Sphingosine-1-phosphate receptors: biology and therapeutic potential in kidney disease.

The major sphingolipid metabolite, sphingosine-1-phosphate (S1P), has important biological functions. S1P is the ligand for a family of five G-protein-coupled receptors with distinct signaling pathways that regulate angiogenesis, vascular maturation, immunity, chemotaxis, and other important biological pathways. Recently, clinical trials have targeted S1P receptors (S1PRs) for autoimmune diseases and transplantation and have generated considerable interest in developing additional, more selective compounds. This review summarizes current knowledge on the biology of S1P and S1PRs that forms the basis for future drug development and the treatment of kidney disease.

Adenosine 2A receptors in acute kidney injury.

Acute kidney injury (AKI) is an important clinical problem that may lead to death and for those who survive, the sequelae of AKI include loss of quality of life, chronic kidney disease and end-stage renal disease. The incidence of AKI continues to rise without clear successes in humans for the pharmacological prevention of AKI or treatment of established AKI. Dendritic cells and macrophages are critical early initiators of innate immunity in the kidney and orchestrate inflammation subsequent to ischaemia-reperfusion injury. These innate cells are the most abundant leucocytes present in the kidney, and they represent a heterogeneous population of cells that are capable of responding to cues from the microenvironment derived from pathogens or endogenous inflammatory mediators such as cytokines or anti-inflammatory mediators such as adenosine. Lymphocyte subsets such as natural killer T cells and Tregs also play roles in regulating ischaemic injury by promoting and suppressing inflammation respectively. Adenosine, produced in response to IR, is generally considered as a protective signalling molecule and elicits its physiological responses through four distinct adenosine receptors. However, its short half-life, lack of specificity and rapid metabolism limit the use of adenosine as a therapeutic agent. These adenosine receptors play various roles in regulating the activity of the aforementioned hematopoietic cells in elevated levels of adenosine such as during hypoxia. This review focuses on the importance of one receptor, the adenosine 2A subtype, in blocking inflammation associated with AKI.

Clinical manifestations and management of acute lithium intoxication.

Acute lithium intoxication is a frequent complication of chronic lithium therapy for manic depressive disorders. Because of lithium's narrow therapeutic index and widespread use, lithium intoxication remains prevalent in 1994. This review summarizes information on the renal handling of lithium and the physiologic basis for toxicity. Recent reports that describe previously unrecognized side effects of lithium intoxication are discussed. We also present management guidelines based upon our understanding of the renal handling of lithium. In this review we compare the effectiveness of lithium removal by various dialysis methods, including bicarbonate dialysis, peritoneal dialysis and continuous arteriovenous hemofiltration. Hemodialysis remains the cornerstone for the treatment of acute lithium toxicity.

Immunohistochemical localization of alpha 2A-adrenergic receptors in catecholaminergic and other brainstem neurons in the rat.

alpha 2-Adrenergic receptors mediate a large portion of the known inhibitory effects of catecholamines on central and peripheral neurons. Molecular cloning studies have established the identity of three alpha 2-adrenergic receptor genes from several species that encode the A, B and C subtypes of the receptor. The rat alpha 2A-adrenergic receptor, as defined by sequence similarity, is the orthologue of the human alpha 2A-adrenergic receptor. In this paper, we report the development of rabbit antisera directed against a portion of the third intracellular loop of the rat alpha 2A-adrenergic receptor and the histochemical localization of alpha 2A-adrenergic receptor-like immunoreactive material in the brainstem and spinal cord of the adult rat. Our antisera detected alpha 2A-adrenergic receptor-specific punctate staining associated with neuronal perikarya. alpha 2A-adrenergic receptor-like immunoreactivity was widely, but heterogeneously, distributed in the brainstem and spinal cord, predominantly in areas involved in the control of autonomic function. Double labelling with antisera to tyrosine hydroxylase or phenylethanolamine-N-methyl-transferase revealed that alpha 2A-adrenergic receptor-like immunoreactivity is present in most, perhaps all, noradrenergic and adrenergic cells of the brainstem. alpha 2A-Adrenergic receptor-like immunoreactivity was detected in a small percentage of the dopaminergic cells of the A9 and A10 groups. This study provides the first description of the specific immunohistochemical localization of alpha 2A-adrenergic receptors using a subtype-specific polyclonal antibody. The results support the view that alpha 2-adrenergic receptors are involved in central cardiovascular control and suggest that the catecholaminergic autoreceptors of central noradrenergic and adrenergic neurons are the A subtype of the alpha 2-adrenergic receptors.

Pseudohypertension in a patient with diffuse scleroderma.

Pseudohypertension is the artifactual elevation of blood pressure that occurs secondary to noncompressible blood vessels. It has been described in patients with uremia, diabetes mellitus, and severe atherosclerosis. If unrecognized, the condition may lead to inappropriate and potentially harmful therapy. We report a case of pseudohypertension in a 65-year-old man with diffuse scleroderma. His blood pressure as assessed by conventional sphygmomanometry was at least 240/135 to 145 mm Hg. Intra-arterial blood pressure was found to be 107/52 mm Hg. The severe rise in blood pressure as measured by sphygmomanometry led to the concern of scleroderma renal crisis and potentially harmful therapy. Intra-arterial pressure monitoring confirmed the presence of pseudohypertension, however. This is the first reported case of pseudohypertension in a patient with diffuse scleroderma.

Hypokalemia, metabolic alkalosis, and hypertension: Cushing's syndrome in a patient with metastatic prostate adenocarcinoma.

Ectopic adrenocorticotropic hormone (ACTH) and/or corticotropin-releasing hormone (CRH) are associated with a growing list of tumors. We report a 69-year-old white man with a history of high-grade prostate carcinoma and widely metastatic adenocarcinoma who presented with metabolic alkalosis, hypokalemia, and hypertension secondary to ectopic ACTH and CRH secretion. Laboratory values were consistent with hypokalemia and metabolic alkalosis. Markedly elevated serum cortisol (135 microg/dL), ACTH (1,387 pg/dL), CRH (69 pg/dL), and urine free cortisol (16,276 microg/24 h) levels were found. Chest computed tomographic (CT) scan showed small noncalcified parenchymal densities; however, bronchoscopy and bronchoalveolar lavage washings were unremarkable for a neoplastic process. Abdominal CT scan and magnetic resonance imaging showed multiple small liver lesions and multiple thoracic and lumbar intensities consistent with diffuse metastatic disease. Histological analysis of a biopsy specimen from the thoracic spine showed an undifferentiated adenocarcinoma consistent with a prostate primary tumor. The severe metabolic alkalosis secondary to glucocorticoid-induced excessive mineralocorticoid activity was treated with potassium supplements, spironolactone, and ketoconazole. In this case report, we describe an unusual tumor associated with ectopic ACTH and CRH production and the pharmacodynamic relationship of plasma cortisol levels and urinary cortisol excretion with ketoconazole treatment.

The effect of acetate and bicarbonate dialysis on orthostatic blood pressure regulation.

Dialysis-induced hypotension is probably due to multiple factors which reduce cardiac output and peripheral vascular resistance. That acetate, used in dialysate as a source of bicarbonate, may be one of these factors is suggested from several studies which show improved blood pressure regulation with bicarbonate-containing dialysate. We have further examined this hypothesis by comparing supine and standing blood pressure changes in nine stable patients during dialysis with acetate or bicarbonate solutions. Although mean weight changes were not different, standing blood pressure was lower after acetate dialysis. Such a hypotensive effect of acetate could result from depression of the force of myocardial contraction and/or from a reduction in peripheral vascular resistance.

Effect of Na-channel blockers and lumen Ca on K secretion by rat renal distal tubule.

In previous studies the effectiveness of amiloride (AML) in reducing K secretion has been variable. Based on studies by Cuthbert and Wong (Mol. Pharmacol. 8: 222-229, 1972) in which the Na-channel-blocking action of AML in frog skin was found to require the availability of Ca ions in extracellular fluid, we postulated that the ability of AML and its analogue, benzamil (BZA), to inhibit distal tubule K secretion depends on the presence of Ca in luminal fluid. We found that addition of Ca to a perfusion solution containing 50 microM BZA did reduce K secretion more than BZA alone. Maximal inhibition was observed with 2.5 mM free ionic Ca. Graded increases in luminal Ca in presence of AML or BZA reduced K transport in a dose-dependent manner. The decrease in K secretion with increasing luminal Ca was paralleled by a decrease in transepithelial voltage. These results support our hypothesis that the effectiveness of Na-channel blockers to reduce K secretion by the rat distal tubule depends on presence of luminal Ca and suggest an interaction between luminal Ca and Na-channel blockers on the Na channel.

Chlorothiazide effect on feedback-mediated control of glomerular filtration rate.

We examined the effect of chlorothiazide (CTZ) on the tubuloglomerular (TG) feedback system in anesthetized Sprague-Dawley rats. During infusion of CTZ (0.25 mg.kg body wt-1.min-1) we found that whole kidney glomerular filtration rate (GFR) decreased by 19% (1.0 +/- 0.1 vs. 0.8 +/- 0.1 ml/min; P less than 0.005). To asses the activity of the TG feedback system during CTZ administration we compared measurements of single-nephron (SN)GFR from tubule fluid sampled separately at proximal and distal sites. During CTZ administration, distally measured SNGFR decreased significantly by 16% (27.3 +/- 1.3 vs. 22.9 +/- 1.1 nl/min; P less than 0.025), whereas proximally measured SNGFR was unchanged. Thus the difference in SNGFR between proximal and distal determination increased during CTZ infusion (4.7 +/- 0.7 vs. 7.7 +/- 0.7 nl/min; P less than 0.025), indicating that CTZ suppresses GFR by TG feedback. Na, K, and Cl concentrations measured in the late proximal tubule fluid during control and CTZ infusions were similar. In early distal tubule fluid samples K and Cl concentrations were unaffected by CTZ infusion, whereas Na concentrations increased by 32% (47.9 +/- 2.7 vs. 63.1 +/- 2.4 mM; P less than 0.001). Proximal tubule microperfusion with 1.0 mM CTZ decreased transport rates of Na and water by approximately 40%, whereas the transport rate of Cl was not affected. In conclusion our results indicate that CTZ reduces GFR by activating TG feedback. The mechanism by which this occurs is in part due to an increase in the strength of the signal.

Selective A2A adenosine receptor activation reduces ischemia-reperfusion injury in rat kidney.

A2A adenosine receptors (A2A-ARs) are known modulators of renal hemodynamics and potent inhibitors of inflammation. We sought to determine whether selective activation of A2A-ARs protects kidneys from ischemia-reperfusion injury. The ester derivative of DWH-146 (DWH-146e), a selective A2A agonist, was found to be more potent and selective for A2A-ARs than the prototype compound CGS-21680. Osmotic minipumps were implanted subcutaneously to infuse into rats either vehicle or DWH-146e (0.004 microg. kg(-1). min(-1)), during and after ischemia-reperfusion injury. Following 24 and 48 h of reperfusion, the rise in serum creatinine and blood urea nitrogen for vehicle-treated rats was substantially elevated compared with DWH-146e-treated rats. Histological examination revealed widespread tubular epithelial necrosis and vascular congestion in the outer medulla of vehicle-treated compared with DWH-146e-treated animals. ZM-241385, a selective A(2A) antagonist, blocked the protective effect of DWH-146e. Delaying administration of DWH-146e until the initiation of reperfusion also decreased serum creatinine. We conclude that 1) selective A2A-AR activation by DWH-146e reduces ischemia-reperfusion injury in rat kidneys, 2) the effect of DWH-146e is A2A receptor mediated, and 3) the protective effects are mediated by preventing injury during the reperfusion period.

Active potassium absorption by the renal distal tubule.

Maintenance of potassium homeostasis during potassium depletion appears to involve an active potassium absorptive mechanism in the distal nephron. Direct demonstration of such a pathway in the distal tubule of the rat has been lacking. The purpose of the current study was to examine the hypothesis that an ATP-dependent active transport mechanism plays a role in potassium absorption by the rat distal tubule. We utilized in vivo microperfusion techniques in Sprague-Dawley rats maintained on a regular diet of low-potassium diet for 3-4 wk. The effect of a selective inhibitor of the gastric H-K-adenosinetriphosphatase (ATPase) (Sch 28080, 0.1 mM) was tested in distal tubules of both groups of rats. Distal tubules of normal rats secreted potassium. Sch 28080 had no effect on this net potassium flux. In contrast, distal tubules of potassium-deficient rats absorbed potassium. Sch 28080 abolished this potassium absorption and produced a small hyperpolarization of the lumen-negative transepithelial voltage (VTE). The change in VTE can be explained by a concomitant increase in potassium concentration in the late distal tubule. These results are consistent with the presence of an H-K-ATPase in the distal tubule of potassium-deficient rats.

Luminal calcium regulates potassium transport by the renal distal tubule.

We examined the effect of changes in lumen calcium concentration on net potassium transport by distal tubules in anesthetized rats. Tubules were perfused with a control solution that resembled interstitial fluid but lacked calcium. Experimental solutions were prepared by adding varying amounts of CaCl2 to the control solution to produce solutions with free ionic calcium concentration ([Ca2+]) of 0.2, 0.4, and 0.8 mM. In paired comparisons 0.2 mM Ca2+ did not affect net potassium transport, whereas 0.4 and 0.8 mM Ca2+ each reduced potassium secretion by approximately 30%. Unidirectional potassium fluxes using 86Rb as a tracer for potassium and transepithelial voltage (VTE) were measured to characterize further the effect of calcium on potassium transport. Presence of 0.8 mM Ca2+ in the lumen did not affect unidirectional absorptive potassium flux; therefore, the decrease in net potassium flux was accounted for entirely by a decrease in unidirectional secretory potassium flux. The lumen negative VTE measured in the late distal tubule decreased during perfusion with 0.8 mM Ca2+. These results are consistent with the hypothesis that increases in lumen (extracellular) calcium concentration in the range normally present in the distal tubule reduce net potassium secretion by decreasing the electrochemical gradient for potassium secretion.

Regulation of adenylyl cyclase in polarized renal epithelial cells by G protein-coupled receptors.

We employed two guanine nucleotide binding protein (G protein)-coupled receptors known to be targeted to opposite domains in renal epithelial cells to test the hypothesis that the polarized receptor expression of receptors regulates the activity of the receptor's effector molecule, adenylyl cyclase. We used LLC-PK1 cells stably transfected with cDNA encoding the alpha 2B-adrenergic receptor (alpha 2B-AR) or A1-adenosine receptor (A1-AdR). Immunohistochemistry and Western blot analysis confirmed the basolateral and apical expression of alpha 2B-ARs and A1-AdRs, respectively. Adenylyl cyclase activity was assessed by measuring cAMP accumulation following the addition of forskolin (10 microM) in the presence of 3-isobutyl-1-methylxanthine to apical or basolateral chambers of confluent monolayers. A five- to sixfold increase in cAMP accumulation occurred following apical (or basolateral) stimulation of LLC-PK1 cells expressing apical (or basolateral) receptors in comparison to forskolin stimulation of corresponding domains of untransfected cells. We conclude 1) adenylyl cyclase activity is present at or near the apical and basolateral domains of LLC-PK1 cells, and 2) factors that regulate the polarized expression of inhibitory G protein-coupled receptors may also regulate local adenylyl cyclase activity.

Apical membrane and intracellular distribution of endogenous alpha 2A-adrenergic receptors in MDCK cells.

The purpose of the current studies was to characterize the endogenous alpha 2-adrenergic receptor (AR) subtypes present in Madin-Darby canine kidney (MDCK) cells and to determine their level of expression and pattern of distribution. By saturation binding analysis with [3H]MK-912, MDCK cells expressed high levels of alpha 2-ARs with a maximum receptor density (Bmax) of 798 +/- 55 fmol/mg protein and an equilibrium dissociation constant (Kd) of 0.98 +/- 0.32 nM. Competitive binding studies using prazosin, oxymetazoline, phentolamine, and epinephrine to displace [3H]MK-912 demonstrated inhibition constant (Ki) values of 1,270 +/- 250, 5.0 +/- 0.4, 5.5 +/- 0.3, and 392 +/- 150 nM (n = 3), respectively. In Northern blot analysis we found that MDCK cells expressed transcripts encoding alpha 2A-AR and not alpha 2B-AR or alpha 2C-AR. Surface binding experiments suggested that approximately 60% of alpha 2A-ARs are distributed at the cell surface domain. Specific binding of [3H]MK-912 to soluble apical and basolateral surface proteins isolated by surface biotinylation indicated the expression of surface alpha 2A-ARs was limited to the apical domain of MDCK cells. No alpha 2A-ARs were detected on the basolateral surface. We conclude that endogenous alpha 2A-ARs are targeted to the apical domain of MDCK cells and that the intracellular compartment may contain ARs as a reservoir for de novo cell surface expression or, alternatively, may represent internalized receptors.

Regulation of rat renal alpha 2 B-adrenergic receptors by potassium depletion.

Potassium depletion and alpha 2-adrenergic receptor (alpha 2-AR) agonists produce similar physiological effects on renal function. Both stimuli increase Na-H exchange in proximal tubule cells, inhibit water transport in collecting tubule cells, and alter blood pressure regulation. The purpose of this study was to determine whether potassium depletion and renal alpha 2-AR subtype expression were linked. Kidney membrane proteins and RNA were harvested from anesthetized rats fed a potassium-deficient diet for 4-20 days (LK 4 to LK 20). Using a selective alpha 2-AR antagonist, [3H]MK-912, we observed that potassium depletion led to a dramatic increase in maximum binding (270% of control) without a change in dissociation constant. Competitive binding studies in LK 14 kidney membranes employing chlorpromazine, prazosin, and oxymetazoline suggested that the increase in alpha 2-ARs in response to potassium depletion was due primarily to an increase in the B subtype of alpha 2-AR. Northern blot analysis demonstrated that renal alpha 2B-AR mRNA levels increased (190% of control) after 4 or 14 days on a potassium-deficient diet. In contrast, there was no difference in steady-state alpha 2A-receptor protein levels by Western blot analysis. We conclude that potassium depletion selectively increases the expression of the B subtype of alpha 2-AR with no detectable effect on alpha 2A-AR expression.

Alpha 2B-adrenergic receptors: immunolocalization and regulation by potassium depletion in rat kidney.

Regulation of transport processes in renal epithelial cells in response to alpha 2-adrenergic receptor (alpha 2-AR) stimulation is likely due to specific effects of the A-, B-, or C-subtype alpha 2-AR. The purpose of the present study was 1) to examine the precise localization, using subtype-specific antibodies, the B-subtype alpha 2-AR protein, the most abundant alpha 2-AR in rat kidney; and 2) to determine the effect of dietary potassium restriction, a factor known to increase alpha 2B-ARs, on the distribution of receptors. We employed a method to generate a specific antibody to the third intracellular loop of the alpha 2B-AR and demonstrated the specificity of this antibody. Our results demonstrated, for the first time, evidence for the alpha 2B-receptor protein in the basolateral membrane of proximal convoluted and straight tubules. No specific immunoreactivity was detected in other nephron segments. Finally, the increase in alpha 2B-AR expression observed previously was due to the recruitment of alpha 2B-ARs to the basolateral membrane of proximal tubules.

A(2A) adenosine receptor-mediated inhibition of renal injury and neutrophil adhesion.

We sought to determine the mechanisms responsible for the reduced renal tissue injury by agonists of A(2A) adenosine receptors (A(2A)-ARs) in models of ischemia-reperfusion (I/R) injury. DWH-146e, a selective A(2A)-AR agonist, was administered subcutaneously to Sprague-Dawley rats and C57BL/6 mice via osmotic minipumps, and animals were subjected to I/R. I/R led to an increase in plasma creatinine and kidney neutrophil infiltration. Infusion of DWH-146e at 10 ng. kg(-1). min(-1) produced a 70% reduction in plasma creatinine as well as a decrease in neutrophil density in outer medulla and cortex and myeloperoxidase activity in the reperfused kidney. Myeloperoxidase activity in kidney correlated with the degree of renal injury. P-selectin and intercellular adhesion molecule 1 (ICAM-1) immunoreactivity were most prominent in endothelial cells of peritubular capillaries and interlobular arteries of cortex and outer and inner medulla of vehicle-treated mice whose kidneys were subjected to I/R. DWH-146e treatment led to a pronounced decrease in P-selectin- and ICAM-1-like immunoreactivity. These data are consistent with our hypothesis that A(2A)-AR agonists limit I/R injury due to an inhibitory effect on neutrophil adhesion.

Enhanced protection from renal ischemia-reperfusion [correction of ischemia:reperfusion] injury with A(2A)-adenosine receptor activation and PDE 4 inhibition.

BACKGROUND: We previously demonstrated in rats and mice that agonists of A(2A)-adenosine receptors (A(2A)-ARs) reduce renal injury following ischemia-reperfusion. We now extend these studies and examine the effects of ATL-146e (formerly DWH-146e), an A(2A)-AR agonist, and rolipram, a type IV phosphodiesterase (PDE 4) inhibitor, on murine renal injury following ischemia-reperfusion. METHODS: C57BL/6 mice were treated with rolipram, ATL-146e, or both compounds combined and were subjected to renal ischemia for 32 minutes and reperfusion for 24 to 48 hours. In vitro studies were performed on suspended and adhering human neutrophils. RESULTS: Continuous delivery of rolipram or ATL-146e during reperfusion reduced renal injury in a dose-dependent manner. Maximal protection was observed when ATL-146e was infused for six hours during reperfusion. Elevated plasma creatinine and myeloperoxidase activity produced by ischemia-reperfusion were reduced by rolipram (0.1 ng/kg/min) and ATL-146e (10 ng/kg/min) by up to approximately 60% and 70%, respectively. Co-infusion of both compounds produced a maximum reduction of plasma creatinine of approximately 90% and myeloperoxidase activity. In vitro studies on suspended and adhering human neutrophils demonstrated that selective stimulation of A(2A)-ARs by ATL-146e increased cAMP accumulation, reduced oxidative activity of activated neutrophils, and decreased activated neutrophil adherence. These responses were potentiated by rolipram. CONCLUSIONS: We conclude that the combined infusion of ATL-146e and rolipram leads to enhanced renal tissue protection from ischemia-reperfusion by mechanisms that may include reduced neutrophil adherence/recruitment and release of reactive oxygen species.