventricular organs neurons that function in the detection of blood borne signals associated with changes in extracellular fluid volume.

Effect of electrolytic lesion of the dorsal raphe nucleus on water intake and sodium appetite

The present study determined the effect of an electrolytic lesion of the dorsal raphe nucleus (DRN) on water intake and sodium appetite. Male Wistar rats weighing 290-320 g with a lesion of the DRN (L-DRN), performed two days before experiments and confirmed by histology at the end of the experiments, presented increased sensitivity to the dehydration induced by fluid deprivation. The cumulative water intake of L-DRN rats reached 23.3 1.9 ml (a 79% increase, N = 9) while sham-lesioned rats (SL-DRN) did not exceed 13.0 1.0 ml (N = 11, P < 0.0001) after 5 h. The L-DRN rats treated with isoproterenol (300 g kg-1 ml-1, sc) exhibited an increase in water intake that persisted throughout the experimental period (L-DRN, 15.7 1.47 ml, N = 9 vs SL-DRN, 9.3 1.8 ml, N = 11, P < 0.05). The L-DRN rats also showed an increased spontaneous sodium appetite during the entire period of assessment. The intake of 0.3 M NaCl after 12, 24, 36 and 72 h by the L-DRN rats was always higher than 20.2 4.45 ml (N = 10), while the intake by SL-DRN was always lower than 2.45 0.86 ml (N = 10, P < 0.00001). Sodium- and water-depleted L-DRN rats also exhibited an increased sodium appetite (13.9 2.0 ml, N = 11) compared to SL-DRN (4.6 0.64 ml, N = 11) after 120 min of observation (P < 0.02). The sodium preference of L-DRN rats in both conditions was always higher than that of SL-DRN rats. These results suggest that electrolytic lesion of the DRN overcomes a tonic inhibitory component of sodium appetite.

Transplantation of porcine umbilical cord matrix cells into the rat brain

Immune rejection of transplanted material is a potential complication of organ donation. In response to tissue transplantation, immune rejection has two components: a host defense directed against the grafted tissue and an immune response from the grafted tissue against the host (graft vs host disease). To treat immune rejection, transplant recipients are typically put on immunosuppression therapy. Complications may arise from immune suppression or from secondary effects of immunosuppression drugs. Our preliminary work indicated that stem cells may be xenotransplanted without immunosuppression therapy. Here, we investigated the survival of pig stem cells derived from umbilical cord mucous connective tissue (UCM) after transplantation into rats. Our data demonstrate that UCM cells survive at least 6 weeks without immune suppression of the host animals after transplantation into either the brain or the periphery. In the first experiment, UCM cells were transplanted into the rat brain and recovered in that tissue 2-6 weeks posttransplantation. At 4 weeks posttransplantation, the UCM cells engrafted into the brain along the injection tract. The cells were small and roughly spherical. The transplanted cells were positively immunostained using a pig-specific antibody for neuronal filament 70 (NF70). In contrast, 6 weeks posttransplantation, about 10% of the UCM cells that were recovered had migrated away from the injection site into the region just ventral to the corpus callosum; these cells also stained positively for NF70. In our second experiment, UCM cells that were engineered to constitutively express enhanced green fluorescent protein (eGFP) were transplanted. These cells were recovered 2-4 weeks after brain transplantation. Engrafted cells expressing eGFP and positively staining for NF70 were recovered. This finding indicates a potential for gene therapy. In the third experiment, to determine whether depositing the graft into the brain protected UCM cells from immune detection/clearance, UCM cells were injected into the tail vein and/or the semitendinosis muscle in a group of animals. UCM cells were recovered from the muscle or within the kidney 3 weeks posttransplantation. In control experiments, rat brains were injected with PKH 26-labeled UCM cells that had been lysed by repeated sonic disruption. One and 2 weeks following injection, no PKH 26-labeled neurons or glia were observed. Taken together, these data indicate that UCM cells can survive xenotransplantation and that a subset of the UCM cells respond to local signals to differentiate along a neural lineage.

Acute sodium deficit triggers plasticity of the brain angiotensin type 1 receptors

The brain renin-angiotensin system (bRAS) is involved in the control of hydromineral balance. However, little information is available on the functional regulation of the bRAS as a consequence of sodium deficit in the extracellular fluid compartments. We used a pharmacological model of acute Na+ depletion (furosemide injections) to investigate changes of a major component of the bRAS, the hypothalamic angiotensin type 1A (AT(1A)) receptors. Furosemide induced a rapid and long-lasting expression of the AT(1A) mRNA in the subfornical organ, the median preoptic nucleus (MnPO), and the parvocellular division of the paraventricular nucleus (pPVN). Na+ depletion increased the number of cells expressing AT(1A) mRNA in the pPVN, but not in the MnPO. The enhancement of AT(1A) mRNA expression was associated with an increase in AT(1) binding sites in all the regions studied. It is of interest that in the paraventricular nucleus, the majority of the neurons expressing AT(1A) mRNA also showed an increase in metabolic activity (Fos-related antigen immunoreactivity [FRA-ir]). By contrast, in the MnPO, we observe two distinct cell populations. Our data demonstrated that an acute Na+ deficit induced a functional regulation of the hypothalamic AT(1A) receptors, indicating that these receptors are subject to plasticity in response to hydromineral perturbations.

Activation of renal afferent pathways following furosemide treatment. I. Effects Of survival time and renal denervation

Three experiments were performed to determine whether renal afferent pathways were activated by the diuretic drug, furosemide. It was hypothesized that activated neurons of the renal afferent pathway would express the protein product Fos of the c-fos immediate early gene and be identified by immunocytochemical staining for Fos in the cell nucleus. In the first two experiments, rats were injected with either furosemide (5 mg) or vehicle solution (sterile isotonic saline) and sacrificed either 1.75 h (short-survival experiment) or 3.5 h (long-survival experiment) after injection. In both experiments, the furosemide-treated rats had significantly more Fos-positive cell nuclei than vehicle-treated rats in the subfornical organ (SFO), organum vasculosum lamina terminalis (OVLT), supraoptic nuclei (SON), and magnocellular region of the paraventricular nuclei (PVN) - areas previously shown to be activated by hypovolemia or peripheral angiotensin. In the short-survival experiment, the furosemide-treated rats had more Fos-positive cell nuclei in the nucleus of the solitary tract (NTS) and in the dorsal horn of the spinal cord at spinal levels T(11), T(12), and T(13). In contrast, furosemide treatment did not produce more Fos-positive cell nuclei in the NTS and dorsal horn of the spinal cord in the long-survival experiment. These results suggest that the activation of the SFO, OVLT, SON and PVN may be via a different mechanism than that of NTS or spinal cord dorsal horn. Based upon our previous work, we hypothesized that the NTS and spinal cord dorsal horn labeling was due to activation of sympathetic afferents originating in the kidney and labeling in forebrain structures was due to stimulation by angiotensin generated by renal renin release. To test this hypothesis, a third experiment was devised that was identical to the short-survival experiment, except that all rats had bilateral renal denervation surgery 1 week previously. In this experiment, furosemide administration increased the number of Fos-positive cells in the SFO, OVLT, SON and PVN, but not in the caudal thoracic spinal cord or NTS. These results together with the results of first two experiments lend support to our hypothesis that furosemide-induced neuronal activation in the thoracic spinal cord and NTS is due to activation of second- and/or third-order neurons of a renal sympathetic afferent pathway. Furosemide-induced activation in the SFO, OVLT, SON and PVN does not depend on renal innervation. It is hypothesized that activation in these forebrain regions depends on the action of angiotensin II that is generated after furosemide treatment. Our results indicate that both a hormonal pathway and a renal sympathetic afferent pathway conduct information from the kidney to the central nervous system (CNS) after furosemide treatment.