Expansion of extracellular volume in early polycystic kidney disease

The vasopressin-aquaporin-2 pathway syndromes

Vasopressin is the key hormone involved in water conservation and regulation of water balance, essential for life. In the renal collecting duct, vasopressin binds to the V2 receptor, increasing water permeability through activation of aquaporin-2 redistribution to the luminal membrane. This mechanism promotes rapid water reabsorption, important for immediate survival; however, only recently it has become clear that long-term adverse effects are associated with alterations of the vasopressin-aquaporin-2 pathway, leading to several syndromes associated with water balance disorders. The kidney resistance to the vasopressin action may cause severe dehydration for patients and, conversely, nonosmotic release of vasopressin is associated with water retention and increasing the circulatory blood volume. This chapter discusses the relevance of the altered vasopressin-aquaporin-2 pathway in some diseases associated with water balance disorders, including congenital nephrogenic diabetes insipidus, syndrome of inappropriate secretion of antidiuretic hormone, nephrogenic syndrome of inappropriate antidiuresis, and autosomal dominant polycystic kidney disease. The emerging picture suggests that targeting the vasopressin-AQP2 axis can provide therapeutic benefits in those patients.

Vasopressin and disruption of calcium signalling in polycystic kidney disease

Autosomal dominant polycystic kidney disease (ADPKD) is the most common monogenic kidney disease and is responsible for 5-10% of cases of end-stage renal disease worldwide. ADPKD is characterized by the relentless development and growth of cysts, which cause progressive kidney enlargement associated with hypertension, pain, reduced quality of life and eventual kidney failure. Mutations in the PKD1 or PKD2 genes, which encode polycystin-1 (PC1) and polycystin-2 (PC2), respectively, cause ADPKD. However, neither the functions of these proteins nor the molecular mechanisms of ADPKD pathogenesis are well understood. Here, we review the literature that examines how reduced levels of functional PC1 or PC2 at the primary cilia and/or the endoplasmic reticulum directly disrupts intracellular calcium signalling and indirectly disrupts calcium-regulated cAMP and purinergic signalling. We propose a hypothetical model in which dysregulated metabolism of cAMP and purinergic signalling increases the sensitivity of principal cells in collecting ducts and of tubular epithelial cells in the distal nephron to the constant tonic action of vasopressin. The resulting magnified response to vasopressin further enhances the disruption of calcium signalling that is initiated by mutations in PC1 or PC2, and activates downstream signalling pathways that cause impaired tubulogenesis, increased cell proliferation, increased fluid secretion and interstitial inflammation.

Calmodulin-sensitive adenylyl cyclases mediate AVP-dependent cAMP production and Cl- secretion by human autosomal dominant polycystic kidney cells

In autosomal dominant polycystic kidney disease (ADPKD), binding of AVP to the V2 receptor (V2R) increases cAMP and accelerates cyst growth by stimulating cell proliferation and Cl(-)-dependent fluid secretion. Basal cAMP is elevated in human ADPKD cells compared with normal human kidney (NHK) cells. V2R mRNA levels are elevated in ADPKD cells; however, AVP caused a greater increase in global cAMP in NHK cells, suggesting an intrinsic difference in cAMP regulation. Expression, regulatory properties, and receptor coupling of specific adenylyl cyclases (ACs) provide temporal and spatial regulation of the cAMP signal. ADPKD and NHK cells express mRNAs for all nine ACs. Ca(2+)-inhibited ACs 5 and 6 are increased in ADPKD cells, while Ca(2+)/CaM-stimulated ACs 1 and 3 are downregulated. ACs 1, 3, 5, and 6 were detected in cyst cells in situ, and codistribution with aquaporin-2 suggests that these cysts were derived from collecting ducts. To determine the contribution of CaM-sensitive ACs to AVP signaling, cells were treated with W-7, a CaM inhibitor. W-7 decreased AVP-induced cAMP production and Cl(-) secretion by ADPKD cells. CaMKII inhibition increased AVP-induced cAMP, suggesting that cAMP synthesis is mediated by AC3. In contrast, CaM and CaMKII inhibition in NHK cells did not affect AVP-induced cAMP production. Restriction of intracellular Ca(2+) switched the response in NHK cells, such that CaM inhibition decreased AVP-induced cAMP production. We suggest that a compensatory response to decreased Ca(2+) in ADPKD cells switches V2R coupling from Ca(2+)-inhibited ACs 5/6 to Ca(2+)/CaM-stimulated AC3, to mitigate high cAMP levels in response to continuous AVP stimulation.

Vasopressin, copeptin, and renal concentrating capacity in patients with autosomal dominant polycystic kidney disease without renal impairment

BACKGROUND AND OBJECTIVES

Autosomal dominant polycystic kidney disease (ADPKD) is the most prevalent hereditary renal disease, characterized by cyst formation in the kidneys leading to end stage kidney failure. It is clinically acknowledged that ADPKD patients have impaired urine concentrating capacity, but the mechanism behind this observation is unknown.

DESIGN, SETTING, PARTICIPANTS, & MEASUREMENTS

Fifteen ADPKD patients (estimated GFR ≥60 ml/min per 1.73 m(2)) and 15 age- and sex-matched healthy controls underwent a standard prolonged water deprivation test in which urine and plasma osmolality, vasopressin, and copeptin were measured. The effect of a synthetic vasopressin analog (desmopressin) injected at the moment of maximal urine concentrating capacity was also studied.

RESULTS

After 14 hours of water deprivation, ADPKD patients tended to have higher plasma osmolality (P=0.07) and significantly higher vasopressin and copeptin levels (both P<0.05), whereas urine osmolality was similar in ADPKD patients and controls (710 versus 742 mOsmol/kg; P=0.61). Maximal urine concentrating capacity was lower in ADPKD patients (758 versus 915 mOsmol/kg in controls; P<0.001). At maximal urine concentrating capacity, plasma osmolality, vasopressin, and copeptin levels were significantly higher in ADPKD patients. The median increase in urine osmolality after desmopressin administration in ADPKD patients was less than in healthy controls.

CONCLUSIONS

Already early in their disease, ADPKD patients have impaired maximal urine concentrating capacity brought out upon dehydration, with no evidence of impaired hypothalamic response. To maintain fluid balance, vasopressin concentration increases, which is hypothesized to play a role in ADPKD disease progression.

Collecting duct cells that lack normal cilia have mislocalized vasopressin-2 receptors

Polycystic kidney disease (PKD) is a ciliopathy characterized by renal cysts and hypertension. These changes are presumably due to altered fluid and electrolyte transport in the collecting duct (CD). This is the site where vasopressin (AVP) stimulates vasopressin-2 receptor (V2R)-mediated aquaporin-2 (AQP2) insertion into the apical membrane. Since cysts frequently occur in the CD, we studied V2R and AQP2 trafficking and function in CD cell lines with stunted and normal cilia [cilia (-), cilia (+)] derived from the orpk mouse (hypomorph of the Tg737/Ift88 gene). Interestingly, only cilia (-) cells grown on culture dishes formed domes after apical AVP treatment. This observation led to our hypothesis that V2R mislocalizes to the apical membrane in the absence of a full-length cilium. Immunofluorescence indicated that AQP2 localizes to cilia and in a subapical compartment in cilia (+) cells, but AQP2 levels were elevated in both apical and basolateral membranes in cilia (-) cells after apical AVP treatment. Western blot analysis revealed V2R and glycosylated AQP2 in biotinylated apical membranes of cilia (-) but not in cilia (+) cells. In addition, apical V2R was functional upon apical desmopressin (DDAVP) treatment by demonstrating increased cAMP, water transport, and benzamil-sensitive equivalent short-circuit current (I(sc)) in cilia (-) cells but not in cilia (+) cells. Moreover, pretreatment with a PKA inhibitor abolished DDAVP stimulation of I(sc) in cilia (-) cells. Thus we propose that structural or functional loss of cilia leads to abnormal trafficking of AQP2/V2R leading to enhanced salt and water absorption. Whether such apical localization contributes to enhanced fluid retention and hypertension in PKD remains to be determined.

Tolvaptan inhibits ERK-dependent cell proliferation, Cl⁻ secretion, and in vitro cyst growth of human ADPKD cells stimulated by vasopressin

In autosomal dominant polycystic kidney disease (ADPKD), arginine vasopressin (AVP) accelerates cyst growth by stimulating cAMP-dependent ERK activity and epithelial cell proliferation and by promoting Cl(-)-dependent fluid secretion. Tolvaptan, a V2 receptor antagonist, inhibits the renal effects of AVP and slows cyst growth in PKD animals. Here, we determined the effect of graded concentrations of tolvaptan on intracellular cAMP, ERK activity, cell proliferation, and transcellular Cl(-) secretion using human ADPKD cyst epithelial cells. Incubation of ADPKD cells with 10(-9) M AVP increased intracellular cAMP and stimulated ERK and cell proliferation. Tolvaptan caused a concentration-dependent inhibition of AVP-induced cAMP production with an apparent IC(50) of ∼10(-10) M. Correspondingly, tolvaptan inhibited AVP-induced ERK signaling and cell proliferation. Basolateral application of AVP to ADPKD cell monolayers grown on permeable supports caused a sustained increase in short-circuit current that was completely blocked by the Cl(-) channel blocker CFTR(inh-172), consistent with AVP-induced transepithelial Cl(-) secretion. Tolvaptan inhibited AVP-induced Cl(-) secretion and decreased in vitro cyst growth of ADPKD cells cultured within a three-dimensional collagen matrix. These data demonstrate that relatively low concentrations of tolvaptan inhibit AVP-stimulated cell proliferation and Cl(-)-dependent fluid secretion by human ADPKD cystic cells.

Rationale and design of the TEMPO (Tolvaptan Efficacy and Safety in Management of Autosomal Dominant Polycystic Kidney Disease and its Outcomes) 3-4 Study

BACKGROUND

Current management of autosomal dominant polycystic kidney disease (ADPKD) is focused on treating disease complications, not on slowing cyst development or preventing progression to kidney failure. Tolvaptan, a selective vasopressin V2 (vasopressin 2) receptor antagonist, has been proved to inhibit kidney cyst growth and preserve kidney function in multiple animal models of polycystic kidney disease. The TEMPO (Tolvaptan Efficacy and Safety in Management of Autosomal Dominant Polycystic Kidney Disease and Its Outcomes) 3-4 Study will examine the long-term effectiveness and safety of tolvaptan in patients with ADPKD. We report baseline characteristics and revised power calculations for the trial.

STUDY DESIGN

A prospective, 3-year, multicenter, double-blind, placebo-controlled trial of tolvaptan, a selective V2 receptor antagonist. Primary outcome is total kidney volume percentage of change from baseline for tolvaptan relative to placebo. Secondary outcome parameters include time to ADPKD-associated complications (kidney function decrease, blood pressure control, renal pain, and albuminuria) and safety end points.

SETTING & PARTICIPANTS

This trial includes patients with ADPKD with relatively preserved kidney function (baseline estimated creatinine clearance ≥60 mL/min), aged 50 years or younger, and with total kidney volume measured using magnetic resonance imaging ≥750 mL.

INTERVENTION

Administration of placebo or tolvaptan, dose titrated to tolerance.

OUTCOMES

Number of subjects enrolled and baseline characteristics.

MEASUREMENTS

Total kidney volume, kidney function, albuminuria, kidney pain, and vital signs.

RESULTS

1,445 patients with ADPKD were enrolled between March 2007 and January 2009. Preliminary baseline median total kidney volume was 1.46 L, and estimated creatinine clearance was 105 ± 34 mL/min. A prespecified blinded sample-size recalculation at two-thirds enrollment confirmed the likely power of the study to detect 20% differences from placebo in the primary and key secondary end points at P < 0.05.

LIMITATIONS

This is a preselected ADPKD population chosen for its risk of progression to kidney failure and may not represent the general ADPKD population. If study results are positive with regard to the primary end point, positive effects on other secondary clinical outcomes will be required to assess overall benefit.

CONCLUSIONS

This randomized trial is the largest clinical study of a proposed ADPKD intervention to date. It targets patients with ADPKD with early disease who are projected to have rapid cyst growth and accelerated outcomes. Blockade of vasopressin V2 receptor is hypothesized to inhibit cyst growth, thereby delaying additional adverse clinical outcomes.

Polycystic kidney disease: pathogenesis and potential therapies

Autosomal dominant polycystic kidney disease (ADPKD) is a prevalent, inherited condition for which there is currently no effective specific clinical therapy. The disease is characterized by the progressive development of fluid-filled cysts derived from renal tubular epithelial cells which gradually compress the parenchyma and compromise renal function. Current interests in the field focus on understanding and exploiting signaling mechanisms underlying disease pathogenesis as well as delineating the role of the primary cilium in cystogenesis. This review highlights the pathogenetic pathways underlying renal cyst formation as well as novel therapeutic targets for the treatment of PKD. This article is part of a Special Issue entitled: Polycystic Kidney Disease.

Cyclic AMP-mediated cyst expansion

In polycystic kidney disease (PKD), intracellular cAMP promotes cyst enlargement by stimulating mural epithelial cell proliferation and transepithelial fluid secretion. The proliferative effect of cAMP in PKD is unique in that cAMP is anti-mitogenic in normal renal epithelial cells. This phenotypic difference in the proliferative response to cAMP appears to involve cross-talk between cAMP and Ca(2+) signaling to B-Raf, a kinase upstream of the MEK/ERK pathway. In normal cells, B-Raf is repressed by Akt (protein kinase B), a Ca(2+)-dependent kinase, preventing cAMP activation of ERK and cell proliferation. In PKD cells, disruption of intracellular Ca(2+) homeostasis due to mutations in the PKD genes relieves Akt inhibition of B-Raf, allowing cAMP stimulation of B-Raf, ERK and cell proliferation. Fluid secretion by cystic cells is driven by cAMP-dependent transepithelial Cl(-) secretion involving apical cystic fibrosis transmembrane conductance regulator (CFTR) Cl(-) channels. This review summarizes the current knowledge of cAMP-dependent cyst expansion, focusing on cell proliferation and Cl(-)-dependent fluid secretion, and discusses potential therapeutic approaches to inhibit renal cAMP production and its downstream effects on cyst enlargement. This article is part of a Special Issue entitled: Polycystic Kidney Disease.

Copeptin, a surrogate marker of vasopressin, is associated with disease severity in autosomal dominant polycystic kidney disease

BACKGROUND AND OBJECTIVES

Experimental studies suggest a detrimental role for vasopressin in the pathogenesis of autosomal dominant polycystic kidney disease (ADPKD). However, it is unknown whether endogenous vasopressin concentration is associated with disease severity in patients with ADPKD.

DESIGN, SETTING, PARTICIPANTS, & MEASUREMENTS

Plasma copeptin concentration (a marker of endogenous vasopressin levels) was measured in 102 ADPKD patients (diagnosis based on Ravine criteria) by an immunoassay. Plasma and urinary osmolarity were also measured. To assess disease severity, GFR and effective renal blood flow were measured by continuous infusion of 125I-iothalamate and 131I-hippuran, total renal volume by magnetic resonance imaging, and 24-hour urinary albumin excretion by nephelometry.

RESULTS

In these ADPKD patients, copeptin was associated with the various markers of disease severity in ADPKD (positively with total renal volume [R=0.47] and albuminuria [R=0.39] and negatively with GFR [R=-0.58] and effective renal blood flow [R=-0.52], all P<0.001). These associations were independent of age, gender, and use of diuretics. Copeptin was furthermore associated with plasma osmolarity (P<0.001) but not with 24-hour urinary volume, 24-hour urinary osmolarity or fractional urea excretion (P=0.7, 0.9, and 0.3, respectively).

CONCLUSIONS

On cross-sectional analysis, copeptin is associated with disease severity in ADPKD patients, supporting the results of experimental studies that suggest that vasopressin antagonists have a renoprotective effect in ADPKD and offering a good prospect for clinical studies with these agents.

Polycystic kidney disease: inheritance, pathophysiology, prognosis, and treatment

Both autosomal dominant and recessive polycystic kidney disease are conditions with severe associated morbidity and mortality. Recent advances in the understanding of the genetic and molecular pathogenesis of both ADPKD and ARPKD have resulted in new, targeted therapies designed to disrupt cell signaling pathways responsible for the abnormal cell proliferation, dedifferentiation, apoptosis, and fluid secretion characteristic of the disease. Herein we review the current understanding of the pathophysiology of these conditions, as well as the current treatments derived from our understanding of the mechanisms of these diseases.

Protein kinase D stabilizes aldosterone-induced ERK1/2 MAP kinase activation in M1 renal cortical collecting duct cells to promote cell proliferation

Aldosterone elicits transcriptional responses in target tissues and also rapidly stimulates the activation of protein kinase signalling cascades independently of de novo protein synthesis. Here we investigated aldosterone-induced cell proliferation and extra-cellular regulated kinase 1 and 2 (ERK1/2) mitogen activated protein (MAP) kinase signalling in the M1 cortical collecting duct cell line (M1-CCD). Aldosterone promoted the proliferative growth of M1-CCD cells, an effect that was protein kinase D1 (PKD1), PKCdelta and ERK1/2-dependent. Aldosterone induced the rapid activation of ERK1/2 with peaks of activation at 2 and 10 to 30 min after hormone treatment followed by sustained activation lasting beyond 120 min. M1-CCD cells suppressed in PKD1 expression exhibited only the early, transient peaks in ERK1/2 activation without the sustained phase. Aldosterone stimulated the physical association of PKD1 with ERK1/2 within 2 min of treatment. The mineralocorticoid receptor (MR) antagonist RU28318 inhibited the early and late phases of aldosterone-induced ERK1/2 activation, and also aldosterone-induced proliferative cell growth. Aldosterone induced the sub-cellular redistribution of ERK1/2 to the nuclei at 2 min and to cytoplasmic sites, proximal to the nuclei after 30 min. This sub-cellular distribution of ERK1/2 was inhibited in cells suppressed in the expression of PKD1.

Cardiovascular abnormalities in autosomal-dominant polycystic kidney disease

Cardiovascular problems are a major cause of morbidity and mortality in patients with autosomal-dominant polycystic kidney disease (ADPKD). Hypertension is a common early symptom of ADPKD, and occurs in approximately 60% of patients before renal function has become impaired. Hypertension is associated with an increased rate of progression to end-stage renal disease and is the most important potentially treatable variable in ADPKD. Left ventricular hypertrophy, which is a powerful, independent risk factor for cardiovascular morbidity and mortality, also occurs frequently in patients with ADPKD. Both hypertension and left ventricular hypertrophy have important roles in cardiovascular complications in these individuals. Moreover, biventricular diastolic dysfunction, endothelial dysfunction, increased carotid intima-media thickness, and impaired coronary flow velocity reserve are present even in young patients with ADPKD who have normal blood pressure and well-preserved renal function. These findings suggest that cardiovascular involvement starts very early in the course of ADPKD. Intracranial and extracranial aneurysms and cardiac valvular defects are other potential cardiovascular problems in patients with ADPKD. Early diagnosis and treatment of hypertension, with drugs that block the renin-angiotensin-aldosterone system, has the potential to decrease the cardiovascular complications and slow the progression of renal disease in ADPKD.

Role of hypervolemia and renin in the blood pressure control of patients with pyelonephritis renal scarring

Patients with pyelonephritic renal scarring are at risk of developing renal failure and hypertension. We studied glomerular filtration rate (GFR), renal plasma flow (RPF), filtration fraction (FF), systolic (SBP) and diastolic (DBP) blood pressure, fractional sodium, potassium and phosphate excretion, peripheral renin activity (PRA), plasma aldosterone (p-Aldo), urinary albumin excretion (U-Alb) and urinary beta 2-microglobulin excretion (beta 2-M) in hydropenia and during transition to 3% volume expansion with isotonic saline infusion in 22 female patients with renal scarring due to pyelonephritis and 9 healthy controls. The patients had significantly lower GFR, higher SBP and higher PRA in hydropenia, but there was no significant difference in RPF, FF, DBP or p-Aldo. After volume expansion, SBP, DBP, PRA and p-Aldo were significantly higher in patients than in controls. Transition to 3% volume expansion was associated with a similar increase in SBP in both patients and controls, whereas DBP increased significantly more in the patients (p less than 0.01). Volume expansion resulted in a significant suppression of PRA and p-Aldo in both patients and controls. The patients with renal scarring had the same capacity to excrete sodium and water during transition to volume expansion as the healthy controls. The renin-aldosterone system seems abnormally activated and is probably more important than hypervolemia in the development of hypertension in this group of patients.

Vasopressin antagonists in polycystic kidney disease

Increased cell proliferation and fluid secretion, probably driven by alterations in intracellular calcium homeostasis and cyclic adenosine 3,5-phosphate, play an important role in the development and progression of polycystic kidney disease. Hormone receptors that affect cyclic adenosine monophosphate and are preferentially expressed in affected tissues are logical treatment targets. There is a sound rationale for considering the arginine vasopressin V2 receptor as a target. The arginine vasopressin V2 receptor antagonists OPC-31260 and tolvaptan inhibit the development of polycystic kidney disease in cpk mice and in three animal orthologs to human autosomal recessive polycystic kidney disease (PCK rat), autosomal dominant polycystic kidney disease (Pkd2/WS25 mice), and nephronophthisis (pcy mouse). PCK rats that are homozygous for an arginine vasopressin mutation and lack circulating vasopressin are markedly protected. Administration of V2 receptor agonist 1-deamino-8-D-arginine vasopressin to these animals completely recovers the cystic phenotype. Administration of 1-deamino-8-D-arginine vasopressin to PCK rats with normal arginine vasopressin aggravates the disease. Suppression of arginine vasopressin release by high water intake is protective. V2 receptor antagonists may have additional beneficial effects on hypertension and chronic kidney disease progression. A number of clinical studies in polycystic kidney disease have been performed or are currently active. The results of phase 2 and phase 2-3 clinical trials suggest that tolvaptan is safe and well tolerated in autosomal dominant polycystic kidney disease. A phase 3, placebo-controlled, double-blind study in 18- to 50-yr-old patients with autosomal dominant polycystic kidney disease and preserved renal function but relatively rapid progression, as indicated by a total kidney volume >750 ml, has been initiated and will determine whether tolvaptan is effective in slowing down the progression of this disease.

Potential pharmacological interventions in polycystic kidney disease

Polycystic kidney diseases (autosomal dominant and autosomal recessive) are progressive renal tubular cystic diseases, which are characterised by cyst expansion and loss of normal kidney structure and function. Autosomal dominant polycystic kidney disease (ADPKD) is the most common life- threatening, hereditary disease. ADPKD is more prevalent than Huntington's disease, haemophilia, sickle cell disease, cystic fibrosis, myotonic dystrophy and Down's syndrome combined. Early diagnosis and treatment of hypertension with inhibitors of the renin-angiotensin-aldosterone system (RAAS) and its potential protective effect on left ventricular hypertrophy has been one of the major therapeutic goals to decrease cardiac complications and contribute to improved prognosis of the disease. Advances in the understanding of the genetics, molecular biology and pathophysiology of the disease are likely to facilitate the improvement of treatments for these diseases. Developments in describing the role of intracellular calcium ([Ca(2+)](i)) and its correlation with cellular signalling systems, Ras/Raf/mitogen extracellular kinase (MEK)/extracellular signal-regulated protein kinase (ERK), and interaction of these pathways with cyclic adenosine monophosphate (cAMP) levels, provide new insights on treatment strategies. Blocking the vasopressin V(2) receptor, a major adenylyl cyclase agonist, demonstrated significant improvements in inhibiting cytogenesis in animal models. Because of activation of the mammalian target of rapamycin (mTOR) pathway, the use of sirolimus (rapamycin) an mTOR inhibitor, markedly reduced cyst formation and decreased polycystic kidney size in several animal models. Caspase inhibitors have been shown to decrease cytogenesis and renal failure in rats with cystic disease. Cystic fluid secretion results in cyst enlargement and somatostatin analogues have been shown to decrease renal cyst progression in patients with ADPKD. The safety and efficacy of these classes of drugs provide potential interventions for experimental and clinical trials.

Autosomal dominant polycystic kidney disease: role of the renin-angiotensin system in raised blood pressure in progression of renal and cardiovascular disease

Raised blood pressure (BP) is extremely common in individuals with autosomal dominant polycystic kidney disease (ADPKD) and is almost invariably raised once they develop renal failure. The underlying mechanisms for the rise in BP in individuals with ADPKD are unclear. The progressive number and enlargement of renal cysts, causing structural damage to the kidneys and, thereby, affecting tubular function as well as causing distortion of the glomeruli and renal ischaemia, is likely to be of primary importance. There is some evidence from animal models that there may be over-activity of the intra-renal renin-angiotensin system (RAS) that could account for the rise in BP. Studies in man have shown conflicting results, but a recent more carefully controlled study using both measurements of activity and pharmacological blockade of the RAS clearly demonstrated no evidence of over-activity of the circulating RAS in ADPKD compared to matched individuals with essential hypertension. A more likely explanation for the rise in BP that occurs in ADPKD is retention of sodium and water due to tubular damage. Disappointingly, in spite of good evidence that RAS blocking drugs slow the progression of other renal, particularly glomerular, diseases, there is little evidence to suggest this is true for patients with ADPKD. Nevertheless, there is no doubt that lowering BP in ADPKD is just as important, if not more important, as in essential hypertension to prevent cardiovascular disease and strokes, with a recommended BP target of < 120/80 mmHg.

Polycystic kidney disease: genes, proteins, animal models, disease mechanisms and therapeutic opportunities

An increased understanding of the genetic, molecular and cellular mechanisms responsible for the development of polycystic kidney disease has laid out the foundation for the development of rational therapies. Many animal models where these therapies can be tested are currently available. This review summarizes the rationale for these treatments, the results of preclinical trials and the prospects for clinical trials, some already in early phases of implementation.

Mechanisms of Disease: autosomal dominant and recessive polycystic kidney diseases

Autosomal dominant polycystic kidney disease and autosomal recessive polycystic kidney disease are the best known of a large family of inherited diseases characterized by the development of renal cysts of tubular epithelial cell origin. Autosomal dominant and recessive polycystic kidney diseases have overlapping but distinct pathogeneses. Identification of the causative mutated genes and elucidation of the function of their encoded proteins is shedding new light on the mechanisms that underlie tubular epithelial cell differentiation. This review summarizes recent literature on the role of primary cilia, intracellular calcium homeostasis, and signaling involving Wnt, cyclic AMP and Ras/MAPK, in the pathogenesis of polycystic kidney disease. Improved understanding of pathogenesis and the availability of animal models orthologous to the human diseases provide an excellent opportunity for the development of pathophysiology-based therapies. Some of these have proven effective in preclinical studies, and clinical trials have begun.

Renal Vascular Resistance and Renin-Angiotensin System in the Pathogenesis of Early Hypertension in Autosomal Dominant Polycystic Kidney Disease

Activation of the renin-angiotensin system (RAS) has been proposed to increase renal vascular resistance (RVR) and to play a role in the development of hypertension in autosomal dominant polycystic kidney disease (ADPKD). The aim of this study was to investigate the relationship among RVR, RAS and blood pressure (BP) profile in patients without renal impairment. Thirty-four ADPKD patients underwent ambulatory blood pressure monitoring (ABPM) over a 24-h period and were divided into two groups: 17 hypertensive (group A, day-systolic BP > or = 135 mmHg and/or day-diastolic BP > or = 85 mmHg) and 17 normotensive (group B, day-BP < 135/85 mmHg) patients. The two groups were comparable with respect to age, sex, and renal function. None of the patients assumed therapy. In all subjects the plasma renin activity (PRA) was measured, and the RVR was assessed by measuring resistivity indices (RI). RI was significantly higher in the hypertensive than in normotensive patients (0.67 +/- 0.05 vs. 0.62 +/- 0.03), while PRA was normal in all subjects, and showed no statistical difference between the two groups. Taking all the patients together (group A + group B), a significant positive correlation between RI and 24-h mean arterial pressure (MAP) was discovered, but no correlation was found between RI and PRA or between MAP and PRA. We conclude that in ADPKD patients without renal impairment the MAP values are strictly correlated with the RVR, but not with PRA. Thus factors other than RAS probably contribute to the increase of the RVR and to the early development of hypertension.

Effects of growth hormone on renal tubular handling of sodium in healthy humans

To investigate the mechanisms behind the water- and sodium-retaining effects of growth hormone (GH), we studied the effect of GH on 1) water and sodium homeostasis, 2) the renin-angiotensin-aldosterone system (RAAS), and 3) lithium clearance (C(Li)) with and without concomitant prostaglandin (PG) synthesis inhibition with ibuprofen. GH administration for 6 days induced a significant increase in plasma renin, which was abolished by coadministration of ibuprofen (mU x l(-1) x 24 h(-1): control: 22.4 +/- 4.3; GH: 37.7 +/- 8.8; ibuprofen: 15.2 +/- 3.0; GH + ibuprofen: 19.7 +/- 2.5; ANOVA: P < 0.01). Comparable increments in extracellular volume were seen after 6-day treatment with GH alone and in combination with ibuprofen [liters: control, 19.57 +/- 0.92; GH, 20.80 +/- 1.00 (ANOVA: P < 0.0005); ibuprofen, 19.38 +/- 0.90; GH + ibuprofen, 21.63 +/- 1.37 (ANOVA: P < 0.0005)]. Treatment with GH increased C(Li) and changed the tubular handling of sodium and water. The absolute distal sodium reabsorption was increased, and this was only partially counterbalanced by decreased reabsorption in the proximal tubules. The data demonstrate that GH-induced activation of the RAAS can be blocked by concomitant PG synthesis inhibition and that the tubular effects of GH include increased distal nephron sodium and water reabsorption.

Insulin-like growth factor I administration induces fluid and sodium retention in healthy adults: possible involvement of renin and atrial natriuretic factor

OBJECTIVE

Growth hormone induces fluid and sodium retention. The underlying mechanism is, however, incompletely understood. A possible mediator could be IGF-I. To investigate the impact of IGF-I administration on body fluid distribution and sodium homeostasis in healthy subjects, we examined normal subjects during six days IGF-I treatment and during a six-day control period.

DESIGN AND MEASUREMENTS

Eight normal male subjects aged 23-30 years were randomised to receive IGF-I 50 microg/kg subcutaneously thrice daily during a six day study period, and to a six day control period. After each study period, extracellular volume and plasma volume (ECV, PV) were determined using 82Br and 125I-albumin. Blood samples, urinary sodium excretion, and bioimpedance were measured every second day of each study period.

RESULTS

Serum IGF-I (microg/l) increased during active treatment (control, 293 +/- 9; IGF-I, 628 +/- 42; P < 0.01). ECV (l) was expanded by IGF-I (control, 18.42 +/- 0.28; IGF-I, 19.72 +/- 0.50; P < 0.05) whereas PV (l) remained unaffected (control, 3.76 +/- 0.11; IGF-I, 3.80 +/- 0.16; n.s.). Likewise, bioimpedance and body weight were unchanged by IGF-I. Plasma renin (mU/l) increased but not significantly during IGF-I (control, 28.7 +/- 2.7; IGF-I, 39.9 +/- 4.3; P = 0.08), and plasma aldosterone was unaffected by IGF-I. N-Terminal proANF (pmol/l) was suppressed during IGF-I administration (control, 422 +/- 32; IGF-I, 330 +/- 20; P < 0.05). Diurnal sodium excretion (mmol) was reduced during IGF-I administration (control, 151 +/- 8; IGF-I, 124 +/- 7; P < 0.05).

CONCLUSION

IGF-I treatment causes fluid and sodium retention. This may be mediated by increased renin release and suppression of atrial natriuretic factor. The present data suggest that the fluid and sodium retaining effect of GH is at least partly mediated through IGF-I.

Growth hormone treatment improves body fluid distribution in patients undergoing elective abdominal surgery

OBJECTIVE

To investigate the possible beneficial effects of growth hormone (GH) in catabolic patients we examined the impact of GH on body fluid distribution in patients with ulcerative colitis undergoing elective abdominal surgery.

DESIGN AND MEASUREMENTS

Twenty-four patients (14 female, 10 male) aged 19-47 years were in a double-blinded study randomly assigned to receive either placebo (n = 12) or GH (n = 12) 6 i.u. s.c. twice daily from 2 days before until 7 days after ileo-anal J pouch surgery. Extracellular and plasma volume (ECV, PV) were determined using 82Br and 125I albumin dilution at day -2 and at day 7, and body composition was estimated by dual X-ray absorptiometry and bioimpedance. Changes in body weight and fluid balance were recorded and hence intracellular volume was assessed.

RESULTS

During placebo treatment body weight decreased 4.3 +/- 0.6 kg; during GH treatment body weight was constant (P < 0.01). There was a positive fluid balance in the GH-treated patients compared to the placebo group (GH: 3.6 +/- 0.7 l; plc: -0.7 +/- 1.2 l, P < 0.01). ECV increased 2.12 +/- 0.70 l during GH and was unaffected during placebo (P = 0.02). PV was unchanged by GH and decreased 0.39 +/- 0.08 l during placebo administration (P = 0.03). Intracellular volume (ICV) decreased less during GH than during placebo (GH: -1.42 +/- 0.45; plc: -3.70 +/- 0.76; P = 0.02). Bioimpedance remained constant during GH administration and increased 60 +/- 9 ohm in the placebo-treated group (P < 0.05). Plasma renin and aldosterone remained unchanged in both study groups.

CONCLUSION

Body weight, plasma volume and intracellular volume is preserved during GH treatment in catabolic patients and ECV is increased. From a therapeutic point of view these effects may be desirable under conditions of surgical stress.

The pathogenesis of hypertension in autosomal dominant polycystic kidney disease

BACKGROUND

Hypertension is a common and serious complication of autosomal dominant polycystic kidney disease (ADPKD), often occurring early in the disease before the renal function starts to decrease. The pathogenesis of this early hypertension is controversial.

OBJECTIVE

To review studies on the pathogenesis of early and late hypertension in ADPKD.

STUDY SELECTION

Studies on ADPKD and hypertension were retrieved from Medline from the last 20 years, with an emphasis on the last 10 years. These studies, together with selected published abstracts from recent hypertension and nephrology meetings, were reviewed critically.

RESULTS

Cyst growth, renal handling of sodium, activation of the renin-angiotensin-aldosterone system, volume expansion, an elevated plasma volume, and increased plasma atrial natriuretic peptide and plasma endothelin levels have all been found to be associated with hypertension in ADPKD. In some studies an inappropriate activity of the renin-angiotensin-aldosterone system that could be related to cyst growth and intrarenal ischemia was found. An increase in renal vascular resistance has been demonstrated and might be caused by intrarenal release of angiotensin II. Interestingly, the protective effect of angiotensin converting enzyme inhibitors on the renal function could not be demonstrated in ADPKD patients with a moderately decreased renal function. The importance, if any, of endothelial vasodilatory factors is not known. Sympathetic nervous activity seems to be increased in ADPKD, but the importance of this for the blood pressure level is not known.

CONCLUSION

The pathogenesis of hypertension in ADPKD is complex and likely to be dependent on the interaction of hemodynamic, endocrine and neurogenic factors.

Blockade of the renin-angiotensin-aldosterone system prevents growth hormone-induced fluid retention in humans

To test if the renin-angiotensin-aldosterone system (RAAS) is involved in growth hormone (GH)-associated fluid retention, we examined the effect of GH administration in the presence or absence of RAAS blockade at different levels on body fluid homeostasis. Eight subjects were examined in a controlled, randomized double-blinded trial. During four 6-day periods they received subcutaneous GH (6 IU-m-2) or placebo injections and tablets as follows: 1) placebo and placebo, 2) GH and placebo, 3) GH and captopril, and 4) GH and spironolactone. GH increased extracellular volume (liters; placebo 18.87 +/- 0.85; GH + placebo 20.43 +/- 1.01) but this effect was abolished by captopril (GH + captopril 18.82 +/- 0.67) and spironolactone (GH + spironolactone 18.99 +/- 0.85). Correspondingly, the GH-induced reduction in bioimpedance was blocked by captopril and spironolactone. Plasma renin and angiotensin II concentrations increased during all three GH treatment regimens, whereas plasma aldosterone was increased only after GH plus spironolactone. The data demonstrate that GH activates the RAAS and that blockade of the RAAS by two separate mechanisms prevents fluid retention normally encountered after GH exposure. These observations suggest that the RAAS plays a key role in GH-induced regulation of fluid homeostasis.

Body fluids, circadian blood pressure and plasma renin during growth hormone administration: a placebo-controlled study with two growth hormone doses in healthy adults

Side effects that can be related to fluid retention are common during the initial phases of growth hormone (GH) administration. The aim of this study was to examine the changes in body fluid compartments, diurnal blood pressure and plasma renin concentration during GH administration with two different dosages in healthy adults. Eight healthy male subjects aged 24-32 years were examined during three 2-week study periods in a double-blind placebo controlled study. They received, in random order, GH (3 or 6 IU m-2 daily) or placebo during 2 weeks. Bio-impedance was measured every 2nd day, and extracellular volume (ECV) and plasma volume (PV) were isotopically determined at day 6. Blood samples were obtained regularly. Diurnal blood pressure was recorded and 24-h urinary samples were collected at days 0, 6 and 14. ECV (l) was increased by GH (placebo, 19.58 +/- 0.82; 3 IU m-2, 20.77 +/- 1.22; 6 IU m-2, 20.65 +/- 0.94; p<0.01), whereas PV (l) was unaffected (placebo, 3.91+/- 0.20; 3 IU m-2, 4.04 +/- 0.22; 6 IU m-2, 3.90 +/- 0.27). Total body water (l) increased significantly during GH administration (placebo, 50.8 +/- 2.6; 3 IU m-2, 52.6 +/- 2.3; 6 IU m-2, 53.9 +/- 1.8, p<0.05). After 6 days of treatment a significant increase in renin (p = 0.03) was observed. Mean diurnal blood pressure levels remained unchanged, whereas mean diurnal heart rate (min-1) increased significantly (placebo, 75 +/- 3.6; 3 IU m-2, 79 +/- 3.2; 6 IU m-2, 79 +/- 3.7; p<0.01). In conclusion, GH administration induces an elevation in total body water which may involve a stimulation of plasma renin and an increased ECV without any changes in PV or diurnal blood pressure.

Effect of inhibition of converting enzyme on renal hemodynamics and sodium management in polycystic kidney disease

We compared the tubular transport of sodium and the erythrocyte sodium-lithium countertransport activity in hypertensive patients with autosomal dominant polycystic kidney disease (ADPKD) and in normotensive control subjects. In addition, we assessed the effects of inhibition of converting enzyme on renal hemodynamics and sodium excretion in hypertensive patients with ADPKD to provide information on mechanisms responsible for the increased renal vascular resistance and filtration fraction and the adjustment of the pressure-natriuresis relationship during saline expansion, observed in patients with ADPKD, hypertension, and preserved renal function. In comparison with normotensive control subjects, the hypertensive patients with ADPKD had lower renal plasma flows, higher renal vascular resistances and filtration fractions, and similar proximal and distal fractional reabsorptions of sodium. The administration of enalapril resulted in significant increases in the renal plasma flow and significant reductions in mean arterial pressure, renal vascular resistance, and filtration fraction, but the glomerular filtration rate remained unchanged. Despite the significant reduction in mean arterial pressure during inhibition of converting enzyme, the distal fractional reabsorption of sodium decreased while the total fractional excretion of sodium remained unchanged or increased slightly. No significant differences were detected between the normotensive control subjects and the hypertensive patients with ADPKD in erythrocyte sodium-lithium countertransport activity, plasma renin activity, plasma aldosterone concentration, or atrial natriuretic factor. These results suggest that the renal renin-angiotensin system plays a central role in the alterations in renal hemodynamics and sodium management associated with the development of hypertension in ADPKD.

Renal structure and hypertension in autosomal dominant polycystic kidney disease

Hypertension has been reported to occur in 50 to 75 percent of subjects with autosomal dominant polycystic kidney disease (ADPKD) prior to the onset of marked renal insufficiency but concurrent with cystic deformation of the renal parenchyma. The present study was undertaken to examine whether the renal structural abnormalities are greater in hypertensive (HBP) versus normotensive (NBP) male and female patients with ADPKD who were matched within gender groups for age, body surface area, serum creatinine concentration (males HBP 1.2 +/- 0.02 vs. NBP 1.1 +/- 0.03 mg/dl. NS: females HBP 0.9 +/- 0.03 vs. NBP 0.9 +/- 0.02 mg/dl, NS) and creatinine clearance (males HBP 100 +/- 3 vs. NBP 108 +/- 3 ml/min/1.73 m2, NS: females HBP 97 +/- 3 vs. NBP 96 +/- 2 ml/min/1.73 m2, NS). Renal volume was significantly greater in the HBP compared to the NBP group (males HBP 624 +/- 47 vs. NBP 390 +/- 43 cm3, P less than 0.0005; females HBP 446 +/- 32 vs. NBP 338 +/- 24 cm3, P less than 0.002). Since increased renal volume is due to increased cysts, the results indicate that the early high incidence of hypertension in ADPKD correlates with the renal structural abnormalities in this disorder.

The renin-angiotensin-aldosterone system and autosomal dominant polycystic kidney disease

BACKGROUND

A high incidence of hypertension (50 to 75 percent) occurs early in the course of autosomal dominant polycystic kidney disease. Cyst enlargement, causing bilateral renal ischemia and subsequent release of renin, is proposed as the cause of this form of hypertension.

METHODS

To investigate this hypothesis, we measured plasma renin activity and aldosterone concentrations during short-term and long-term converting-enzyme inhibition in 14 patients with hypertension due to polycystic kidney disease, 9 patients with essential hypertension, 11 normotensive patients with polycystic kidney disease, and 13 normal subjects. The groups were comparable with respect to age, sex, body-surface area, degree of hypertension, sodium excretion, and renal function.

RESULTS

During the short-term study, the mean (+/- SE) plasma renin activity was significantly higher in the hypertensive patients with polycystic kidney disease than in the patients with essential hypertension, in the supine (0.36 +/- 0.06 vs. 0.22 +/- 0.06 ng per liter.second, P = 0.05) and upright positions (1.03 +/- 0.14 vs. 0.61 +/- 0.08 ng per liter.second, P less than 0.03) and after converting-enzyme inhibition (1.97 +/- 0.28 vs. 0.67 +/- 0.17 ng per liter.second, P less than 0.0006). The mean arterial pressures measured in the supine and upright positions and the plasma aldosterone concentrations measured in the upright position were significantly higher in the normotensive patients with polycystic kidney disease than in the normal subjects. After six weeks of converting-enzyme inhibition, renal plasma flow increased (P less than 0.005), and both renal vascular resistance (P less than 0.007) and the filtration fraction (P less than 0.02) decreased significantly in the hypertensive patients with polycystic kidney disease but not in the patients with essential hypertension.

CONCLUSIONS

The renin-angiotensin-aldosterone system is stimulated significantly more in hypertensive patients with polycystic kidney disease than in comparable patients with essential hypertension. The increased renin release, perhaps due to renal ischemia caused by cyst expansion, probably contributes to the early development of hypertension in polycystic kidney disease.

Natriuretic response to volume expansion in polycystic kidney disease

Hypertension is an early manifestation of autosomal dominant polycystic kidney disease (ADPKD). Whether polycystic kidneys have an intrinsic abnormality that leads to sodium retention, volume expansion, and hypertension is uncertain. We studied the natriuretic response to a 4-hour infusion of physiologic saline at a rate of 6.5 ml/kg per hour in 10 patients with ADPKD who had normal renal function and 10 gender- and age-matched control subjects. Baseline 24-hour urinary excretions of sodium and potassium were similar in both groups. The baseline filtration fraction was significantly higher in the patients with ADPKD than in the control subjects. During the infusion of saline, no significant changes in blood pressure, clearance of inulin, or clearance of p-aminohippuric acid were detected. The increase in fractional excretion of sodium over baseline was significantly higher in the patients with ADPKD than in the control subjects. The pressure-natriuresis regression line was significantly shifted to the right in patients with ADPKD who had hypertension. The fractional excretion of potassium was significantly lower in patients with ADPKD than in control subjects. No significant differences in plasma renin activity, aldosterone, or atrial natriuretic factor were detected between the two groups. These observations suggest the presence of subtle abnormalities in the management of renal sodium that might contribute to the development and maintenance of hypertension in patients with ADPKD.

Atrial natriuretic factor in mild to moderate chronic renal failure

The relationship between kidney function and plasma immunoreactive atrial natriuretic factor (irANF) levels as well as the effects of synthetic human ANF-(99-126) were investigated in 13 patients with mild to moderate chronic renal failure. Under basal conditions, glomerular filtration rate averaged 39 +/- 5 (SEM) ml/min/1.73 m2 and blood pressure (BP) averaged 166/107 +/- 7/2 mm Hg; 12 patients were hypertensive. Plasma irANF levels were significantly increased (98 +/- 16 vs 42 +/- 4 pg/ml in healthy control subjects; p less than 0.001) and correlated (p less than 0.05-0.005) inversely with hematocrit (r = -0.65) and positively with systolic BP (r = 0.75) or fractional sodium excretion (r = 0.75). Human ANF-(99-126) infusion for 45 minutes at 0.034 microgram/kg/min augmented (p less than 0.05-0.01) diuresis and urinary sodium, chloride, calcium, phosphate, and magnesium excretion. During the subsequent 45 minutes of human ANF-(99-126) infusion at a rate of 0.077 microgram/kg/min, diuresis and electrolyte excretion remained elevated (p less than 0.05-0.01). Glomerular filtration rate and effective renal plasma flow were not significantly modified, but filtration fraction rose progressively (p less than 0.01). Human ANF-(99-126) infusion decreased BP (p less than 0.05-0.01), produced hemoconcentration (hematocrit + 7%; p less than 0.01) without negative body fluid balance, and increased (p less than 0.01-0.001) plasma norepinephrine, insulin, and serum free fatty acids; plasma aldosterone and renin activity were unaltered during but rose after cessation of human ANF-(99-126) infusion.(ABSTRACT TRUNCATED AT 250 WORDS)