Effects of angiotensin II and nonpeptide receptor antagonists on transduction pathways in rat proximal tubule

Landscape of GPCR expression along the mouse nephron

Kidney transport and other renal functions are regulated by multiple G protein-coupled receptors (GPCRs) expressed along the renal tubule. The rapid, recent appearance of comprehensive unbiased gene expression data in the various renal tubule segments, chiefly RNA sequencing and protein mass spectrometry data, has provided a means of identifying patterns of GPCR expression along the renal tubule. To allow for comprehensive mapping, we first curated a comprehensive list of GPCRs in the genomes of mice, rats, and humans (https://hpcwebapps.cit.nih.gov/ESBL/Database/GPCRs/) using multiple online data sources. We used this list to mine segment-specific and cell type-specific expression data from RNA-sequencing studies in microdissected mouse tubule segments to identify GPCRs that are selectively expressed in discrete tubule segments. Comparisons of these mapped mouse GPCRs with other omics datasets as well as functional data from isolated perfused tubule and micropuncture studies confirmed patterns of expression for well-known receptors and identified poorly studied GPCRs that are likely to play roles in the regulation of renal tubule function. Thus, we provide data resources for GPCR expression across the renal tubule, highlighting both well-known GPCRs and understudied receptors to provide guidance for future studies.

Respiratory-Swallow Pattern Following Total Laryngectomy

The goal of this study was to further characterize respiratory patterns in total laryngectomees with attention to respiratory-swallow pattern as it relates to presence/absence of tracheoesophageal puncture (TEP) and bolus consistency. It was hypothesized that participants with TEP would exhibit respiratory-swallow patterns that were significantly different than those without TEP and that bolus consistency (thin or solid) would modulate respiratory-swallow pattern. Data were collected from 12 adults (8 male), aged 46-67 years (mean 57) status post total laryngectomy (1-30 years; average 6 years). Those actively receiving chemoradiation, with history of esophageal cancer, with neurologic disease, with history of lung cancer, with known or suspected recurrence of head and neck cancer (HNC), or with severe cognitive deficits were excluded. Laryngectomy participants were asked to swallow three sips of water and three bites of graham cracker. Submental surface EMG activity was used to detect swallows and a custom stoma mask in line with a pneumotachograph measured airflow during the swallows. Non-parametric Mann-Whitney test for differences was used to detect significance for our dependent variables, TEP or bolus consistency and independent variables, respiratory-swallow pattern. Laryngectomee's showed preference for swallow during inspiration which is inconsistent with the expiratory pattern preference found in healthy adults with intact larynges by McFarland et al. (Respir Physiol Neurobiol 234:89-96, 2016) but consistent with the pattern preference for inspiration (or non-dominant respiratory-swallow phase pattern) found in the HNC population at-large by Brodsky et al. (J Appl Physiol 112(10):1698-1705, 2012). No significant difference was found in swallow pattern with regards to presence/absence of TEP or bolus consistency.

Relationship Between Laryngeal Sensation, Length of Intubation, and Aspiration in Patients with Acute Respiratory Failure

Dysphagia is common in hospitalized patients post-extubation and associated with poor outcomes. Laryngeal sensation is critical for airway protection and safe swallowing. However, current understanding of the relationship between laryngeal sensation and aspiration in post-extubation populations is limited. Acute respiratory failure patients requiring intensive care unit admission and mechanical ventilation received a Flexible Endoscopic Evaluation of Swallowing (FEES) within 72 h of extubation. Univariate and multivariable analyses were performed to examine the relationship between laryngeal sensation, length of intubation, and aspiration. Secondary outcomes included pharyngolaryngeal secretions, pneumonia, and diet recommendations. One-hundred and three patients met inclusion criteria. Fifty-one patients demonstrated an absent laryngeal adductor reflex (LAR). Altered laryngeal sensation correlated with the presence of secretions (p = 0.004). There was a significant interaction between the LAR, aspiration, and duration of mechanical ventilation. Altered laryngeal sensation was significantly associated with aspiration on FEES only in patients with a shorter length of intubation (p = 0.008). Patients with altered laryngeal sensation were prescribed significantly more restricted liquid (p = 0.03) and solid (p = 0.001) diets. No relationship was found between laryngeal sensation and pneumonia. There is a high prevalence of laryngeal sensory deficits in mechanically ventilated patients post-extubation. Altered laryngeal sensation was associated with secretions, aspiration, and modified diet recommendations especially in those patients with a shorter length of mechanical ventilation. These results demonstrate that laryngeal sensory abnormalities impact the development of post-extubation dysphagia.

The role of renal proximal tubule transport in the regulation of blood pressure

The electrogenic sodium/bicarbonate cotransporter 1 (NBCe1) on the basolateral side of the renal proximal tubule plays a pivotal role in systemic acid-base homeostasis. Mutations in the gene encoding NBCe1 cause severe proximal renal tubular acidosis accompanied by other extrarenal symptoms. The proximal tubule reabsorbs most of the sodium filtered in the glomerulus, contributing to the regulation of plasma volume and blood pressure. NBCe1 and other sodium transporters in the proximal tubule are regulated by hormones, such as angiotensin II and insulin. Angiotensin II is probably the most important stimulator of sodium reabsorption. Proximal tubule AT_1A receptor is crucial for the systemic pressor effect of angiotensin II. In rodents and rabbits, the effect on proximal tubule NBCe1 is biphasic; at low concentration, angiotensin II stimulates NBCe1 via PKC/cAMP/ERK, whereas at high concentration, it inhibits NBCe1 via NO/cGMP/cGKII. In contrast, in human proximal tubule, angiotensin II has a dose-dependent monophasic stimulatory effect via NO/cGMP/ERK. Insulin stimulates the proximal tubule sodium transport, which is IRS2-dependent. We found that in insulin resistance and overt diabetic nephropathy, stimulatory effect of insulin on proximal tubule transport was preserved. Our results suggest that the preserved stimulation of the proximal tubule enhances sodium reabsorption, contributing to the pathogenesis of hypertension with metabolic syndrome. We describe recent findings regarding the role of proximal tubule transport in the regulation of blood pressure, focusing on the effects of angiotensin II and insulin.

Angiotensin II Type 1 Receptor-Dependent GLP-1 and PYY Secretion in Mice and Humans

Angiotensin II (Ang II) is the key hormone mediator of the renin angiotensin system, which regulates blood pressure and fluid and electrolyte balance in the body. Here we report that in the colonic epithelium, the Ang II type 1 receptor is highly and exclusively expressed in enteroendocrine L cells, which produce the gut hormones glucagon-like peptide-1 and peptide YY (PYY). Ang II stimulated glucagon-like peptide-1 and PYY release from primary cultures of mouse and human colon, which was antagonized by the specific Ang II type 1 receptor blocker candesartan. Ang II raised intracellular calcium levels in L cells in primary cultures, recorded by live-cell imaging of L cells specifically expressing the fluorescent calcium sensor GCaMP3. In Ussing chamber recordings, Ang II reduced short circuit currents in mouse distal colon preparations, which was antagonized by candesartan or a specific neuropeptide Y1 receptor inhibitor but insensitive to amiloride. We conclude that Ang II stimulates PYY secretion, in turn inhibiting epithelial anion fluxes, thereby reducing net fluid secretion into the colonic lumen. Our findings highlight an important role of colonic L cells in whole-body fluid homeostasis by controlling water loss through the intestine.

A noninvasive swallowing measurement system using a combination of respiratory flow, swallowing sound, and laryngeal motion

The assessment of swallowing function is important for the prevention of aspiration pneumonia. We developed a new swallowing monitoring system that uses respiratory flow, swallowing sound, and laryngeal motion. We applied this device to 11 healthy volunteers and 10 patients with dysphagia. Videofluoroscopy (VF) was conducted simultaneously with swallowing monitoring using our device. We measured laryngeal rising time (LRT), the time required for the larynx to elevate to the highest position, and laryngeal activation duration (LAD), the duration between the onset of rapid laryngeal elevation and the time when the larynx returned to the lowest position. In addition, we evaluated the coordination between swallowing and breathing. We found that LAD was correlated with a VF-derived parameter, pharyngeal response duration (PRD) in healthy subjects (LAD: 959 ± 259 ms vs. PRD: 1062 ± 149 ms, r = 0.60); however, this correlation was not found in the dysphagia patients. LRT was significantly prolonged in patients (healthy subjects: 320 ± 175 ms vs. patients: 465 ± 295 ms, P < 0.001, t test). Furthermore, frequency of swallowing immediately after inspiration was significantly increased in patients. Therefore, the new device may facilitate the assessment of some aspects of swallowing dysfunction.

Angiotensin II counteracts the effects of cAMP/PKA on NHE3 activity and phosphorylation in proximal tubule cells

Binding of angiotensin II (ANG II) to the AT₁ receptor (AT₁R) in the proximal tubule stimulates Na⁺/H⁺ exchanger isoform 3 (NHE3) activity through multiple signaling pathways. However, the effects of ANG II/AT₁R-induced inihibitory G protein (G_i) activation and subsequent decrease in cAMP accumulation on NHE3 regulation are not well established. We therefore tested the hypothesis that ANG II reduces cAMP/PKA-mediated phosphorylation of NHE3 on serine 552 and, in doing so, stimulates NHE3 activity. Under basal conditions, ANG II stimulated NHE3 activity but did not affect PKA-mediated NHE3 phosphorylation at serine 552 in opossum kidney (OKP) cells. However, in the presence of the cAMP-elevating agent forskolin (FSK), ANG II blocked FSK-induced NHE3 inhibition, reduced intracellular cAMP concentrations, lowered PKA activity, and prevented the FSK-mediated increase in NHE3 serine 552 phosphorylation. All effects of ANG II were blocked by pretreating OKP cells with the AT₁R antagonist losartan, highlighting the contribution of the AT₁R/G_i pathway in ANG II-mediated NHE3 upregulation under cAMP-elevating conditions. Accordingly, G_i inhibition by pertussis toxin treatment decreased NHE3 activity both in vitro and in vivo and, more importantly, prevented the stimulatory effect of ANG II on NHE3 activity in rat proximal tubules. Collectively, our results suggest that ANG II counteracts the effects of cAMP/PKA on NHE3 phosphorylation and inhibition by activating the AT₁R/G_i pathway. Moreover, these findings support the notion that NHE3 dephosphorylation at serine 552 may represent a key event in the regulation of renal proximal tubule sodium handling by ANG II in the presence of natriuretic hormones that promote cAMP accumulation and transporter phosphorylation.

Respiratory disease and the oesophagus: reflux, reflexes and microaspiration

Gastro-oesophageal reflux is associated with a wide range of respiratory disorders, including asthma, isolated chronic cough, idiopathic pulmonary fibrosis, chronic obstructive pulmonary disease and cystic fibrosis. Reflux can be substantial and reach the proximal margins of the oesophagus in some individuals with specific pulmonary diseases, suggesting that this association is more than a coincidence. Proximal oesophageal reflux in particular has led to concern that microaspiration might have an important, possibly even causal, role in respiratory disease. Interestingly, reflux is not always accompanied by typical reflux symptoms, such as heartburn and/or regurgitation, leading many clinicians to empirically treat for possible gastro-oesophageal reflux. Indeed, costs associated with use of acid suppressants in pulmonary disease far outweigh those in typical GERD, despite little evidence of therapeutic benefit in clinical trials. This Review comprehensively examines the possible mechanisms that might link pulmonary disease and oesophageal reflux, highlighting the gaps in current knowledge and limitations of previous research, and helping to shed light on the frequent failure of antireflux treatments in pulmonary disease.

Angiotensin II type 1 receptor antagonists in animal models of vascular, cardiac, metabolic and renal disease

We have reviewed the effects of angiotensin II type 1 receptor antagonists (ARBs) in various animal models of hypertension, atherosclerosis, cardiac function, hypertrophy and fibrosis, glucose and lipid metabolism, and renal function and morphology. Those of azilsartan and telmisartan have been included comprehensively whereas those of other ARBs have been included systematically but without intention of completeness. ARBs as a class lower blood pressure in established hypertension and prevent hypertension development in all applicable animal models except those with a markedly suppressed renin-angiotensin system; blood pressure lowering even persists for a considerable time after discontinuation of treatment. This translates into a reduced mortality, particularly in models exhibiting marked hypertension. The retrieved data on vascular, cardiac and renal function and morphology as well as on glucose and lipid metabolism are discussed to address three main questions: 1. Can ARB effects on blood vessels, heart, kidney and metabolic function be explained by blood pressure lowering alone or are they additionally directly related to blockade of the renin-angiotensin system? 2. Are they shared by other inhibitors of the renin-angiotensin system, e.g. angiotensin converting enzyme inhibitors? 3. Are some effects specific for one or more compounds within the ARB class? Taken together these data profile ARBs as a drug class with unique properties that have beneficial effects far beyond those on blood pressure reduction and, in some cases distinct from those of angiotensin converting enzyme inhibitors. The clinical relevance of angiotensin receptor-independent effects of some ARBs remains to be determined.

[Effect of body position on coordination of breathing and swallowing]

BACKGROUND

To allow passage of food, the swallowing process closes off the larynx and interrupts respiratory flow. Both the timing of the interruption of respiratory flow and the body position can affect the results of the swallowing process.

OBJECTIVE

The effect of body position on the swallowing process and the coordination of breathing and swallowing is investigated.

MATERIALS AND METHOD

A combined EMG/bioimpedance measurement system and a piezoelectric sensor were used to investigate coordination of breathing and swallowing of a range of food consistencies in three different body positions (90°, 45° and 0°) in healthy subjects.

RESULTS

Investigations were carried out on 21 healthy subjects (12 ♂, 9 ♀). 762 swallows were recorded. Changing body position was found to have a statistically significant effect on swallow-related parameters (maximum laryngeal elevation and speed of laryngeal elevation) and breathing pattern (pre- and post-swallow breathing phases). The laryngeal elevation as well as the speed of the laryngeal elevation is influenced significantly by the consistency to be swallowed. The breathing pattern changes from saliva to solid food of inspiration/swallow/inspiration to expiration/swallow/expiration. A change of body position influences the parameters specific for swallowing and the breathing patterns significantly.

CONCLUSIONS

This study demonstrates that body position affects coordination of breathing and swallowing and swallow-related parameters in healthy subjects. Our results indicate that patients should be enabled to adopt a position in which they are sitting at an angle of at least 45°.

AVP-induced increase in AQP2 and p-AQP2 is blunted in heart failure during cardiac remodeling and is associated with decreased AT1R abundance in rat kidney

AIM

The objective was to examine the renal effects of long-term increased angiotensin II and vasopressin plasma levels in early-stage heart failure (HF). We investigated the regulations of the V2 vasopressin receptor, the type 1A angiotensin II receptor, the (pro)renin receptor, and the water channels AQP2, AQP1, AQP3, and AQP4 in the inner medulla of rat kidney.

METHODS

HF was induced by coronary artery ligation. Sixty-eight rats were allocated to six groups: Sham (N = 11), HF (N = 11), sodium restricted sham (N = 11), sodium restricted HF (N = 11), sodium restricted sham + DDAVP (N = 12), and sodium restricted HF + DDAVP (N = 12). 1-desamino-8-D-arginine vasopressin (0.5 ng h-1 for 7 days) or vehicle was administered. Pre- and post-treatment echocardiographic evaluation was performed. The rats were sacrificed at day 17 after surgery, before cardiac remodeling in rat is known to be completed.

RESULTS

HF rats on standard sodium diet and sodium restriction displayed biochemical markers of HF. These rats developed hyponatremia, hypo-osmolality, and decreased fractional excretion of sodium. Increase of AQP2 and p(Ser256)-AQP2 abundance in all HF groups was blunted compared with control groups even when infused with DDAVP and despite increased vasopressin V2 receptor and Gsα abundance. This was associated with decreased protein abundance of the AT1A receptor in HF groups vs. controls.

CONCLUSION

Early-stage HF is associated with blunted increase in AQP2 and p(Ser256)-AQP2 despite of hyponatremia, hypo-osmolality, and increased inner medullary vasopressin V2 receptor expression. Decreased type 1A angiotensin II receptor abundance likely plays a role in the transduction of these effects.

Estrogen directly and specifically downregulates NaPi-IIa through the activation of both estrogen receptor isoforms (ERα and ERβ) in rat kidney proximal tubule

We have previously demonstrated that estrogen (E2) downregulates phosphate transporter NaPi-IIa and causes phosphaturia and hypophosphatemia in ovariectomized rats. In the present study, we examined whether E2 directly targets NaPi-IIa in the proximal tubule (PT) and studied the respective roles of estrogen receptor isoforms (ERα and ERβ) in the downregulation of NaPi-IIa using both in vivo and an in vitro expression systems. We found that estrogen specifically downregulates NaPi-IIa but not NaPi-IIc or Pit2 in the kidney cortex. Proximal tubules incubated in a "shake" suspension with E2 for 24 h exhibited a dose-dependent decrease in NaPi-IIa protein abundance. Results from OVX rats treated with specific agonists for either ERα [4,4',4″;-(4-propyl-[1H]-pyrazole-1,3,5-triyl) trisphenol, PPT] or ERβ [4,4',4″-(4-propyl-[1H]-pyrazole-1,3,5-triyl) trisphenol, DPN] or both (PPT + DPN), indicated that only the latter caused a sharp downregulation of NaPi-IIa, along with significant phosphaturia and hypophosphatemia. Lastly, heterologous expression studies demonstrated that estrogen downregulated NaPi-IIa only in U20S cells expressing both ERα and ERβ, but not in cells expressing either receptor alone. In conclusion, these studies demonstrate that rat PT cells express both ERα and ERβ and that E2 induces phosphaturia by directly and specifically targeting NaPi-IIa in the PT cells. This effect is mediated via a mechanism involving coactivation of both ERα and ERβ, which likely form a functional heterodimer complex in the rat kidney proximal tubule.

Roles of renal proximal tubule transport in acid/base balance and blood pressure regulation

Sodium-coupled bicarbonate absorption from renal proximal tubules (PTs) plays a pivotal role in the maintenance of systemic acid/base balance. Indeed, mutations in the Na⁺-HCO₃⁻ cotransporter NBCe1, which mediates a majority of bicarbonate exit from PTs, cause severe proximal renal tubular acidosis associated with ocular and other extrarenal abnormalities. Sodium transport in PTs also plays an important role in the regulation of blood pressure. For example, PT transport stimulation by insulin may be involved in the pathogenesis of hypertension associated with insulin resistance. Type 1 angiotensin (Ang) II receptors in PT are critical for blood pressure homeostasis. Paradoxically, the effects of Ang II on PT transport are known to be biphasic. Unlike in other species, however, Ang II is recently shown to dose-dependently stimulate human PT transport via nitric oxide/cGMP/ERK pathway, which may represent a novel therapeutic target in human hypertension. In this paper, we will review the physiological and pathophysiological roles of PT transport.

Noninvasive ventilation and breathing-swallowing interplay in chronic obstructive pulmonary disease*

OBJECTIVES

To investigate breathing-swallowing interactions in patients with chronic obstructive pulmonary disease requiring noninvasive mechanical ventilation and, if needed, to develop a technical modification of the ventilator designed to eliminate ventilator insufflations during swallowing.

DESIGN

We conducted a prospective, open-label, interventional study.

PATIENTS

Fifteen consecutive chronic obstructive pulmonary disease patients with exacerbations requiring ICU admission and NIV.

INTERVENTIONS

Swallowing performance and breathing-swallowing interactions were investigated noninvasively by chin electromyography, cervical piezoelectric sensor, and inductive respiratory plethysmography. Two water-bolus sizes (5 and 10 mL) were tested in random order. Swallowing was tested with and without noninvasive mechanical ventilation, in random order. First, a standard mechanical ventilator capable of delivering noninvasive mechanical ventilation was used. Second, a marketed device was equipped with an off-switch for use during swallowing.

MEASUREMENTS AND MAIN RESULTS

Swallowing performance and breathing-swallowing interactions were investigated noninvasively by chin electromyography, cervical piezoelectric sensor, and inductive respiratory plethysmography. Two water bolus sizes (5 and 10 mL) were tested in random order. Swallowing was tested with and without noninvasive mechanical ventilation in random order. First, a standard mechanical ventilator capable of delivering noninvasive mechanical ventilation was used. Swallowing efficiency, breathing-swallowing synchronization, and Borg Scale dyspnea scores improved significantly with noninvasive mechanical ventilation. However, swallowing induced ventilator triggering followed by autotriggering. To improve patient-ventilator synchrony, a marketed device was equipped with an off-switch for use during swallowing. This device completely eliminated swallowing-induced ventilator triggering and postswallow autotriggering.

CONCLUSION

Patients with chronic obstructive pulmonary disease admitted to the ICU for acute exacerbations had abnormal breathing-swallowing interactions and dyspnea, which improved with noninvasive mechanical ventilation. Furthermore, a ventilator device with a simple switch-off pushbutton to eliminate insufflations during swallows prevented swallowing-induced ventilator triggering and postswallow autotriggering.

Angiotensin II dose-dependently stimulates human renal proximal tubule transport by the nitric oxide/guanosine 3',5'-cyclic monophosphate pathway

Stimulation of renal proximal tubule (PT) transport by angiotensin II (Ang II) is critical for regulation of BP. Notably, in rats, mice, and rabbits, the regulation of PT sodium transport by Ang II is biphasic: transport is stimulated by picomolar to nanomolar concentrations of Ang II but inhibited by nanomolar to micromolar concentrations of Ang II. However, little is known about the effects of Ang II on human PT transport. By functional analysis with isolated PTs obtained from nephrectomy surgery, we found that Ang II induces a dose-dependent profound stimulation of human PT transport by type 1 Ang II receptor (AT1)-dependent phosphorylation of extracellular signal-regulated kinase (ERK). In PTs of wild-type mice, the nitric oxide (NO) /cGMP/cGMP-dependent kinase II (cGKII) pathway mediated the inhibitory effect of Ang II. In PTs of cGKII-deficient mice, the inhibitory effect of Ang II was lost, but activation of the NO/cGMP pathway failed to phosphorylate ERK. Conversely, in human PTs, the NO/cGMP pathway mediated the stimulatory effect of Ang II by phosphorylating ERK independently of cGKII. These contrasting responses to the NO/cGMP pathway may largely explain the different modes of PT transport regulation by Ang II, and the unopposed marked stimulation of PT transport by high intrarenal concentrations of Ang II may be an important factor in the pathogenesis of human hypertension. Additionally, the previously unrecognized stimulatory effect of the NO/cGMP pathway on PT transport may represent a human-specific therapeutic target in hypertension.

ICU-acquired swallowing disorders

OBJECTIVES

Patients hospitalized in the ICU can frequently develop swallowing disorders, resulting in an inability to effectively transfer food, liquids, and pills from their mouth to stomach. The complications of these disorders can be devastating, including aspiration, reintubation, pneumonia, and a prolonged hospital length of stay. As a result, critical care practitioners should understand the optimal diagnostic strategies, proposed mechanisms, and downstream complications of these ICU-acquired swallowing disorders.

DATA SOURCES

Database searches and a review of the relevant medical literature.

DATA SYNTHESIS

A significant portion of the estimated 400,000 patients who annually develop acute respiratory failure, require endotracheal intubation, and survive to be extubated are determined to have dysfunctional swallowing. This group of swallowing disorders has multiple etiologies, including local effects of endotracheal tubes, neuromuscular weakness, and an altered sensorium. The diagnosis of dysfunctional swallowing is usually made by a speech-language pathologist using a bedside swallowing evaluation. Major complications of swallowing disorders in hospitalized patients include aspiration, reintubation, pneumonia, and increased hospitalization. The national yearly cost of swallowing disorders in hospitalized patients is estimated to be over $500 million. Treatment modalities focus on changing the consistency of food, changing mealtime position, and/or placing feeding tubes to prevent aspiration.

CONCLUSIONS

Swallowing disorders are costly and clinically important in a large population of ICU patients. The development of effective screening strategies and national diagnostic standards will enable further studies aimed at understanding the precise mechanisms for these disorders. Further research should also concentrate on identifying modifiable risk factors and developing novel treatments aimed at reducing the significant burden of swallowing dysfunction in critical illness survivors.

A systematic comparison of the properties of clinically used angiotensin II type 1 receptor antagonists

Angiotensin II type 1 receptor antagonists (ARBs) have become an important drug class in the treatment of hypertension and heart failure and the protection from diabetic nephropathy. Eight ARBs are clinically available [azilsartan, candesartan, eprosartan, irbesartan, losartan, olmesartan, telmisartan, valsartan]. Azilsartan (in some countries), candesartan, and olmesartan are orally administered as prodrugs, whereas the blocking action of some is mediated through active metabolites. On the basis of their chemical structures, ARBs use different binding pockets in the receptor, which are associated with differences in dissociation times and, in most cases, apparently insurmountable antagonism. The physicochemical differences between ARBs also manifest in different tissue penetration, including passage through the blood-brain barrier. Differences in binding mode and tissue penetration are also associated with differences in pharmacokinetic profile, particularly duration of action. Although generally highly specific for angiotensin II type 1 receptors, some ARBs, particularly telmisartan, are partial agonists at peroxisome proliferator-activated receptor-γ. All of these properties are comprehensively reviewed in this article. Although there is general consensus that a continuous receptor blockade over a 24-hour period is desirable, the clinical relevance of other pharmacological differences between individual ARBs remains to be assessed.

Alpha2-adrenoceptors coordinate swallowing and respiration

Because the discoordination between swallowing and respiration may cause severe respiratory disorders such as aspiration pneumonia, understanding the neuronal mechanisms underlying such coordination is important. Recently, it was reported that medullary noradrenergic neurons are involved in evoking esophageal-gastric relaxation reflex, leading to a hypothesis that such neurons are also involved in swallowing-respiration coordination. We tested this hypothesis using an in vitro brain-stem preparation obtained from neonatal rats. A temporal inhibition of respiratory rhythm was consistently observed when swallowing activity was induced by electrical stimulations to the supralaryngeal nerve. We found that a broad adrenergic receptor agonist, norepinephrine, markedly blocked the swallowing-induced temporal inhibition of respiration. Further studies revealed that swallowing-induced respiratory inhibition is blocked by an alpha2-adrenergic receptor agonist and enhanced by an alpha2-adrenergic receptor antagonist, indicating an important role of alpha2-adrenergic receptors in regulation of the coordination between swallowing and respiration in vitro.

Arachidonic acid metabolites inhibit the stimulatory effect of angiotensin II in renal proximal tubules

Angiotensin II (Ang II) regulates renal proximal transport in a biphasic way via Ang II type 1 receptor (AT1). Whereas extracellular signal-regulated kinase (ERK) activation mediates the stimulatory effect, cytosolic phospholipase A2 (cPLA2) mediates the inhibitory effect independently of ERK. In this study, we tested the hypothesis that the cPLA2/P450 epoxygenase pathway might work to suppress the Ang II-mediated ERK activation. In the presence of arachidonic acid or 5,6-epoxyeicosatrienoic acid (EET), Ang II failed to stimulate the Na-HCO3 cotransporter activity in renal proximal tubules isolated from wild-type, AT1A-deficient, and cPLA2-alpha-deficient mice. In addition, Ang II failed to induce a significant ERK phosphorylation in the presence of arachidonic acid or 5,6-EET. Arachidonic acid or 5,6-EET also suppressed the stimulatory effect of Ang II on net proximal tubule bicarbonate absorption without changing cell Ca2+ concentrations. These results indicate that the cPLA2-alpha/P450/EET pathway blocks the stimulatory effect of Ang II by suppressing the ERK activation. Thus, the cPLA2-alpha/P450/EET pathway may operate as a unique negative feedback mechanism to attenuate excessive Ang II activity in the renal proximal tubules, where extremely high concentrations of Ang II are found.

Management of communication and swallowing in intensive care: the role of the speech pathologist

Patients in adult, pediatric, and neonatal intensive care settings often require the services of a speech-language pathologist. It is common practice to consult a speech pathologist to determine whether a patient is ready to initiate oral intake or help a patient with an artificial airway communicate. Assessments for dysphagia are initially clinical and conducted at bedside. Results from the clinical evaluation help determine if and when an instrumental examination should be completed. Patients who are tracheostomized, or had been, and those who were intubated for a prolonged period are at risk for aspiration. Speaking valves allow patients to communicate orally. Some studies have shown that speaking valves might also reduce the risk of aspiration with oral intake. Collaboration between speech-language pathologists and critical care nurses is a vital component for ensuring patient safety and success in both communication and eating.

Adenosine and renal tubular function

PURPOSE OF REVIEW

Intrarenal adenosine is present in the cytoplasm of renal epithelial cells and in the extracellular space. Adenosine is generated at high levels in response to imbalance between energy demand and supply (e.g. increased tubular sodium chloride transport or hypoxia) and activates cell membrane adenosine receptors to affect renal vascular and tubular functions. Adenosine regulates renal sodium and water excretion via a myriad of effects on renal hemodynamic, glomerular filtration rate, renin secretion and direct effects on the renal tubule epithelium. This review examines the direct effects of adenosine on renal tubular epithelial transport in light of the most recent evidence and discusses some physiologic and pathophysiologic implications.

RECENT FINDINGS

Intrarenal adenosine affects proximal fluid and solute transport in a biphasic fashion. Under physiological conditions adenosine stimulates proximal tubular re-absorption, thus reducing the load delivered to the distal nephron. A supra-physiologic increase in adenosine such as in ischemia reduces reabsorption in the proximal tubule, thus reducing renal oxygen consumption.

SUMMARY

Intrarenal adenosine and its receptors have important regulatory functions in the renal epithelium. A complete understanding of this autocrine/paracrine system holds great potential for novel therapeutic strategies, such as the use of nucleoside analogues for reno-protection in renal ischemia.

Roles of ERK and cPLA2 in the angiotensin II-mediated biphasic regulation of Na+-HCO3(-) transport

Regulation of renal proximal transport by angiotensin II (Ang II) is biphasic: low concentrations (picomolar to nanomolar) stimulate reabsorption, but higher concentrations (nanomolar to micromolar) inhibit reabsorption. Traditionally, the stimulatory effect has been attributed to activation of protein kinase C and/or a decrease in intracellular cAMP, whereas the inhibitory action has been attributed to the activation of phospholipase A2 (PLA2) and the subsequent release of arachidonic acid. The Ang II receptor subtype responsible for these effects and the intracellular signaling pathways involved are not completely understood. We isolated proximal tubules from wild-type, Ang II type 1A receptor (AT1A)-deficient, and group IVA cytosolic phospholipase A2 (cPLA2alpha)-deficient mice, and compared their responses to Ang II. In wild-type mice, we found that the stimulatory and inhibitory effects of Ang II on Na+-HCO3(-) cotransporter activity are both AT1-mediated but that ERK activation only plays a role in the former. The stimulatory effect of Ang II was also observed in AT1A-deficient mice, suggesting that this occurs through AT1B. In contrast, the inhibitory effects of Ang II appeared to be mediated by cPLA2alpha activation because high-concentration Ang II stimulated Na+-HCO3(-) cotransporter activity when cPLA2alpha activity was abrogated by pharmacological means or genetic knockout. Consistent with this observation, we found that activation of the cPLA2alpha/P450 pathway suppressed ERK activation. We conclude that Ang II activates ERK and cPLA2alpha in a concentration-dependent manner via AT1, and that the balance between ERK and cPLA2alpha activities determines the ultimate response to Ang II in intact proximal tubules.

Effect of reclining and chin-tuck position on the coordination between respiration and swallowing

Chin-tuck position and reclining posture have been used in dysphagia patients to prevent aspiration during swallowing. However, both behavioural treatments may affect respiratory function. This study was carried out to test the hypothesis that if chin-tuck posture and body reclining affected respiratory function, this would be associated with altered coordination between respiration and swallowing. To investigate this hypothesis, respiratory parameters and manometry were used in each of four combinations of reclining posture and chin-tuck position. In the 60 degrees reclining with 60 degrees chin-tuck position, duration of swallowing apnea (0.89 s.d. 0.17 s) and submental electromyography burst (2.34 s.d. 0.84 s) were significantly longer when compared to both upright sitting and 30 degrees reclining positions. We conclude that 60 degrees reclining from vertical with 60 degrees chin-tuck may affect oral processing stages which delay and reduce a variety of oropharyngeal movements. These in turn significantly influence the coordination between respiration and swallowing.

Signaling pathways involved with the stimulatory effect of angiotensin II on vacuolar H+-ATPase in proximal tubule cells

It has been documented that angiotensin II (ANG II) (10(-9) M) stimulates proton extrusion via H(+)-adenosine triphosphatase (ATPase) in proximal tubule cells. In the present study, we investigated the signaling pathways involved in the effects of ANG II on H(+)-ATPase activity and on the cytosolic free calcium concentration in immortalized rat proximal tubule cells, a permanent cell line derived from rat proximal tubules. The effects of ANG on pH(i) and [Ca(+2)](i) were assessed by the fluorescent probes, 2',7-bis (2-carboxyethyl)-5(6)-carboxyfluorescein-acetoxy-methyl ester and fluo-4-acetoxy-methyl ester, in the absence of Na(+) to block the Na(+)/H(+) exchanger. In the control situation, the pH recovery rate following intracellular acidification with NH(4)Cl was 0.073+/-0.011 pH units/min (n=12). This recovery was significantly increased with ANG II (10(-9 )M), to 0.12+/-0.015 pH units/min, n=10. This last effect was also followed by a significant increase of Ca(+2) (i), from 99.72+/-1.704 nM (n=21) to 401.23+/-33.91 nM (n=39). The stimulatory effect of ANG II was blocked in the presence of losartan, an angiotensin II subtype 1 (AT(1)) receptor antagonist. H89 [protein kinase A (PKA) inhibitor] plus ANG II had no effect on the pH recovery. Staurosporine [protein kinase C (PKC) inhibitor] impaired the effect of ANG II. Phorbol myristate acetate (PKC activator) mimicked in part the stimulatory effect of ANG II, but reduced Ca(+2) (i). 1,2-Bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (intracellular calcium chelator) alone reduced the pH(i) recovery rate below control levels and impaired the effect of ANG II, in a way similar to that of trimethoxy benzoate (a blocker of Ca(+2) (i) mobilization). We conclude that ANG II regulates rat proximal tubule vacuolar H(+)-ATPase by a PKA-independent mechanism and that PKC and intracellular calcium play a critical role in this regulation.

Direct regulation of insulin secretion by angiotensin II in human islets of Langerhans

AIMS/HYPOTHESIS

This study aimed to identify the expression of angiotensin II receptors in isolated human islets and beta cells and to examine the functional consequences of their activation.

MATERIALS AND METHODS

Single-cell RT-PCR was used to identify whether human islet cells express mRNA for type 1 angiotensin II receptors (AT(1)), and western blotting was used to determine AT(1) protein expression by human islets and MIN6 beta cells. We measured changes in intracellular calcium by microfluorimetry using Fura 2-loaded MIN6 cells and human islet cells. Dynamic insulin secretory responses were determined by RIA following perifusion of human islets and MIN6 cells.

RESULTS

Human islets expressed mRNAs for both the angiotensin precursor, angiotensinogen, and for angiotensin-converting enzyme. In addition, human and mouse beta cells expressed AT(1). These were functionally coupled to increases in intracellular calcium, which occurred at least in part through phospholipase-C-sensitive mechanisms and calcium influx through voltage-operated calcium channels. Short-term exposure of human islets and MIN6 cells to angiotensin II caused a rapid, short-lived initiation of insulin secretion at 2 mmol/l glucose and potentiation of insulin secretion induced by glucose (at 8 and 16.7 mmol/l).

CONCLUSIONS/INTERPRETATION

These data demonstrate that the AT(1) is expressed by beta cells and that angiotensin II effects a short-lived and direct stimulation of human and mouse beta cells to promote insulin secretion, most probably through elevations in intracellular calcium. Locally produced angiotensin II may be important in regulating a coordinated insulin secretory response from beta cells.

Signaling pathways in the biphasic effect of ANG II on Na+/H+ exchanger in T84 cells

The effect of ANG II on pH(i), [Ca(2+)](i) and cell volume was investigated in T84 cells, a cell line originated from colon epithelium, using the probes BCECF-AM, Fluo 4-AM and acridine orange, respectively. The recovery rate of pH(i) via the Na(+)/H(+) exchanger was examined in the first 2 min following the acidification of pH(i) with a NH(4)Cl pulse. In the control situation, the pH(i) recovery rate was 0.118 +/- 0.001 (n = 52) pH units/min and ANG II (10(-12) M or 10(-9) M) increased this value (by 106% or 32%, respectively) but ANG II (10(-7) M) decreased it to 47%. The control [Ca(2+)](i) was 99 +/- 4 (n = 45) nM and ANG II increased this value in a dose-dependent manner. The ANG II effects on cell volume were minor and late and should not interfere in the measurements of pH(i) recovery and [Ca(2+)](i). To document the signaling pathways in the hormonal effects we used: Staurosporine (a PKC inhibitor), W13 (a calcium-dependent calmodulin antagonist), H89 (a PKA inhibitor) or Econazole (an inhibitor of cytochrome P450 epoxygenase). Our results indicate that the biphasic effect of ANG II on Na(+)/H(+) exchanger is a cAMP-independent mechanism and is the result of: 1) stimulation of the exchanger by PKC signaling pathway activation (at 10(-12) - 10(-7) M ANG II) and by increases of [Ca(2+)](i) in the lower range (at 10(-12) M ANG II) and 2) inhibition of the exchanger at high [Ca(2+)](i) levels (at 10(-9) - 10(-7) M ANG II) through cytochrome P450 epoxygenase-dependent metabolites of the arachidonic acid signaling pathway.

Angiotensin II AT1receptor blockade decreases vasopressin-induced water reabsorption and AQP2 levels in NaCl-restricted rats

Vasopressin and ANG II, which are known to play a major role in renal water and sodium reabsorption, are mainly coupled to the cAMP/PKA and phosphoinositide pathways, respectively. There is evidence for cross talk between these intracellular signaling pathways. We therefore hypothesized that vasopressin-induced water reabsorption could be attenuated by ANG II AT1receptor blockade in rats. To address this, three protocols were used: 1) DDAVP treatment (20 ng/h sc for 7 days, n = 8); 2) DDAVP (20 ng/h sc for 7 days) and candesartan (1 mg·kg−1·day−1sc for 7 days) cotreatment ( n = 8); and 3) vehicle infusion as the control ( n = 8). All rats were maintained on a NaCl-deficient diet (0.1 meq Na+·200 g body wt−1·day−1) during the experiment. DDAVP treatment alone resulted in a significant decrease in urine output (3.1 ± 0.2 ml/day) compared with controls (11.5 ± 2.2 ml/day, P < 0.05), whereas the urine output was significantly increased in response to DDAVP and candesartan cotreatment (9.8 ± 1.0 ml/day, P < 0.05). Consistent with this, rats cotreated with DDAVP and candesartan demonstrated decreased urine osmolality (1,319 ± 172 mosmol/kgH2O) compared with rats treated with DDAVP alone (3,476 ± 182 mosmol/kgH2O, P < 0.05). Semiquantitative immunoblotting revealed significantly decreased expression of medullary aquaporin-2 (AQP2) and AQP2 phosphorylated in the PKA phosphorylation consensus site Ser-256 (p-AQP2) in response to DDAVP and candesartan cotreatment compared with DDAVP treatment alone. In addition, cortical and medullary AQP1 was also downregulated. Fractional sodium excretion (FENa) and plasma potassium levels were markedly increased, and the expressions of the cortical type 3 Na+/H+exchanger (NHE3), thiazide-sensitive Na-Cl cotransporter (NCC), and Na-K-ATPase were significantly decreased in response to DDAVP and candesartan cotreatment. Moreover, medullary type 1 bumetanide-sensitive Na-K-2Cl cotransporter expression showed a marked gel mobility shift from 160 to ∼180 kDa corresponding to enhanced glycosylation, whereas expression was unchanged. In conclusion, ANG II AT1receptor blockade in DDAVP-treated rats was associated with decreased urine concentration and decreased AQP2 and AQP1 expression. Moreover, FENawas increased in parallel with decreased expression of NHE3, NCC, and Na-K-ATPase. These results suggest that ANG II AT1receptor activation plays a significant role in regulating aquaporin and sodium transporter expression and modulating urine concentration in vivo.

Angiotensin II directly stimulates activity and alters the phosphorylation of Na-K-ATPase in rat proximal tubule with a rapid time course

We present evidence that Na-K-ATPase in the rat proximal tubule is directly activated by ANG II much faster than previously observed. Specifically, we show that a 2-min exposure to 0.1 and 1 nM ANG II slowed the rate of intracellular sodium accumulation in response to an increase in extracellular sodium added in the presence of gramicidin D. From these data, we show that ANG II directly stimulates Na-K-ATPase activity at rate-limiting concentrations of intracellular sodium. Under these same conditions, exposing proximal tubules to ANG II altered the amount of 32P incorporated into multiple phosphopeptides generated from a tryptic digest of the alpha-subunit of Na-K-ATPase. Na-K-ATPase was isolated from whole cell lysates by means of a ouabain-affinity column and then separated into its individual subunits by SDS-PAGE. Na-K-ATPase bound to the column in its E2 conformation and was eluted by altering its conformation to E1 using Na+ATP. Na-K-ATPase isolated from cells treated with ANG II eluted more easily from the ouabain-affinity column than Na-K-ATPase isolated from control cells, suggesting that ANG II decreased the affinity of Na-K-ATPase for ouabain. Thus ANG II rapidly stimulated the activity of Na-K-ATPase in 2 min or less by a mechanism that could involve changes in phosphorylation and conformation of Na-K-ATPase. We suggest that the physiological role for rapid direct activation of Na-K-ATPase is greater control of intracellular sodium during sodium reabsorption.

An autocrine role for endothelin-1 in the regulation of proximal tubule NHE3

BACKGROUND

Chronic metabolic acidosis leads to an increase in NHE3 activity that is mediated by endothelin-1 (ET-1) expression and activation of the proximal tubule endothelin B receptor. Chronic metabolic acidosis increases preproET-1 mRNA abundance in kidney cortex, but the cell responsible has not been identified.

METHODS

PreproET-1 mRNA abundance was quantified by competitive reverse transcription-polymerase chain reaction (RT-PCR) on tissue harvested from control rats or rats in which chronic metabolic acidosis was induced by addition of NH(4)Cl to the drinking water.

RESULTS

Chronic metabolic acidosis leads to an increase in preproET-1 mRNA expression in kidney cortex, proximal tubules, and glomeruli. The increase in preproET-1 expression correlates with the decrease in blood [HCO3(-)]. ET-1 expression is also increased by acidosis in abdominal aorta, but not in cardiac muscle.

CONCLUSION

In the renal proximal tubule, chronic metabolic acidosis induces an increase in preproET-1 expression, providing a mechanism for autocrine regulation of proximal tubule NHE3 activity. This response is not unique to the proximal tubule cell, but is also not ubiquitous.

Angiotensin II stimulates NHE3 activity by exocytic insertion of the transporter: role of PI 3-kinase

BACKGROUND

Low-concentration angiotensin II (Ang II) stimulates Na+/H+ exchanger 3 (NHE3) activity in renal proximal tubule mainly via angiotensin II type 1 (AT1) receptors. The mechanisms that mediate the increase in NHE3 activity elicited by Ang II remain incompletely settled.

METHODS

To assess a potential role of NHE3 trafficking in the Ang II effect, NHE3 activity was measured by H+-driven initial rate of 22Na uptake resistant to 50 micromol/L of the Na+/H+ exchange inhibitor cariporide (HOE642), and sensitive to 300 micromol/L ethyl isopropyl amiloride (EIPA), in a model of cultured proximal tubular cells (MKCC), in which functional apical NHE3 and AT receptors are normally present. Apical expression of NHE3 protein was determined by cell surface biotinylation and immunoblotting.

RESULTS

Ang II (10-10 mol/L, 43 minutes) increased NHE3 activity and biotinylated NHE3 protein without any change in total amount of NHE3 protein. Both effects were suppressed by specific AT1 receptor antagonists. When 2-mercaptoethanesulphonic acid (MESNA) was used to cleave biotin from all apical proteins, intracellular biotinylated NHE3 protein remained unchanged after Ang II incubation compared to control. When sulfo-N-hydrosuccinimide (NHS)-acetate was used first to block all apical reactive sites, an increase in biotinylated NHE3 protein was observed following Ang II incubation. To evaluate the role of phosphatidylinositol 3-kinase (PI 3-kinase), the specific inhibitor wortmannin was used. It suppressed Ang II-induced increase in NHE3 activity and trafficking. Furthermore, latrunculin B, inhibitor of actin filament polymerization, prevented both Ang II stimulatory effects.

CONCLUSION

Ang II stimulates NHE3 activity, at least in part, by exocytic insertion of the protein into the apical membrane. This effect is mediated by PI 3-kinase and required integrity of actin cytoskeleton.

Biphasic regulation of renal proximal bicarbonate absorption by luminal AT(1A) receptor

Angiotensin II (AngII) regulates renal proximal transport in a biphasic way. It has been recently shown that the basolateral type 1A receptor (AT(1A)) mediates the biphasic regulation of Na(+)-HCO(3)(-) cotransporter (NBC) by AngII. However, the receptor subtype(s) responsible for the luminal AngII actions remained to be established. To clarify this issue, the luminal AngII effects in isolated proximal tubules from wild-type (WT) and AT(1A)-deficient mice (AT(1A) KO) were compared. In WT, the rate of bicarbonate absorption (JHCO(3)(-)), analyzed with a stop-flow microspectrofluorometric method, was stimulated by 10(-10) mol/L luminal AngII but was inhibited by 10(-6) mol/L luminal AngII. Both stimulatory and inhibitory effects of AngII were completely blocked by valsartan (AT(1) antagonist) but unaffected by PD 123,319 (AT(2) antagonist). In AT(1A) KO, in contrast, luminal AngII (10(-10) - 10(-6) mol/L) did not change JHCO(3)(-). In WT, 10(-6) mol/L luminal AngII increased cell Ca(2+) concentrations ([Ca(2+)](i)), which was again blocked by valsartan but not by PD 123,319. However, luminal AngII did not increase [Ca(2+)](i) in AT(1A) KO. On the other hand, the addition of arachidonic acid similarly inhibited JHCO(3)(-) in WT and AT(1A) KO. Furthermore, the acute activation of protein kinase C by phorbol 12-myristate 13-acetate similarly stimulated JHCO(3)(-) in WT and AT1A KO, indicating that the inhibitory and stimulatory pathways necessary for the AngII actions were preserved in AT(1A) KO. These results indicate that the luminal AT(1A) mediates the biphasic regulation of bicarbonate absorption by luminal AngII, while no evidence was obtained for a role of AT(2).

Role of PKC and calcium in modulation of effects of angiotensin II on sodium transport in proximal tubule

It has been well documented that low concentrations of ANG II (10(-11) to 10(-10) M) stimulate, whereas high concentrations of ANG II (10(-8) to 10(-5) M) inhibit Na(+) transport in proximal tubules of rat and rabbit kidneys. Measured ANG II concentration in proximal tubular fluid is in the nanomolar range. In the present study, we investigated the role of PKC, intracellular Ca(2+), and cAMP in modulating the effects of luminal ANG II on Na(+) absorption by microperfusion techniques in rabbit superficial segment of proximal tubules in vitro. We confirmed that ANG II (10(-9) M) had no change on fluid absorption (J(v)); however, fluid absorption increased significantly when 10(-9) M ANG II and 3,4,5-trimethoxybenzoic acid-8-(diethylamino)octyl ester (TMB-8), a blocker of intracellular calcium mobilization, were added together. In contrast, ANG II significantly decreased J(v) when PKC was inhibited. When 10(-9) M ANG II was present together with 1-(5-isoquinolinesulfonyl)-2-mehtylpiperazine and TMB-8, no significant change of J(v) occurred. Inhibition of endogenous cAMP activity by a PKA inhibitor did not change either basal or ANG II-stimulated fluid absorption. Our results indicate that ANG II regulates Na(+) absorption by a cAMP-independent mechanism and that PKC and intracellular calcium both play a critical role in modulating the effects of physiological concentration of ANG II on proximal tubule transport. Balance between these two cytosolic messengers modulates the effects of ANG II on fluid absorption in the proximal tubule.

Physiologic effects of open and closed tracheostomy tubes on the pharyngeal swallow

Studies linking aspiration and dysphagia to an open tracheostomy tube exemplify the possibility that the larynx may have an influence on oropharyngeal swallow function. Experiments addressing the effects of tracheostomy tube occlusion during the swallow have looked at the presence and severity of aspiration, but few have included measurements that capture the changes in swallowing physiology. Also, hypotheses for the importance of near-normal subglottic air pressure during the swallow have not been offered to date. As such, the aim of this study was to compare the depth of laryngeal penetration, bolus speed, and duration of pharyngeal muscle contraction during the swallow in individuals with tracheostomy tubes while their tubes were open and closed. The results of this series of experiments indicate that within the same tracheostomized patient, pharyngeal swallowing physiology is measurably different in the absence of subglottic air pressure (open tube) as compared to the closed tube condition.

Biphasic regulation of Na+-HCO3- cotransporter by angiotensin II type 1A receptor

Although angiotensin (Ang) II is known to regulate renal proximal transport in a biphasic way, the receptor subtype(s) mediating these Ang II effects remained to be established. To clarify this issue, we compared the effects of Ang II in wild-type mice (WT) and Ang II type 1A receptor-deficient mice (AT(1A) KO). The Na+-HCO3- cotransporter (NBC) activity, analyzed in isolated nonperfused tubules with a fluorescent probe, was stimulated by 10(-10) mol/L Ang II but was inhibited by 10(-6) mol/L Ang II in WT. Although valsartan (AT1 antagonist) blocked both stimulation and inhibition by Ang II, PD 123,319 (AT2 antagonist) did not modify these effects of Ang II. In AT1A KO, in contrast, this biphasic regulation was lost, and only stimulation of NBC activity by 10(-6) mol/L Ang II was observed. This stimulation was blocked by valsartan but not by PD 123,319. More than 10(-8) mol/L Ang II induced a transient increase in cell Ca2+ concentrations in WT, which was again blocked by valsartan but not by PD 123,319. However, up to 10(-5) mol/L Ang II did not increase cell Ca2+ concentrations in AT1A KO. Finally, the addition of arachidonic acid inhibited the NBC activity similarly in WT and AT(1A) KO, suggesting that the inhibitory pathway involving P-450 metabolites is preserved in AT(1A) KO. These results indicate that AT(1A) mediates the biphasic regulation of NBC. Although low-level expression of AT(1B) could be responsible for the stimulation by 10(-6) mol/L Ang II in AT1A KO, no evidence was obtained for AT2 involvement.

The luminal membrane of rat thick limb expresses AT1 receptor and aminopeptidase activities

BACKGROUND

Endogenous intratubular angiotensin II (Ang II) supports an autocrine tonic stimulation of NaCl absorption in the proximal tubule, and its production may be regulated independently of circulating Ang II. In addition, endogenous Ang II activity may be regulated at the brush border membrane (BBM), by the rate of aminopeptidase A and N (APA and APN) activities and the rate of Ca2+-independent phospholipase A2 (PLA2-dependent endocytosis and recycling of the complex Ang II subtype 1 (AT1) receptor (AT1-R). The aim of the present study was to look for subcellular localization of AT1-R, and APA and APN activities in the medullary thick ascending limb of Henle (mTAL), as well as search for an asymmetric coupling of AT1-R to signal transduction pathways.

METHODS

Preparations of isolated basolateral membrane (BLMV) and luminal (LMV) membrane vesicles from rat mTAL were used to localize first, AT1-R by 125I-[Sar1, Ile8] Ang II binding studies and immunoblot experiments with a specific AT1-R antibody, and second, APA and APN activities. Microfluorometric monitoring of cytosolic Ca2+ with a Fura-2 probe was performed in mTAL microperfused in vitro, after apical or basolateral application of Ang II.

RESULTS

AT1-R were present in both LMV and BLMV, with a similar Kd (nmol/L range) and Bmax. Accordingly, BLMV and LMV preparations similarly stained specific AT1-R antibody. APA and APN activities were selectively localized in LMV, although to a lesser extent than those measured in BBM. In the in vitro microperfused mTAL, basolateral but not apical Ang II induced a transient increase in cytosolic [Ca2+].

CONCLUSIONS

Besides the presence of basolateral AT1-R in mTAL coupled to the classical Ca2+-dependent transduction pathways, AT1-R are present in LMV, not coupled with Ca2+ signaling, and co-localized with APA and APN activities. Thus, apical APA and APN may play an important role in modulating endogenous Ang II activity on NaCl reabsorption in mTAL.

Angiotensin II stimulates calcineurin activity in proximal tubule epithelia through AT-1 receptor-mediated tyrosine phosphorylation of the PLC-gamma1 isoform

Angiotensin II (AngII) contributes to the maintenance of extracellular fluid volume by regulating sodium transport in the nephron. In nonepithelial cells, activation of phospholipase C (PLC) by AT-1 receptors stimulates the generation of 1,4,5-trisphosphate (IP(3)) and the release of intracellular calcium. Calcineurin, a serine-threonine phosphatase, is activated by calcium and calmodulin, and both PLC and calcineurin have been linked to sodium transport in the proximal tubule. An examination of whether AngII activates calcineurin in a model of proximal tubule epithelia (LLC-PK1 cells) was performed; AngII increased calcineurin activity within 30 s. An examination of whether AngII activates PLC in proximal tubule epithelia was also performed after first showing that all three families of PLC isoforms are present in LLC-PK1 cells. Application of AngII increased IP(3) generation by 60% within 15 s, which coincided with AngII-induced tyrosine phosphorylation of the PLC-gamma1 isoform also observed at 15 s. AngII-induced tyrosine phosphorylation was blocked by the AT-1 receptor antagonist, Losartan. Subsequently, an inhibitor of tyrosine phosphorylation blocked the AngII-induced activation of calcineurin, as did coincubation with an inhibitor of PLC activity and with an antagonist of the AT-1 receptor. It is therefore concluded that AngII stimulates calcineurin phosphatase activity in proximal tubule epithelial cells through a mechanism involving AT-1 receptor-mediated tyrosine phosphorylation of the PLC isoform.

NHE3 activity and trafficking depend on the state of actin organization in proximal tubule

The present study was addressed to define the contribution of cytoskeleton elements in the kidney proximal tubule Na+/H+ exchanger 3 (NHE3) activity under basal conditions. We used luminal membrane vesicles (LMV) isolated from suspensions of rat cortical tubules pretreated with either colchicine (Colch) or cytochalasin D (Cyto D). Colch pretreatment of suspensions (200 microM for 60 min) moderately decreased LMV NHE3 activity. Cyto D pretreatment (1 microM for 60 min) elicited an increase in LMV NHE3 transport activity but did not increase Na-glucose cotransport activity. Cyto D pretreatment of suspensions did not change the apparent affinity of NHE3 for internal H+. In contrast, after Cyto D pretreatment of the suspensions, NHE3 protein abundance was increased in LMV and remained unchanged in cortical cell homogenates. The effect of Cyto D on NHE3 was further assessed with cultures of murine cortical cells. The amount of surface biotinylated NHE3 increased on Cyto D treatment, whereas NHE3 protein abundance was unchanged in cell homogenates. In conclusion, under basal conditions NHE3 activity depends on the state of actin organization possibly involved in trafficking processes between luminal membrane and intracellular compartment.

Sodium-potassium-adenosinetriphosphatase-dependent sodium transport in the kidney: hormonal control

Tubular reabsorption of filtered sodium is quantitatively the main contribution of kidneys to salt and water homeostasis. The transcellular reabsorption of sodium proceeds by a two-step mechanism: Na(+)-K(+)-ATPase-energized basolateral active extrusion of sodium permits passive apical entry through various sodium transport systems. In the past 15 years, most of the renal sodium transport systems (Na(+)-K(+)-ATPase, channels, cotransporters, and exchangers) have been characterized at a molecular level. Coupled to the methods developed during the 1965-1985 decades to circumvent kidney heterogeneity and analyze sodium transport at the level of single nephron segments, cloning of the transporters allowed us to move our understanding of hormone regulation of sodium transport from a cellular to a molecular level. The main purpose of this review is to analyze how molecular events at the transporter level account for the physiological changes in tubular handling of sodium promoted by hormones. In recent years, it also became obvious that intracellular signaling pathways interacted with each other, leading to synergisms or antagonisms. A second aim of this review is therefore to analyze the integrated network of signaling pathways underlying hormone action. Given the central role of Na(+)-K(+)-ATPase in sodium reabsorption, the first part of this review focuses on its structural and functional properties, with a special mention of the specificity of Na(+)-K(+)-ATPase expressed in renal tubule. In a second part, the general mechanisms of hormone signaling are briefly introduced before a more detailed discussion of the nephron segment-specific expression of hormone receptors and signaling pathways. The three following parts integrate the molecular and physiological aspects of the hormonal regulation of sodium transport processes in three nephron segments: the proximal tubule, the thick ascending limb of Henle's loop, and the collecting duct.

Regulation by PKC isoforms of Na(+)/H(+) exchanger in luminal membrane vesicles isolated from cortical tubules

The present study was designed to determine the Na/H exchanger isoforms present in luminal membrane vesicles (LMV) isolated from rat kidney cortical tubule suspensions, as well as the effects of acute phorbol ester (phorbol myristate acetate, PMA) and angiotensin II (ANG II) pretreatment of suspensions on NHE activity and protein kinase C (PKC) isoform abundance. In LMV, both NHE3 and NHE2 proteins were found by Western blot analysis, but only ethylisopropylamiloride-sensitive and almost completely Hoe-694-resistant Na/H exchange activity was observed from (22)Na uptake and thus attributed to NHE3. PMA pretreatment increased Na/H exchange activity and PKC isoforms alpha, delta, and epsilon abundance in LMV, and these effects were prevented by PKC inhibition. Low-dose ANG II (10(-11) M) pretreatment increased Na/H exchange activity and only PKC-zeta abundance in LMV, and these effects were also prevented by PKC inhibition. After high-dose ANG II (10(-7) M), Na/H exchange activity was decreased in LMV. PKC inhibition did not prevent this effect. In conclusion, the stimulating effects of PMA and low-dose ANG II are explained by the translocation of different isoforms of PKC in LMV, whereas the inhibitory effect of high-dose ANG II is not PKC dependent.