Tubular hypertrophy due to work load induced by furosemide is associated with increases of IGF-1 and IGFBP-1

Cell Hypertrophy: A “Biophysical Roadblock” to Reversing Kidney Injury

In anamniotes cell loss can typically be compensated for through proliferation, but in amniotes, this capacity has been significantly diminished to accommodate tissue complexity. In order to cope with the increased workload that results from cell death, instead of proliferation highly specialised post-mitotic cells undergo polyploidisation and hypertrophy. Although compensatory hypertrophy is the main strategy of repair/regeneration in various parenchymal tissues, the long-term benefits and its capacity to sustain complete recovery of the kidney has not been addressed sufficiently. In this perspective article we integrate basic principles from biophysics and biology to examine whether renal cell hypertrophy is a sustainable adaptation that can efficiently regenerate tissue mass and restore organ function, or a maladaptive detrimental response.

Loop Diuretics in Infants with Heart Failure

Tremendous advances have been made in the last 5 decades in the surgical management of congenital heart disease (CHD). Most infants affected by clinically significant CHD are at risk for developing heart failure (HF). Adult HF management is mostly evidence-based and our knowledge in this field has expanded significantly in the past decade. However, data on management approaches for HF in infants are limited. The indications and implications for various medications and interventions in patients with HF secondary to CHD are an upcoming area of interest. It is critical that we expand our ability to prevent, detect, and manage HF in the pediatric population.

Distal convoluted tubule

The distal convoluted tubule (DCT) is a short nephron segment, interposed between the macula densa and collecting duct. Even though it is short, it plays a key role in regulating extracellular fluid volume and electrolyte homeostasis. DCT cells are rich in mitochondria, and possess the highest density of Na+/K+-ATPase along the nephron, where it is expressed on the highly amplified basolateral membranes. DCT cells are largely water impermeable, and reabsorb sodium and chloride across the apical membrane via electroneurtral pathways. Prominent among this is the thiazide-sensitive sodium chloride cotransporter, target of widely used diuretic drugs. These cells also play a key role in magnesium reabsorption, which occurs predominantly, via a transient receptor potential channel (TRPM6). Human genetic diseases in which DCT function is perturbed have provided critical insights into the physiological role of the DCT, and how transport is regulated. These include Familial Hyperkalemic Hypertension, the salt-wasting diseases Gitelman syndrome and EAST syndrome, and hereditary hypomagnesemias. The DCT is also established as an important target for the hormones angiotensin II and aldosterone; it also appears to respond to sympathetic-nerve stimulation and changes in plasma potassium. Here, we discuss what is currently known about DCT physiology. Early studies that determined transport rates of ions by the DCT are described, as are the channels and transporters expressed along the DCT with the advent of molecular cloning. Regulation of expression and activity of these channels and transporters is also described; particular emphasis is placed on the contribution of genetic forms of DCT dysregulation to our understanding.

Treatment of hypertension in obese patients

Obesity is a global pandemic and with its rise, its associated co-morbidities are increasing in prevalence, particularly uncontrolled hypertension. Lifestyle changes should be an anchor for the management of obesity-related hypertension; however, they are difficult to sustain. Drug therapy is often necessary to achieve blood pressure control. Diuretics, inhibitors of the renin-angiotensin system, and dihydropyridine calcium channel blockers are often used as first trio, with subsequent additions of mineralocorticoid receptor antagonists and/or dual alpha/beta blocking agents. While a number of agents are currently available, 50 % of hypertensive patients remain uncontrolled. A number of novel drug and invasive therapies are in development and hold significant potential for the effective management of obesity-related hypertension.

Diuretics in heart failure: practical considerations

This review discusses the role of diuretics in heart failure by focusing on different classifications and mechanisms of action. Pharmacodynamic and pharmacokinetic properties of diuretics are elucidated. The predominant discussion highlights the use of loop diuretics, which are the most commonly used drugs in heart failure. Different methods of using this therapy in different settings along with a comprehensive review of the side-effect profile are highlighted. Special situations necessitating adjustment and the phenomenon of diuretic resistance are explained.

Update in diuretic therapy: clinical pharmacology

All diuretics except spironolactone exert their effects from the lumen of the nephron. Thus, to exert an effect, they must reach the urine. Pharmacokinetics (PK) describes this access. Different edematous disorders can affect access to this site of action and therein affect response to a diuretic. In addition, once a diuretic reaches the site of action, a response ensues. The characteristics of this response that can be affected by a patient's clinical condition are described by the pharmacodynamics (PD) of a diuretic. To understand the mechanisms of abnormal response to a diuretic one must dissect its PK and PD in different edematous disorders. For example, in patients with renal insufficiency, the mechanism of poor diuretic response is PK. In contrast, in patients with cirrhosis or in those with congestive heart failure, it is PD. In patients with nephrotic syndrome, both PK and PD are operative. These different mechanisms mandate differences in therapeutic strategy, as explained in this article.

Resistant hypertension: concepts and approach to management

Resistant hypertension (RH), defined simply, is blood pressure (BP) requiring the use of four or more antihypertensive agents, whether controlled or uncontrolled. RH is an increasingly common problem in elderly patients and may affect as many as 20% of the hypertensive population. Unfortunately, at least 30% of patients evaluated for RH are actually adequately controlled when more carefully assessed by home BP monitoring or ambulatory BP monitoring, thus representing a white coat effect. It is also essential to exclude pseudoresistance resulting from improper BP recording techniques or failure of the patient to adhere to the prescribed treatment regimen. Concurrent use of drugs that may interfere with prescribed antihypertensive agents, including many over the counter herbal preparations, must also be excluded. The underlying mechanisms principally driving true RH include pathophysiologic abnormalities of aldosterone signaling, sodium and water retention, excessive sympathetic nervous system activity, and obstructive sleep apnea. Appropriate treatment regimens will usually include an inhibitor of the renin-angiotensin-aldosterone system, a calcium channel blocker, and a diuretic. An aldosterone receptor blocker can be instituted at any step, and is very effective as a fourth drug. Beta-blockers can also be integrated into these treatment plans and may be especially helpful when excessive sympathetic nervous system activity is suspected. Novel device therapies that interrupt sympathetic nerve stimulation at the carotid sinus and kidney are under investigation, and may add entirely new directions in the management of RH. What is most important is that treatment regimens should be targeted to specific patient profiles.

Successful drug development despite adverse preclinical findings part 1: processes to address issues and most important findings

Unexpected adverse preclinical findings (APFs) are not infrequently encountered during drug development. Such APFs can be functional disturbances such as QT prolongation, morphological toxicity or carcinogenicity. The latter is of particular concern in conjunction with equivocal genotoxicity results. The toxicologic pathologist plays an important role in recognizing these effects, in helping to characterize them, to evaluate their risk for man, and in proposing measures to mitigate the risk particularly in early clinical trials. A careful scientific evaluation is crucial while termination of the development of a potentially useful drug must be avoided. This first part of the review discusses processes to address unexpected APFs and provides an overview over typical APFs in particular classes of drugs. If the mode of action (MoA) by which a drug candidate produces an APF is known, this supports evaluation of its relevance for humans. Tailor-made mechanistic studies, when needed, must be planned carefully to test one or several hypotheses regarding the potential MoA and to provide further data for risk evaluation. Safety considerations are based on exposure at no-observed-adverse-effect levels (NOAEL) of the most sensitive and relevant animal species and guide dose escalation in clinical trials. The availability of early markers of toxicity for monitoring of humans adds further safety to clinical studies. Risk evaluation is concluded by a weight of evidence analysis (WoE) with an array of parameters including drug use, medical need and alternatives on the market. In the second part of this review relevant examples of APFs will be discussed in more detail.

Inhibition of mTOR signaling with rapamycin attenuates renal hypertrophy in the early diabetic mice

Early diabetic nephropathy is characterized by renal hypertrophy that is mainly due to proximal tubular hypertrophy. Mammalian target of rapamycin (mTOR) is a serine/threonine protein kinase, and its signaling has been reported to regulate protein synthesis and cellular growth, specifically, hypertrophy. Therefore, we examined the effect of mTOR signaling on diabetic renal hypertrophy by using the specific inhibitor for mTOR, rapamycin. Ten days after streptozotocin-induced diabetes, mice showed kidney hypertrophy with increases in the phosphorylation of p70S6kinase and the expression of cyclin kinase inhibitors, p21(Cip1) and p27(Kip1), in the kidneys. The intraperitoneal injection of rapamycin (2 mg/kg/day) markedly attenuated the enhanced phosphorylation of p70S6kinase, the increment of cyclin-dependent kinase inhibitors, and renal enlargement without any changes of clinical parameters, including blood glucose, blood pressure, and food intake. Overexpression of a constitutive active form of p70S6kinase resulted in increased cell size of cultured mouse proximal tubule cells; thus, activation of p70S6kinase causes hypertrophy of proximal tubular cells. Our findings suggest that activation of mTOR signaling causes renal hypertrophy at the early stage of diabetes.

The rational use of diuretics in heart failure

Congestive heart failure is a progressive hemodynamic disorder associated with significant morbidity and mortality. Concomitant renal dysfunction is frequently seen in patients with heart failure, and can compromise fluid regulation, leading to acute decompensation, and increased morbidity and mortality. Diuretic therapy has been the mainstay for treatment of congestive symptoms, despite documented mortality benefits. Misuse or overuse of diuretics can have negative consequences in heart failure, and optimizing diuretic efficiency may improve outcomes. In addition, new agents targeting elevated neuropeptides may prove to be beneficial in regulating fluid status and optimizing renal function.

Sodium and calcium transport pathways along the mammalian distal nephron: from rabbit to human

The final adjustment of renal sodium and calcium excretion is achieved by the distal nephron, in which transepithelial ion transport is under control of various hormones, tubular fluid composition, and flow rate. Acquired or inherited diseases leading to deranged renal sodium and calcium balance have been linked to dysfunction of the distal nephron. Diuretic drugs elicit their effects on sodium balance by specifically inhibiting sodium transport proteins in the apical plasma membrane of distal nephron segments. The identification of the major apical sodium transport proteins allows study of their precise distribution pattern along the distal nephron and helps address their cellular and molecular regulation under various physiological and pathophysiological settings. This review focuses on the topological arrangement of sodium and calcium transport proteins along the cortical distal nephron and on some aspects of their functional regulation. The availability of data on the distribution of transporters in various species points to the strengths, as well as to the limitations, of animal models for the extrapolation to humans.

Loop diuretics: from the Na-K-2Cl transporter to clinical use

The diuretic response to loop diuretics in various disease states has consistently been found to be subnormal. One of the key determinants of the degree of diuretic response is the functional integrity of the sodium-potassium-chloride transporter in the loop of Henle. Studies in animal models suggest that expression/activity of the transporter may be affected by factors such as altered natural splicing events of NKCC2 (the gene encoding for the renal transporter), renal prostanoids, vasopressin, and other autacoids. We have reviewed the pharmacokinetics and pharmacodynamics of loop diuretics in health and in edematous disorders for which they are used. On the basis of evidence reviewed in this paper, we propose that altered expression or activity of the sodium-potassium-chloride transporter in the loop of Henle, in conjunction with events occurring in other segments of the nephron, possibly accounts for the altered diuretic response to these agents. Thus the modulators of this altered expression/activity could serve as important therapeutic targets for alternative diuretic regimens in these conditions.

Human and murine phenotypes associated with defects in cation-chloride cotransport

The diuretic-sensitive cotransport of cations with chloride is mediated by the cation-chloride cotransporters, a large gene family encompassing a total of seven Na-Cl, Na-K-2Cl, and K-Cl cotransporters, in addition to two related transporters of unknown function. The cation-chloride cotransporters perform a wide variety of physiological roles and differ dramatically in patterns of tissue expression and cellular localization. The renal-specific Na-Cl cotransporter (NCC) and Na-K-2Cl cotransporter (NKCC2) are involved in Gitelman and Bartter syndrome, respectively, autosomal recessive forms of metabolic alkalosis. The associated phenotypes due to loss-of-function mutations in NCC and NKCC2 are consistent, in part, with their functional roles in the distal convoluted tubule and thick ascending limb, respectively. Other cation-chloride cotransporters are positional candidates for Mendelian human disorders, and the K-Cl cotransporter KCC3, in particular, may be involved in degenerative peripheral neuropathies linked to chromosome 15q14. The characterization of mice with both spontaneous and targeted mutations of several cation-chloride cotransporters has also yielded significant insight into the physiological and pathophysiological roles of several members of the gene family. These studies implicate the Na-K-2Cl cotransporter NKCC1 in hearing, salivation, pain perception, spermatogenesis, and the control of extracellular fluid volume. Targeted deletion of the neuronal-specific K-Cl cotransporter KCC2 generates mice with a profound seizure disorder and confirms the central role of this transporter in modulating neuronal excitability. Finally, the comparison of human and murine phenotypes associated with loss-of-function mutations in cation-chloride cotransporters indicates important differences in physiology of the two species and provides an important opportunity for detailed physiological and morphological analysis of the tissues involved.

Quantitative assessment of rat kidney function by measuring the clearance of the contrast agent Gd(DOTA) using dynamic MRI

Magnetic resonance imaging (MRI) has been applied to assess kidney function in normal rats by monitoring the passage of the extracellular contrast agent GdDOTA. High-resolution images have been obtained using either the rapid acquisition with relaxation enhancement (RARE) or the snapshot pulse sequence. The latter was superior in anatomic definition due to the shorter echo delays used. The GdDOTA induced signal enhancements in the various renal structures were theoretically modeled and the results of the regression analysis then used to estimate local tissue concentrations in renal cortex, inner medulla and outer medulla/pelvis. The concentration-time curves in vena cava and renal cortex were similar and distinctly different from the ones in medulla and pelvis. This is reflected in the time-to-peak (TTP) values, which were TTP (blood) = 0.18 +/- 0.03 < TTP (cortex) = 0.26 +/- 0.05 < TTP (outer medulla) = 0.62 +/- 0.03 < TTP (inner medulla/pelvis) = 0.92 +/- 0.16 min. The initial tracer uptake rates depended linearly on the dose of GdDOTA administered, the value of the uptake rate in the cortex being significantly higher than those in the outer and inner medulla, which were identical within error limits. The initial medullar tracer uptake followed a first-order kinetics. The rate constant k(cl) = (dc[medulla]/dt)/c[cortex] = 3.4 +/- 0.5 min(-1) for the transition from cortex (predominantly blood signal) to medulla (predominantly urine) was considered a measure for the renal clearance. Intravenous administration of furosemide at doses 2.5, 5, and 10 mg/kg led to a dose-dependent decrease of k(cl). This reflects the inhibitory effect of the diuretic furosemide on medullary water resorption and thus the dilution of the GdDOTA in urine.

Mammalian distal tubule: physiology, pathophysiology, and molecular anatomy

The distal tubule of the mammalian kidney, defined as the region between the macula densa and the collecting duct, is morphologically and functionally heterogeneous. This heterogeneity has stymied attempts to define functional properties of individual cell types and has led to controversy concerning mechanisms and regulation of ion transport. Recently, molecular techniques have been used to identify and localize ion transport pathways along the distal tubule and to identify human diseases that result from abnormal distal tubule function. Results of these studies have clarified the roles of individual distal cell types. They suggest that the basic molecular architecture of the distal nephron is surprisingly similar in mammalian species investigated to date. The results have also reemphasized the role played by the distal tubule in regulating urinary potassium excretion. They have clarified how both peptide and steroid hormones, including aldosterone and estrogen, regulate ion transport by distal convoluted tubule cells. Furthermore, they highlight the central role that the distal tubule plays in systemic calcium homeostasis. Disorders of distal nephron function, such as Gitelman's syndrome, nephrolithiasis, and adaptation to diuretic drug administration, emphasize the importance of this relatively short nephron segment to human physiology. This review integrates molecular and functional results to provide a contemporary picture of distal tubule function in mammals.

Gene expression of the insulin-like growth factor system during mouse kidney development

Expression of the insulin-like growth factor (IGF) system was investigated in mouse renal development and physiology, using non radioactive in situ hybridization and quantitative RT-PCR. IGF-I mRNA levels increased after birth and were confined to distal tubules and peritubular capillaries in the outer medulla. IGF-II mRNA levels were high in developing kidneys and peaked after birth. The type I receptor mRNA expression pattern mostly parallelled those of IGF-I and IGF-II. The IGF binding proteins (IGFBP's) showed weak mRNA expression for IGFBP-1 and -6. High fetal mRNA levels were measured for IGFBP-2, showing a similar profile in time as observed for IGF-II. Low fetal IGFBP-3 and -5 mRNA levels increased after birth. IGFBP-2, -4 and -5 mRNA expression was localized to differentiating cells. In the mature kidney predominant expression was confined to proximal tubules (IGFBP-4), thin limbs of Henle's Loop (IGFBP-2), glomerular mesangial cells (IGFBP-5) and peritubular capillaries of the medulla (IGFBP-5). IGFBP-3 mRNA was exclusively expressed in endothelial cells of the renal capillary system. Distinct mRNA expression for each member of the IGF system may point to specific roles in development and physiology of the mouse kidney.

Insulinlike growth factor I and the kidney

Insulinlike growth factor I (IGF-I), IGF-I receptors, and IGF-binding proteins are expressed in different segments of the nephron in a relationship that suggests autocrine, paracrine, and endocrine modes of action. IGF-I contributes to compensatory nephron growth in a variety of experimental renal diseases with loss in functioning nephron number, and to tissue repair after ischemic acute tubular necrosis. IGF-I causes arteriolar dilatation in the kidney and increases the glomerular filtration rate in experimental animals, in normal subjects, as well as in patients with chronic renal failure, and this effect of the peptide is probably mediated by nitric oxide. IGF-I raises proximal tubular phosphate reabsorption and may increase sodium absorption in distal tubules. In the nephrotic syndrome, IGF-I- and IGF-binding protein complexes are excreted in urine and IGFBP-3 protease activity is increased, causing complex abnormalities in the IGF-system.

Growth hormone and nutrition interact to regulate expressions of kidney IGF-I and IGFBP mRNAs

The effects of growth hormone (GH) and fasting on renal insulin-like growth factor-I (IGF-I) and IGF-binding proteins (IGFBP)-1, -2, -3, -4, -5 were examined in spontaneous dwarf rats (SDR) which have a complete and specific lack of GH among pituitary hormones. Renal expression of the mRNA which encodes IGF-I was reduced in these rats, and IGFBP-1 and IGFBP-4 were found to be elevated. Administration of GH restored expression of IGF-I and IGFBP-1 mRNA, suggesting that GH is, among other pituitary hormones, more specifically associated with renal expression of these genes. The elevation in the IGFBP-4 mRNA level, however, was not attenuated by GH administration, indicating that this hormone may not be directly related to the regulation of expression of this gene. Fasting for 48 hours resulted in a reduction of IGF-I mRNA and an increase in IGFBP-1 mRNA in SDRs as well as in normal rats, suggesting that a cause other than a reduced serum GH is responsible for these fasting-induced changes. Fasting resulted in little change in levels of other IGFBP-2, -3, -4, -5 mRNAs. When these results were compared with those obtained using liver, IGFBP mRNA expression was shown to be regulated differently in different tissues. Based on our finding that IGFBP-1 modulates the mitogenic action of IGF-I, a full understanding of nutrition-related growth processes in the kidney must take this relationship into consideration as well as that which exists between GH and IGF-I.