Inhibition by cyclic AMP and dibutyryl cyclic AMP of transport of organic acids in kidney cortex

Organic anion transporters: discovery, pharmacology, regulation and roles in pathophysiology

Our understanding of the mechanisms behind inter- and intra-patient variability in drug response is inadequate. Advances in the cytochrome P450 drug metabolizing enzyme field have been remarkable, but those in the drug transporter field have trailed behind. Currently, however, interest in carrier-mediated disposition of pharmacotherapeutics is on a substantial uprise. This is exemplified by the 2006 FDA guidance statement directed to the pharmaceutical industry. The guidance recommended that industry ascertain whether novel drug entities interact with transporters. This suggestion likely stems from the observation that several novel cloned transporters contribute significantly to the disposition of various approved drugs. Many drugs bear anionic functional groups, and thus interact with organic anion transporters (OATs). Collectively, these transporters are nearly ubiquitously expressed in barrier epithelia. Moreover, several reports indicate that OATs are subject to diverse forms of regulation, much like drug metabolizing enzymes and receptors. Thus, critical to furthering our understanding of patient- and condition-specific responses to pharmacotherapy is the complete characterization of OAT interactions with drugs and regulatory factors. This review provides the reader with a comprehensive account of the function and substrate profile of cloned OATs. In addition, a major focus of this review is on the regulation of OATs including the impact of transcriptional and epigenetic factors, phosphorylation, hormones and gender.

Extracellular cAMP inhibits proximal reabsorption: are plasma membrane cAMP receptors involved?

Glucagon binding to hepatocytes has been known for a long time to not only stimulate intracellular cAMP accumulation but also, intriguingly, induce a significant release of liver-borne cAMP in the blood. Recent experiments have shown that the well-documented but ill-understood natriuretic and phosphaturic actions of glucagon are actually mediated by this extracellular cAMP, which inhibits the reabsorption of sodium and phosphate in the renal proximal tubule. The existence of this "pancreato-hepatorenal cascade" indicates that proximal tubular reabsorption is permanently influenced by extracellular cAMP, the concentration of which is most probably largely dependent on the insulin-to-glucagon ratio. The possibility that renal cAMP receptors may be involved in this process is supported by the fact that cAMP has been shown to bind to brush-border membrane vesicles. In other cell types (i.e., adipocytes, erythrocytes, glial cells, cardiomyocytes), cAMP eggress and/or cAMP binding have also been shown to occur, suggesting additional paracrine effects of this nucleotide. Although not yet identified in mammals, cAMP receptors (cARs) are already well characterized in lower eukaryotes. The amoeba Dictyostelium discoideum expresses four different cARs during its development into a multicellular organism. cARs belong to the superfamily of seven transmembrane domain G protein-coupled receptors and exhibit a modest homology with the secretin receptor family (which includes PTH receptors). However, the existence of specific cAMP receptors in mammals remains to be demonstrated. Disturbances in the pancreato-hepatorenal cascade provide an adequate pathophysiological understanding of several unexplained observations, including the association of hyperinsulinemia and hypertension, the hepatorenal syndrome, and the hyperfiltration of diabetes mellitus. The observations reviewed in this paper show that cAMP should no longer be regarded only as an intracellular second messenger but also as a first messenger responsible for coordinated hepatorenal functions, and possibly for paracrine regulations in several other tissues.

Regulation of renal tubular secretion of organic compounds

BACKGROUND

Information on the molecular basis underlying organic anion and cation transport in renal tubules has expanded in recent years with the identification and characterization of numerous transporters. However, little is known about the regulation of this transport.

METHODS

Both English and Russian language studies dealing with the regulation of organic ion transport by the kidney have been reviewed.

RESULTS

This review summarizes the literature on the physiological and pharmacological aspects of the regulation of organic ion transport, linking this information where possible to underlying transport mechanisms. Current models of the tubular secretion of organic anions and cations are reviewed. Factors that inhibit or enhance tubular secretion of xenobiotics are described, and their influence on proximal tubule cell transport and function is discussed. Important roles for substrate stimulation, the adrenergic nervous system, numerous hormones, P-glycoprotein, and protein kinase C activity have been identified.

CONCLUSIONS

Despite considerable advances in the understanding of basic transport pathways and mechanisms involved in the tubular secretion of organic compounds, there is still relatively little information on the regulation of this transport. Studies combining the techniques of integrative and cell physiology and molecular biology will provide significant new insights into the pathways regulating the tubular transport of these compounds.

Molecular cloning and functional expression of a multispecific organic anion transporter from human kidney

Recently, we isolated the multispecific organic anion transporter (OAT1) from the rat kidney, which plays important roles in the renal elimination of endogenous and exogenous organic anions including clinically important drugs. In the present study, we cloned and characterized human OAT1. Two cDNA clones, hOAT1-1 cDNA and hOAT1-2 cDNA, were isolated from a human kidney cDNA library, whose amino acid sequences were 86.0% and 87.8% identical to that of rat OAT1, respectively. When expressed in Xenopus laevis oocytes, hOAT1 mediated sodium-independent uptake of p-aminohippurate (PAH) (Km = 9. 3 +/- 1.0 microM). hOAT1-mediated PAH uptake was inhibited by bulky inorganic anions, various xenobiotics, and endogenous substances, including benzylpenicillin, furosemide, indomethacin, probenecid, phenol red, urate, and alpha-ketoglutarate. Northern blot analysis revealed that hOAT1 mRNA is strongly expressed in human kidney; transcripts of different sizes are expressed in skeletal muscle, brain, and placenta. Immunohistochemical analysis using rabbit IgG antibody against the carboxy-terminal 14 peptides of hOAT1 revealed that hOAT1 is expressed at the basolateral membrane of the proximal tubule. hOAT1 gene was located on human chromosome 11q13.1 by fluorescent in situ hybridization analysis. These results indicate that hOAT1 is a multispecific organic anion transporter on the basolateral membrane of the proximal tubule in human kidney.

Cyclic AMP is a hepatorenal link influencing natriuresis and contributing to glucagon-induced hyperfiltration in rats

The effects of glucagon (G) on proximal tubule reabsorption (PTR) and GFR seem to depend on a prior action of this hormone on the liver resulting in the liberation of a mediator and/or of a compound derived from amino acid metabolism. This study investigates in anesthetized rats the possible contribution of cAMP and urea, alone and in combination with a low dose of G, on phosphate excretion (known to depend mostly on PTR) and GFR. After a 60-min control period, cAMP (5 nmol/min x 100 grams of body weight [BW]) or urea (2.5 micromol/min x 100 grams BW) was infused intravenously for 200 min with or without G (1.2 ng/min x 100 grams BW, a physiological dose which, alone, does not influence PTR or GFR). cAMP increased markedly the excretion of phosphate and sodium (+303 and +221%, respectively, P < 0.01 for each) but did not alter GFR. Coinfusion of cAMP and G induced the same tubular effects but also induced a 20% rise in GFR (P < 0.05). Infusion of urea, with or without G, did not induce significant effects on PTR or GFR. After G infusion at increasing doses, the increase in fractional excretion of phosphate was correlated with a simultaneous rise in plasma cAMP concentration and reached a maximum for doubling of plasma cAMP. These results suggest that cAMP, normally released by the liver into the blood under the action of G, (a) is probably an essential hepatorenal link regulating the intensity of PTR, and (b) contributes, in conjunction with specific effects of G on the nephron, to the regulation of GFR.

Mechanisms whereby extracellular adenosine 3',5'-monophosphate inhibits phosphate transport in cultured opossum kidney cells and in rat kidney. Physiological implication

The mechanism of phosphaturia induced by cAMP infusion and the physiological role of extracellular cAMP in modulation of renal phosphate transport were examined. In cultured opossum kidney cells, extracellular cAMP (10-1,000 microM) inhibited Na-dependent phosphate uptake in a time- and concentration-dependent manner. The effect of cAMP was reproduced by ATP, AMP, and adenosine, and was blunted by phosphodiesterase inhibitors or by dipyridamole which inhibits adenosine uptake. [3H]cAMP was degraded extracellularly into AMP and adenosine, and radioactivity accumulated in the cells as labeled adenosine and, subsequently, as adenine nucleotides including cAMP. Radioactivity accumulation was decreased by dipyridamole and by inhibitors of phosphodiesterases and ecto-5'-nucleotidase, assessing the existence of stepwise hydrolysis of extracellular cAMP and intracellular processing of taken up adenosine. In vivo, dipyridamole abolished the phosphaturia induced by exogenous cAMP infusion in acutely parathyroidectomized (APTX) rats, decreased phosphate excretion in intact rats, and blunted phosphaturia induced by PTH infusion in APTX rats. These results indicate that luminal degradation of cAMP into adenosine, followed by cellular uptake of the nucleoside by tubular cells, is a key event which accounts for the phosphaturic effect of exogenous cAMP and for the part of the phosphaturic effect of PTH which is mediated by cAMP added to the tubular lumen under the influence of the hormone.

Contraluminal p-aminohippurate transport in the proximal tubule of the rat kidney. VII. Specificity: cyclic nucleotides, eicosanoids

Using the stop-flow peritubular capillary microperfusion method the inhibitory potency (apparent Ki values) of cyclic nucleotides and prostanoids against contraluminal p-aminohippurate (PAH), dicarboxylate and sulphate transport was evaluated. Conversely the contraluminal transport rate of labelled cAMP, cGMP, prostaglandin E2, and prostaglandin D2 was measured and the inhibition by different substrates was tested. Cyclic AMP and its 8-bromo and dibutyryl analogues inhibited contraluminal PAH transport with an app. Ki,PAH of 3.4, 0.63 and 0.52 mmol/l. The respective app. Ki,PAH values of cGMP and its analogues are with 0.27, 0.04 and 0.05 mmol/l, considerably lower. None of the cyclic nucleotides tested interacted with contraluminal dicarboxylate, sulphate and N1-methylnicotinamide transport. ATP, ADP, AMP, adenosine and adenine as well as GTP, GDP, GMP, guanosine and guanine did not inhibit PAH transport while most of the phosphodiesterase inhibitors tested did. Time-dependent contraluminal uptake of [3H]cAMP and [3H]cGMP was measured at different starting concentrations and showed facilitated diffusion kinetics with the following parameters for cAMP: Km = 1.5 mmol/l, Jmax = 0.34 pmol S-1 cm-1, r (extracellular/intracellular amount at steady state) = 0.91; for cGMP: Km = 0.29 mmol/l, Jmax = 0.31 pmol S-1 cm-1, r = 0.55. Comparison of app. Ki,cGMP with app. Ki,PAH of ten substrates gave a linear relation with a ratio of 1.83 +/- 0.5. All prostanoids applied inhibited the contraluminal PAH transport; the prostaglandins E1, F1 alpha, A1, B1, E2, F2 alpha, D2, A2 and B2 with an app. Ki,PAH between 0.08 and 0.18 mmol/l. The app. Ki of the prostacyclins 6,15-diketo-13,14-dihydroxy-F1 alpha (0.22 mmol/l) and Iloprost (0.17 mmol/l) as well as that of leukotrienes B4 (0.2 mmol/l) was in the same range, while the app. Ki,PAH of the prostacyclins PGI2 (0.55 mmol/l), 6-keto-PGF1 alpha (0.77 mmol/l) and 2,3-dinor-6-keto-PGF1 alpha (0.57 mmol/l) as well as that of thromboxane B2 (0.36 mmol/l) was somewhat higher. None of these prostanoids inhibited contraluminal dicarboxylate transport and only PGB1, E2 and D2 inhibited contraluminal sulphate transport (app. Ki,SO4(2-) 5.4, 11.0, 17.9 mmol/l respectively). Contraluminal influx of labelled PGE2 showed complex transport kinetics with a mixed Km = 0.61 mmol/l and Jmax of 4.26 pmol S-1 cm-1. It was inhibited by probenecid, sulphate and indomethacin. Contraluminal influx of PGD2, however, was only inhibited by probenecid. The data indicate that cyclic nucleotides as well as prostanoids are transported by the contraluminal PAH transporter. For prostaglandin E2 a significant uptake through the sulphate transporter occurs in addition.(ABSTRACT TRUNCATED AT 400 WORDS)

Luminal and peritubular steps in renal transport of p-aminohippurate

Why has the PAH transport system proven so difficult to characterize? The major problems appear to arise in three areas: species differences, differences in methodology, e.g., vesicle preparation techniques, and multiplicity of related transport systems. Species differences are certainly important in some areas, e.g., the luminal membrane where the same investigators have shown the presence of an anion exchanger capable of transporting PAH in rat and dog but not in rabbit. Since all of these species effectively secrete PAH, one must question whether or not the primary luminal component of the PAH secretory system has yet been identified. Other species differences have also been described. For example, the amphibian, Necturus, demonstrates bidirectional organic anion transport, including an uphill luminal step and the urinary bladders of certain species of crustaceans show net reabsorption, whereas the bladders of other species show net secretion of PAH. However, these differences may well prove to be important tools in assessing PAH transport, since amplification of specific pathways and the increased experimental control possible in intact tissue preparations from some of these species, e.g., flounder, snake and amphibian tubules or crustacean urinary bladder, may facilitate resolution of many of the remaining uncertainties. Species differences cannot explain the wide variety of results reported for the basolateral membrane transport step, since many of the conflicting studies were done in the same species. Difficulties inherent in vesicle techniques have been discussed above (Subsection III-A2), and emphasize the need to correlate such data with intact tissue preparations. However, the major source of confusion appears to be related to the ability of PAH to interact with several transport systems, directly or indirectly. Thus, despite the preponderance of evidence showing that the PAH transport system at the basolateral membrane is distinct from those for sulfate, mono- and dicarboxylic acids, acidic amino acids, and uric acid, there remains the real possibility that under physiological conditions: PAH may be a minor substrate for these other systems, substrates for other systems may inhibit PAH transport directly through competition for the PAH carrier or indirectly through competition for the same energy source, and entry of a substrate on one system may trans-stimulate PAH uptake on another. Furthermore, the existence of multiple systems may explain the inability of certain manipulations, e.g., Na gradient dissipation in vivo, to block PAH transport. PAH entry may simply increase via another pathway, e.g., anion exchange.(ABSTRACT TRUNCATED AT 400 WORDS)

Proximal tubule dopamine histofluorescence in renal slices incubated with L-dopa

Renal cortex slices were incubated with amine precursors L-dopa, or L-5-HTP. Localization of synthesized amines, dopamine or serotonin, was examined by means of histofluorescence methods. Formaldehyde-induced fluorescence was present in the proximal convoluted tubule and not in the pars recta or other segments of the tubules after incubation in the presence of 10(-3) to 10(-7) M L-dopa. Tubule-induced fluorescence was not seen in the presence of an inhibitor of dopa-decarboxylase or in the absence of sodium. It was independent of innervation. It is concluded that dopamine and serotonin are accumulated and likely formed within proximal convoluted tubular cells.

Kidney function after unilateral nephrectomy

Immediately after unilateral nephrectomy ( uNX ) some different mechanisms of compensatory adaptation begin to act followed by a restoration of sufficient kidney function in a short time period. Beside biochemical changes early compensatory hypertrophy of the remaining kidney occurs. Simultaneously, functional adaptations of renal blood flow, glomerular filtration and exertion of electrolyte and xenobiotics take place. With a suitable pretreatment it is principally possible to accelerate the regeneration phase. Thus the phase of reduced excretion capacity of tubularly secreted xenobiotics after removal of one kidney can be shortened or prevented.

Inhibitory effect of unconjugated bilirubin on p-aminohippurate transport in rat kidney cortex slices

The effects of unconjugated bilirubin on the accumulation of p-aminohippurate, kinetics of p-aminohippurate uptake, the efflux of pre-accumulated p-aminohippurate and water and electrolyte distribution were investigated in the rat kidney cortical slice. (2) The addition of unconjugated bilirubin to the incubation medium decreased the 60 min slice-to-medium concentration ratio of p-aminohippurate. (3) The decrease in p-aminohippurate accumulation by unconjugated bilirubin was found to be more pronounced by increasing the concentration of pigment in the medium. (4) The rate of uptake of p-aminohippurate as a function of p-aminohippurate concentration differed in aerobiosis and anaerobiosis, and unconjugated bilirubin decreased only the uptake of p-aminohippurate in aerobic conditions. (5) The efflux of pre-accumulated p-aminohippurate decreased when unconjugated bilirubin concentration in the medium was low (10-20 microM) but the efflux increased when the concentration of pigment was much higher (100 microM). (6) The addition of unconjugated bilirubin to the medium (40-100 microM) increased intracellular sodium and total tissue water content, and decreased intracellular potassium and oxygen consumption of tissue. However the slices incubated with low concentration of pigment (20 microM) did not exhibit significative changes in cellular functional parameters. (7) These findings suggest that unconjugated bilirubin impairs p-aminohippurate transport by a complex mechanism that might involve binding of pigment to sites necessary for anion transport, although effects related to pigment toxicity or to its oxidative decomposition are not excluded.

The effect of cyclic AMP on dog renal function

The infusion of dibutyryl-cyclic AMP into the dog renal artery in vivo leads to diuresis, natriuresis and glucosuria. Addition of the nucleotide to the incubation medium bathing dog renal cortex slices in vitro causes inhibition of p-amino-hippurate accumulation and stimulation of glycine and beta-methyl-glucoside transport. The results are interpreted in terms of the development of a blood-lumen flux of sodium and water in the renal proximal tubule, analogous to that seen in the intestine.

Stimulatory and inhibitory effects of sulfhydryl reagent on p-aminohippuric acid transport by isolated renal tubules

Isolated tubules from rabbit kidney cortex were treated with several different sulfhydryl reagents in an attempt to determine whether sulfhydryl groups are involved in organic acid transport. Disulfide reagents such as sodium tetrathionate and 6,6'-dithionicotinic acid were found to exert a biphasic effect on p-aminohippuric acid transport, i.e. transient stimulation followed by inhibition. In contrast, treatment of tubules with the mercaptide-forming reagent, p-chloromercuribenzoate, caused only inhibition of organic acid transport. Treatment of tubules with reductants such as dithiothreitol or mercaptoethanol blocked the stimulatory effect of tetrathionate without affecting the inhibitory effect of this oxidant. The inhibition caused by p-chloromercuribenzoate, however, was largely reversible when tubules were treated with reductants. The results suggest that the renal organic acid transport system contains sulfhydryl groups and that its activity is increased when some of these groups are oxidized.

Mechanism of inhibition of organic acid transport in rabbit renal cortex by cyclic AMP

The mechanism by which isoproterenol and cyclic AMP inhibit organic acid transport was examined. Accumulation of 131I-Hippuran (S/M) was used as an index of organic acid transport. Propranolol, an antagonist of isoproterenol's beta-adrenergic response, not only inhibited accumulation of Hippuran but, when combined with isoproterenol, caused further inhibition. PTH elicited larger increases than isoproterenol in cortical-slice cyclic AMP content but did not inhibit accumulation of Hippuran. This lack of correlation between a positive effect on the adenylate-cyclase-cyclic AMP system and inhibition of transport was also seen when other agents were tested. Cyclic GMP, which has been postulated to work in opposition to cyclic AMP, was as potent as cyclic AMP in decreasing S/M. The decreasing of S/M by adenine and uric acid could not be related to any known effect of these agents on tissue cyclic AMP content. Similarly, although 0.1 mM theophylline significantly decreased S/M, cortical cyclic-AMP content was not increased until a 100-fold greater concentration of theophylline was employed. The data suggest that the inhibitory effect of these agents was attributable to their molecular configurations, which interact directly with the organic acid transport system rather than indirectly via the adenylate-cyclase-cyclic AMP system.