Adenosine stimulation of Na+ transport is mediated by an A1 receptor and a [Ca2+]i-dependent mechanism

Adenosine, type 1 receptors: role in proximal tubule Na+ reabsorption

Adenosine type 1 receptor (A1 -AR) antagonists induce diuresis and natriuresis in experimental animals and humans. Much of this effect is due to inhibition of A1 -ARs in the proximal tubule, which is responsible for 60-70% of the reabsorption of filtered Na(+) and fluid. Intratubular application of receptor antagonists indicates that A1 -AR mediates a portion of Na(+) uptake in PT and PT cells, via multiple transport systems, including Na(+) /H(+) exchanger-3 (NHE3), Na(+) /PO4(-) co-transporter and Na(+) -dependent glucose transporter, SGLT. Renal microperfusion and recollection studies have shown that fluid reabsorption is reduced by A1 -AR antagonists and is lower in A1 -AR KO mice, compared to WT mice. Absolute proximal reabsorption (APR) measured by free-flow micropuncture is equivocal, with studies that show either lower APR or similar APR in A1 -AR KO mice, compared to WT mice. Inhibition of A1 -ARs lowers elevated blood pressure in models of salt-sensitive hypertension, partially due to their effects in the proximal tubule.

The effect of antagonism of adenosine A1 receptor against ischemia and reperfusion injury of the liver

BACKGROUND

Adenosine is known to exert protective roles in hepatic ischemia and reperfusion injury, while all adenosine receptors do not play the cytoprotective roles. We have tested our hypothesis that blockage of adenosine binding to A(1) receptor by its antagonist, KW3902 [8-(noradamantan-3-yl)-1,3-dipropylxanthine] attenuates hepatic ischemia-reperfusion injury.

METHODS

Adult female beagle dogs underwent a 2 h total hepatic vascular exclusion (THVE) with a venovenous bypass. Nontreated animals that underwent THVE with a venovenous bypass alone were used as the control (Group CT, n=6). KW3902 was given to the animals by continuous intraportal infusion for 60 min before ischemia at a dose of 1 microg/kg/min (Group KW, n=6). Two wk survival, hemodynamics, hepatic tissue blood flow (HTBF), liver function, energy metabolism, cAMP concentration, and histopathological findings were studied.

RESULTS

Two wk animal survival was significantly improved in group KW compared with that in group CT (group CT: 16.7% versus group KW: 83.3%). HTBF, liver function, and hepatic adenine nucleotide concentration were remarkably better in group KW than group CT. In addition, cAMP concentration in group KW was maintained significantly higher than group CT. Histopathological examination revealed preservation of hepatic architecture and suppression of neutrophil infiltration into hepatic tissue in group KW.

CONCLUSION

Administration of adenosine A(1) receptor antagonist before ischemia attenuates hepatic ischemia-reperfusion injury. To elicit the beneficial effect of adenosine against ischemia and reperfusion injury of the liver, it is important to oppose adenosine A1 receptor activation.

Adenosine-evoked Na+ transport in human airway epithelial cells

BACKGROUND AND PURPOSE

Absorptive epithelia express apical receptors that allow nucleotides to inhibit Na(+) transport but ATP unexpectedly stimulated this process in an absorptive cell line derived from human bronchiolar epithelium (H441 cells) whilst UTP consistently caused inhibition. We have therefore examined the pharmacological basis of this anomalous effect of ATP.

EXPERIMENTAL APPROACH

H441 cells were grown on membranes and the short circuit current (I(SC)) measured in Ussing chambers. In some experiments, [Ca(2+)](i) was measured fluorimetrically using Fura -2. mRNAs for adenosine receptors were determined by the polymerase chain reaction (PCR).

KEY RESULTS

Cross desensitization experiments showed that the inhibitory response to UTP was abolished by prior exposure to ATP whilst the stimulatory response to ATP persisted in UTP-pre-stimulated cells. Apical adenosine evoked an increase in I(SC) and this response resembled the stimulatory component of the response to ATP, and could be mimicked by adenosine receptor agonists. Pre-stimulation with adenosine abolished the stimulatory component of the response to ATP. mRNA encoding A(1), A(2A) and A(2B) receptor subtypes, but not the A(3) subtype, was detected in H441 cells and adenosine receptor antagonists could abolish the ATP-evoked stimulation of Na(+) absorption.

CONCLUSIONS AND IMPLICATIONS

The ATP-induced stimulation of Na(+) absorption seems to be mediated via A(2A/B) receptors activated by adenosine produced from the extracellular hydrolysis of ATP. The present data thus provide the first description of adenosine-evoked Na(+) transport in airway epithelial cells and reveal a previously undocumented aspect of the control of this physiologically important ion transport process.

Transport characteristics of HL-1 cells: a new model for the study of adenosine physiology in cardiomyocytes

Adenosine is a physiologically important nucleoside in the cardiovascular system where it can act as a cardioprotectant and modulator of energy usage. Adenosine transporters (ATs) modulate cellular adenosine levels, which, in turn, can affect a number of processes such as receptor activation and glucose uptake, but their role in cardiac physiology is poorly understood. Therefore, we have developed a new cell model by determining various adenosine-related characteristics of HL-1, an immortalized atrial cardiomyocyte murine cell line. Adenosine uptake in HL-1 cells is sodium independent, saturable, and inhibitable by nucleoside transport inhibitors (nitrobenzylthioinosine (NBTI), dipyridamole, dilazep). Reverse transcription--polymerase chain reaction analysis confirmed that HL-1 cells possess mouse equilibrative nucleoside transporters 1 and 2 (mENT1, mENT2) and kinetic analyses indicate moderate-affinity (Km = 51.3 +/- 12.9 microM), NBTI-sensitive adenosine transport. NBTI binds at a high-affinity single site (B(max) = 520 +/- 10 fmol/mg protein, Kd = 0.11 +/- 0.04 nM, 1.6 x 10(5) NBTI-binding sites/cell). HL-1 cells possess adenosine receptor, metabolic enzyme, protein kinase C isoform, and insulin-stimulated glucose transport profiles that match normal mouse heart. Therefore, HL-1 is an excellent model to study ATs within cardiomyocytes and the first model for evaluating in detail the role of the ATs in modulating effects of adenosine.

Basolateral and apical A1 adenosine receptors mediate sodium transport in cultured renal epithelial (A6) cells

There are conflicting reports in the literature regarding the adenosine receptor that mediates the increase in sodium transport in the A6 cell. In this study we used specific A1 and A2 adenosine receptor agonists and antagonists, as well as two different subclones of the A6 cell, to determine which adenosine receptor mediates the increase in sodium transport. In the A6S2 subclone, basolateral and apical N6-cyclohexyladenosine (CHA), a selective A1 receptor agonist, stimulated sodium transport at a threshold concentration <10(-7) M, whereas CGS-21680, a selective A2 receptor agonist, had a threshold concentration that was at least 10(-5) M. The A1 receptor antagonist 8-cyclopentyl-1,3-dipropylxanthine (DPCPX) was found to have a nonspecific effect on CHA-stimulated sodium transport, whereas the A2 receptor antagonist 8-(3-chlorostyryl)caffeine (CSC) had no effect. As with the A6S2 subclone, basolateral and apical CHA stimulated sodium transport at a nanomolar concentration in the A6C1 subclone and the threshold concentration for CGS-21680 was in the high micromolar range. Concurrent with the increase in 1 receptor in different subclones of the A6 cell, including a subclone capable of anion secretion.

Annelid epithelia as models for electrogenic Na+ transport

The electrogenic Na(+) absorption across tight epithelia from invertebrates follows the principles analog to the mechanisms found in vertebrates. Extracellular Na(+)-ions pass the apical cell membranes through highly selective Na(+) channels and follow an electrochemical gradient which is sustained by the basolateral Na(+)/K(+)-ATPases. These apical Na(+) channels are selectively blocked by amiloride and represent the rate-limiting target for the control of transcellular Na(+) uptake. Although annelids express ADH-like peptide hormones, they lack the osmoregulatory mineralocorticoid system with the vertebrate-specific key hormone aldosterone. Thus, their epithelia may represent interesting models for investigation of ion transport regulation. While the formation of urine in the nephridia of, for example, leeches had been subject to intensive studies, the investigation of ion transport across their body wall was largely neglected. We use dissected segments of integuments from the limnic leech Hirudo medicinalis and, recently, from the earthworm Lumbricus terrestris for Ussing chamber experiments. We investigate transintegumental ion transport with focus on control of electrogenic Na(+) uptake and the amiloride-sensitive part of it and identified several extracellular factors as peptide hormones, tri- and divalent cations or purinergic molecules with regulatory effects on it. Meanwhile, there exists a macroscopic view on Na(+) absorption; however, other ion transport mechanisms across annelid integuments still await scientific effort. Here we present a concise synopsis about the electrophysiology of annelid integuments to illustrate the state of science and to evaluate whether further studies in this particular field may be of interest.

Adenosine type 1 receptor antagonists in fluid retaining disorders

Adenosine is a vasoactive hormone whose action is mediated through at least four receptors. The most prevalent receptors are type 1, which promote vasoconstriction, and type 2, comprised of 2 subtypes (a,b) that promote vasodilation. In the kidney, type 1 receptors located on preglomerular vessels and in the tubule are involved in the regulation of glomerular filtration. Whole body fluid balance is strongly dependent on the ability of the kidney to maintain stable glomerular filtration. Several antagonists to adenosine type 1 receptors have been developed. These agents generate excess fluid (diuresis) and sodium (natriuresis) excretion in control animals and animal models of fluid retention, as well as in normal and oedematous humans. In both animals and humans, these effects are generally achieved without major changes in glomerular filtration. Animal studies have confirmed the location of adenosine type 1 receptors in relevant tissue sites in the kidney. More highly selective antagonists for adenosine type 1 receptors are regularly developed, improving their use in fluid retaining disorders. Clinical trials with these agents have commenced for the treatment of hypertension, renal failure and congestive heart failure, all disorders that include varying levels of fluid retention. The clinical trial results have been mixed. The early results with congestive heart failure suggest great promise for these agents, whereas trials in hypertension and renal failure have been equivocal.

The effect of caffeine on renal epithelial cells from patients with autosomal dominant polycystic kidney disease

Autosomal dominant polycystic kidney disease (ADPKD) is a hereditary disorder characterized by the progressive enlargement of cysts derived from tubules. Tubule cell proliferation and chloride-dependent fluid accumulation, mechanisms underlying cyst expansion, are accelerated by adenosine 3':5'-cyclic monophosphate (cAMP). This study examined the extent to which caffeine may stimulate the production of cAMP by cyst epithelial cells, thereby adversely increasing proliferation and fluid secretion. Mural epithelial cells from ADPKD cysts and normal human kidney cortex cells (HKC) were cultured, and cAMP levels were determined in response to caffeine and receptor-mediated agonists linked to adenylyl cyclase. Caffeine, a methylxanthine, slightly increased basal levels of cAMP, as did other nonselective phosphodiesterase (PDE) inhibitors, 1-methyl-3- isobutyl xanthine and theophylline and rolipram, a specific PDE IV inhibitor. More importantly, clinically relevant concentrations of caffeine (10 to 50 micro M) potentiated the effects of desmopressin (DDAVP), prostaglandin E(2) (PGE(2)), and isoproterenol to increase cAMP levels in both ADPKD and HKC cells. By contrast, at concentrations that augmented the DDAVP response, caffeine attenuated cAMP accumulation by adenosine, implicating an action apart from the inhibition of PDE. Caffeine enhanced the effect of DDAVP to stimulate transepithelial short-circuit current of polarized ADPKD monolayers, reflecting an increase in chloride secretion. Caffeine potentiated the effect of DDAVP and PGE(2) to increase the levels of phosphorylated extracellular signal-regulated kinase (P-ERK). By contrast, P-ERK levels in HKC cells were not raised by increased intracellular concentrations of cAMP. It is concluded that PDE inhibition by caffeine increases the accumulation of cAMP, and through this mechanism activates the ERK pathway to cellular proliferation and increases transepithelial fluid secretion in ADPKD cystic epithelium. Caffeine is, therefore, a risk factor for the promotion of cyst enlargement in patients with ADPKD.

On the natriuretic effect of verapamil: inhibition of ENaC and transepithelial sodium transport

The natriuretic effect of Ca(2+) channel blockers has been attributed to hemodynamic changes and to poorly defined direct tubular effects. To test the possibility that verapamil may inhibit Na(+) reabsorption at the distal tubule, its effect on transepithelial Na(+) transport in aldosterone-stimulated A6 cells was determined. Cells were grown on permeable supports, and short-circuit current (I(sc)) measured in an Ussing chamber was used as a surrogate marker for transepithelial Na(+) transport. Application of 300 microM verapamil to the apical side inhibited I(sc) by 77% and was nearly as potent as 100 microM amiloride, which inhibited I(sc) by 87%. Verapamil-induced inhibition of I(sc) was accompanied by a significant increase in transepithelial resistance, suggesting blockade of an apical conductance. Its action on transepithelial Na(+) transport does not appear to occur through inhibition of L-type Ca(2+) channels, since I(sc) was unaffected by removal of extracellular Ca(2+). Verapamil also does not appear to inhibit I(sc) by modulating intracellular Ca(2+) stores, since it fails to inhibit transepithelial Na(+) transport when added to the basolateral side. The effect on Na(+) transport is specific for verapamil, since nifedipine, Ba(2+), 4-aminopyridine, and charybdotoxin do not significantly affect I(sc). A direct effect of verapamil on the epithelial Na(+) channel (ENaC) was tested using oocytes injected with the alpha-, beta-, and gamma-subunits. We conclude that verapamil inhibits transepithelial Na(+) transport in A6 cells by blocking ENaC and that the natriuresis observed with administration of verapamil may be due in part to its action on ENaC.

Effects of extracellular purines on ion transport across the integument of Hirudo medicinalis

Little is known about the long-term regulation of epithelial ion transport in invertebrates and the specific mediators involved. For some years, we have been investigating the short-term regulation of transepithelial ion transport across the dorsal integument of the leech Hirudo medicinalis, and we have established a model of Na+ uptake. In the present study, we investigated the effect of long-term acclimation on transintegumental ion transport by adapting leeches to high-salinity conditions. We dissected segments of dorsal integument and measured ion currents in Ussing chamber experiments. Electrophysiological variables, such as transepithelial potential(VT) and short-circuit-current (Isc),were profoundly affected by adaptation to high-salinity conditions. The total transepithelial Na+ current (INa) decreased from 7.66±0.82 to 4.6±0.54 μA cm-2 in preparations adapted to high salinity. The involvement of epithelial Na+channels was determined as current inhibition (Iami) by apical application of amiloride; Na+ channels were equally active in control epithelia and epithelia from leeches adapted to high salinity. Removal of Ca2+ from the apical solutions, which is believed to reduce intracellular Ca2+ concentrations, equalized transepithelial variables between high-salt-adapted integuments and control integuments.

Extracellular purines regulate transepithelial Cl- secretion and Na+ absorption. In a variety of tissues we tested ATP and adenosine for their effects on epithelial transport. Examination of integuments from pondwater- and high-salinity-adapted leeches revealed different sensitivities for these purines. Apical and basolateral application of ATP both stimulated transepithelial Na+ uptake and Iami. Adenosine upregulated non-Na+ currents and acted from the basolateral side only. Apical Ca2+-free conditions attenuated these effects of purines on transepithelial currents. Extracellular UTP had no effect on ion transport.

Effects of adenosine agonists on consumptive behaviour and body temperature

This study was designed to determine the effects of the A1-receptor selective agonist N6-cyclopentyladenosine (CPA), and the A2-selective agonist, 2-p-(2-carboxyethyl)-phenethylamino-5'-N-ethylcarboxamidoadenosine-hydrochloride (CGS-21680) on consumptive behaviour and body temperature in rats in relation to the non-selective A1/A2 adenosine agonist, N-ethylcarboxamidoadenosine (NECA), and to morphine. It was shown that two subcutaneous injections of 0.1 and 0.3 mg kg(-1) CPA caused a similar decrease in food consumption to NECA (2 x 0.03 mg kg(-1)) and morphine (2 x 10 mg kg(-1)). However, two doses of 0.03 mg kg(-1) CPA and 0.1 and 0.3 mg kg(-1)CGS-21680 enhanced feeding. These effects were not directly correlated to faecal output at all doses of the selective agonists, as NECA and morphine induced constipation. The doses of CPA and 0.1 and 0.3 mg kg(-1) of CGS-21680 enhanced water consumption, as did NECA, but not morphine. The stimulation of drinking by CPA was not absolutely associated with diuresis. Instead, urine output was reduced by 0.03 and 0.1 mg kg(-1) and increased by 0.3 mg kg(-1). CGS-21680 at 0.1 and 0.3 mg kg(-1) and NECA also induced diuresis, which was opposite to the effect of morphine. CPA and CGS-21680 both caused significant dose-dependent decreases in body temperature after the two-injection treatment, but their effects were significantly less after 36 h when four doses had been administered. The study indicates that highly selective A1 and A2A adenosine agonists might have the ability to interfere with consumptive behaviour, induce constipation, affect renal function and to lower body temperature.

Adenosine modulates Mg(2+) uptake in distal convoluted tubule cells via A(1) and A(2) purinoceptors

tk;1Adenosine plays a role in the control of water and electrolyte reabsorption in the distal tubule. As the distal convoluted tubule is important in the regulation of renal Mg(2+) balance, we determined the effects of adenosine on cellular Mg(2+) uptake in this segment. The effect of adenosine was studied on immortalized mouse distal convoluted tubule (MDCT) cells, a model of the intact distal convoluted tubule. The rate of Mg(2+) uptake was measured with fluorescence techniques using mag-fura 2. To assess Mg(2+) uptake, MDCT cells were first Mg(2+) depleted to 0.22 +/- 0.01 mM by being cultured in Mg(2+)-free media for 16 h and then placed in 1.5 mM MgCl(2); next, changes in intracellular Mg(2+) concentration ([Mg(2+)](i)) were determined. [Mg(2+)](i) returned to basal levels, 0.53 +/- 0.02 mM, with a mean refill rate, d([Mg(2+)](i))/dt, of 137 +/- 16 nM/s. Adenosine stimulates basal Mg(2+) uptake by 41 +/- 10%. The selective A(1) purinoceptor agonist N(6)-cyclopentyladenosine (CPA) increased intracellular Ca(2+) and decreased parathyroid hormone (PTH)-stimulated cAMP formation and PTH-mediated Mg(2+) uptake. On the other hand, the selective A(2) receptor agonist 2-[p-(2-carbonyl-ethyl)-phenylethylamino]-5'-N-ethylcarboxamidoadenosine (CGS) stimulated Mg(2+) entry in a concentration-dependent fashion. CGS increased cAMP formation and the protein kinase A inhibitor RpcAMPS inhibited CGS-stimulated Mg(2+) uptake. Selective inhibition of phospholipase C, protein kinase C, or mitogen-activated protein kinase enzyme cascades with U-73122, Ro-31-8220, and PD-98059, respectively, diminished A(2) agonist-mediated Mg(2+) entry. Aldosterone potentiated CGS-mediated Mg(2+) entry, and elevation of extracellular Ca(2+) diminished CGS-responsive cAMP formation and Mg(2+) uptake. Accordingly, MDCT cells possess both A(1) and A(2) purinoceptor subtypes with intracellular signaling typical of these respective receptors. We conclude that adenosine has dual effects on Mg(2+) uptake in MDCT cells through separate A(1) and A(2) purinoceptor pathways.

Adenosine activates aromatic L-amino acid decarboxylase activity in the kidney and increases dopamine

Renal sodium handling is important for regulating BP, and renal dopamine and adenosine play an important role in renal sodium handling, however the interaction of these hormones in the kidney was not clarified. In in vivo experiments, adenosine significantly increased water and sodium excretion by 50% compared with vehicle when infused into the left renal artery, accompanied by an increase in urinary dopamine excretion in the left kidney. Neither water-sodium excretion nor dopamine excretion changed in the vehicle-infused kidney. Aromatic L-amino acid decarboxylase activity in the left kidney was significantly higher than that in the noninfused right kidney. The increase in water-sodium excretion induced by adenosine was significantly inhibited by SCH23390, a selective D1 receptor antagonist. In in vitro experiments, porcine renal proximal tubular cells were incubated with 250 microM L-dopa and N(6)-cyclohexyladenosine, an adenosine type 1 receptor agonist, after treatment with adenosine deaminase. N(6)-cyclohexyladenosine significantly increased dopamine formation at a concentration of 10(-9) to 10(-7) M, and this was completely inhibited by 1,3-dipropyl-8-cyclopentylxanthin, an adenosine A1 antagonist. These results show that renal dopamine synthesis is stimulated by adenosine through the activation of aromatic L-amino acid decarboxylase and suggest that adenosine leads to an increase in renal dopamine and natriuresis.

Regulation of the epithelial Na(+) channel by extracellular acidification

The effect of extracellular acidification was tested on the native epithelial Na(+) channel (ENaC) in A6 epithelia and on the cloned ENaC expressed in Xenopus oocytes. Channel activity was determined utilizing blocker-induced fluctuation analysis in A6 epithelia and dual electrode voltage clamp in oocytes. In A6 cells, a decrease of extracellular pH (pH(o)) from 7.4 to 6.4 caused a slow stimulation of the amiloride-sensitive short-circuit current (I(Na)) by 68.4 +/- 11% (n = 9) at 60 min. This increase of I(Na) was attributed to an increase of open channel and total channel (N(T)) densities. Similar changes were observed with pH(o) 5.4. The effects of pH(o) were blocked by buffering intracellular Ca(2+) with 5 microM 1, 2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid. In oocytes, pH(o) 6.4 elicited a small transient increase of the slope conductance of the cloned ENaC (11.4 +/- 2.2% at 2 min) followed by a decrease to 83.7 +/- 11.7% of control at 60 min (n = 6). Thus small decreases of pH(o) stimulate the native ENaC by increasing N(T) but do not appreciably affect ENaC expressed in Xenopus oocytes. These effects are distinct from those observed with decreasing intracellular pH with permeant buffers that are known to inhibit ENaC.

Toward a comprehensive molecular model of active calcium reabsorption

The fine tuning of Ca(2+) excretion in the kidney takes place in the distal nephron, which consists of the distal convoluted tubule, connecting tubule, and initial portion of the cortical collecting duct. In these segments, Ca(2+) is reabsorbed through an active transcellular pathway. The apical influx of Ca(2+) into the distal renal cell is presumably the rate-limiting step in this process, and its molecular identity has remained obscure so far. The recently discovered epithelial Ca(2+) channel (ECaC) exhibits the expected properties for being the gatekeeper in transcellular Ca(2+) reabsorption. The characteristics and potential physiological role of ECaC will be discussed in this review. Our knowledge of the mechanisms involved in the regulation of transcellular Ca(2+) transport has advanced rapidly since the development of cell models originating from distal tubular cells. Studies using these models indicate that hormones including arginine vasopressin, PGE(2), adenosine, ATP, and atrial natriuretic peptide should be considered as calciotropic hormones controlling renal Ca(2+) handling. Evidence is now beginning to emerge that the stimulating calciotropic hormones utilize new cAMP-independent pathways to stimulate Ca(2+) reabsorption. These new findings allow the development of a comprehensive and detailed model of the process of transcellular calcium transport in the kidney whereby the individual contribution of the participating transporters can now be fully appreciated.

Hormone-stimulated Ca2+ reabsorption in rabbit kidney cortical collecting system is cAMP-independent and involves a phorbol ester-insensitive PKC isotype

BACKGROUND

Hormones such as parathyroid hormone (PTH), arginine vasopressin (AVP), and prostaglandin E2 (PGE2) are generally believed to act through cAMP to stimulate active Ca2+ reabsorption in the distal part of the nephron.

METHODS

This study investigates the relationship between intracellular cAMP levels and the rate of Ca2+ reabsorption in immunodissected rabbit connecting and cortical collecting tubules cultured to confluence on permeable supports.

RESULTS

Basolateral PTH, AVP, and PGE2 and apical adenosine dose dependently increased Ca2+ reabsorption from 48 to 110 nmol. hr-1. cm-2. Measurement of intracellular cAMP levels revealed that in the case of PTH and AVP, the dose-response curve for the increase in cAMP virtually matched that for transcellular Ca2+ transport. By contrast, with PGE2, this curve was shifted two decades to the right, whereas in the case of adenosine, no increase in cAMP was observed. The results with the latter two hormones disagree with the classic concept that Ca2+ reabsorption is stimulated via a cAMP-dependent mechanism. Furthermore, the potent adenylyl cyclase inhibitor 2', 5'-dideoxyadenosine (DDA; 100 micrometers) suppressed the PTH- and AVP-induced increase in cAMP completely without affecting Ca2+ reabsorption. Similarly, concentrations of PGE2, which maximally stimulated Ca2+ reabsorption without increasing cAMP, were not inhibited by DDA. The specific protein kinase C (PKC) inhibitor chelerythrine (5 micrometers) inhibited PTH-, AVP-, PGE2-, and adenosine-stimulated Ca2+ reabsorption by 77%, 67%, 79%, and 100%, respectively. Down-regulation of phorbol ester-sensitive PKC isotypes by prolonged (120 hr) treatment with 0.1 micrometers 12-O-tetradecanoylphorbol 13-acetate did not interfere with the inhibitory action of chelerythrine on hormone-stimulated Ca2+ transport.

CONCLUSION

PTH, AVP, PGE2, and adenosine stimulate Ca2+ reabsorption via a pathway that is independent of cAMP and that involves a phorbol ester-insensitive PKC isotype.

TECHNICAL NOTE: measurement of cAMP-dependent protein kinase activity using a fluorescent-labeled kemptide

BACKGROUND

Traditional protein kinase assays include the use of [32P] labeled ATP as phosphate donor and a substrate protein or peptide as phosphoreceptor. Since this approach has a number of drawbacks in addition to generating ionizing radiation, several non-isotopic methods have been developed. Although shown to reflect the activity of purified enzymes, none have been demonstrated to detect physiological changes in endogenous enzyme activity in cell homogenates.

METHODS

Studies were performed to examine the kinetics, reproducibility, and optimal assay conditions of a novel non-radioisotopic kinase assay that detects PKA activity by phosphorylation of the peptide substrate Kemptide covalently bound to a fluorescent molecule (f-Kemptide). Basal and agonist-induced PKA activity in epithelial cell homogenates was measured.

RESULTS

The kinetics of f-Kemptide were similar to the standard radioisotopic method with intraassay and interassay variations of 5.6 +/- 0.8% and 14.3 +/- 2.6%, respectively. Neither fluorescence quenching nor enhancing effects were found with consistent amounts of homogenate protein. Specific PKA activity was determined as the IP20-inhibitable fraction to account for nonspecific phosphorylation, perhaps due to S6 kinase or a similar enzyme. The basal activity of 38% of total PKA in A6 cells increased by 84% after exposure to vasopressin and by 58% after short exposure to forskolin. In T84 cells exposed to VIP there was a 360% increase over basal activity.

CONCLUSIONS

These results show that f-Kemptide exhibits acceptable kinetics, and that the assay system can quantitatively and reproducibly measure basal and stimulated PKA activity in cell homogenates.

Adenosine-stimulated Ca2+ reabsorption is mediated by apical A1 receptors in rabbit cortical collecting system

Confluent monolayers of immunodissected rabbit connecting tubule and cortical collecting duct cells, cultured on permeable supports, were used to study the effect of adenosine on net apical-to-basolateral Ca2+ transport. Apical, but not basolateral, adenosine increased this transport dose dependently from 48 +/- 3 to 110 +/- 4 nmol.h-1.cm-2. Although a concomitant increase in cAMP formation suggested the involvement of an A2 receptor, the A2 agonist CGS-21680 did not stimulate Ca2+ transport, while readily increasing cAMP. By contrast, the A1 agonist N6-cyclopentyladenosine (CPA) maximally stimulated Ca2+ transport without significantly affecting cAMP. Adenosine-stimulated transport was effectively inhibited by the A1 antagonist 1,3-dipropyl-8-cyclopenthylxanthine but not the A2 antagonist 3,7-dimethyl-1-propargylxanthine, providing additional evidence for the involvement of an A1 receptor. Both abolishment of the adenosine-induced transient increase in intracellular Ca2+ concentration by 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid and downregulation of protein kinase C (PKC) by prolonged phorbol ester treatment were without effect on adenosine-stimulated Ca2+ transport. The data presented suggest that adenosine interacts with an apical A1 receptor to stimulate Ca2+ transport via a hitherto unknown pathway that does not involve cAMP formation, PKC activation, and/or Ca2+ mobilization.

Vasopressin-induced activation of protein kinase C in renal epithelial cells

Recent studies indicate that the actions of arginine vasopressin (AVP) and other agonists that stimulate electrogenic sodium transport in renal epithelial A6 cells are linked to a Ca(2+)-mobilizing signal transduction mechanism that involves generation of inositol trisphosphate. Since diacylglycerol is the other product in this pathway, studies were performed to determine the possible role of PKC in the stimulation of sodium transport. AVP induced a biphasic increase in diacylglycerol generation, characterized by an initial rapid rise and then a sustained elevation, and PKC activation, reflected by phosphorylation of a specific 80 kDa myristoylated alanine-rich PKC substrate (MARCKS). To determine the PKC isoform(s) involved in this process, immunoblot analysis was performed using antisera that recognize both classical PKC isoforms, XPKC-I and XPCK-II, cloned from Xenopus oocytes. The transcripts of both isoforms were expressed in the A6 cell. Since protein recognized by antisera was translocated from cytosol to the particulate fraction after exposure to AVP, one or both isoforms were activated in the A6 cell. Further studies showed that cyclohexyladenosine and insulin, additional agonists of sodium transport in A6 cells, also stimulated phosphorylation of MARCKS. These results argue that Ca(2+)-dependent PKC is involved in the action of AVP, and that of other agonists, which stimulate sodium transport.

Cirsimarin and cirsimaritin, flavonoids of Microtea debilis (Phytolaccaceae) with adenosine antagonistic properties in rats: leads for new therapeutics in acute renal failure

In traditional medicine Microtea debilis is used against proteinuria. In ligand-binding studies extracts of Microtea debilis have been shown to inhibit the binding of [3H]1,3-dipropyl-8-cyclopentylxanthine ([3H]DPCPX) to adenosine-A1 receptors in rat forebrain membranes. Subsequently, cirsimarin, a flavonoid, was isolated as the active component and was shown to function as adenosine antagonist at the adenosine-A1 receptor in-vitro. In this study we have investigated the adenosine-A2 receptor activity of cirsimarin the in-vivo inhibition of the effects of adenosine by cirsimarin in rats, the absorption of cirsimarin and the inhibition of the binding of [3H]DPCPX to the adenosine-A1 receptor by urine samples obtained after oral administration of crude extract of Microtea debilis, cirsimarin or cirsimaritin to rats. Cirsimarin inhibited the binding of [3H]5'-N-ethylcarboxamidoadenosine ([3H]NECA) to adenosine-A2 receptors in rat striatum with an inhibition constant, Ki, of 6.5 +/- 0.3 microM. The decrease of heart rate and blood pressure induced by adenosine was significantly inhibited by cirsimarin. After oral administration of 8 and 80 mg kg-1 cirsimarin, the compound could not be detected in either plasma or urine, but the presence of cirsimaritin was established. By use of beta-glucuronidase, glucuronides of cirsimaritin were also detected in the urine. The concentrations of cirsimaritin in the plasma were 0.126 +/- 0.04, 0.138 +/- 0.015, and 0.120 +/- 0.022 microM, respectively, 2, 5 and 12 h after administration of 8 mg kg-1 cirsimarin. The concentrations of cirsimaritin in the urine at the same times after administration of the same dose were 2.05 +/- 1.86, 5.05 +/- 2.6 and 2.06 +/- 0.09 microM, respectively. The inhibition of the binding of [3H]DPCPX to the adenosine-A1 receptor by urine samples collected 2, 5 and 12 h after oral administration of 8 mg kg-1 cirsimarin or a crude extract of Microtea debilis containing approximately 8 mg kg-1 cirsimarin and 2.8 mg kg-1 cirsimaritin, or 6.8 mg kg-1 cirsimaritin, was not significantly different from that of urine samples collected from untreated rats, in contrast with urine samples collected 1 and 2 days after oral administration of 80 mg kg-1 cirsimarin. Approximately 3% of the cirsimarin was excreted in the urine as cirsimaritin. The results indicate that in the kidney and urinary tract the concentrations of cirsimaritin produced after ingestion of more than 8 mg kg-1 cirsimarin can be high enough to inhibit the interaction of adenosine with its receptors; this might explain the effectiveness of Microtea debilis preparations against proteinuria in traditional medicine.

Identification and characterization of MARCKS in Xenopus laevis

MARCKS proteins are widely distributed in mammalian cells and subserve an important role as probes in the examination of signal transduction processes because they are specific endogenous phosphoreceptors for activated protein kinase C. Experiments were performed to determine whether MARCKS proteins are present in amphibia and to show their usefulness as substrates for stimulated PKC activation, using cultured renal epithelial cells (A6) derived from Xenopus laevis as an experimental model.

Novel effect of insulin: insulin-stimulated Na+ transport is mediated by hydrolysis of phosphoinositides

Previous studies showed that insulin stimulation of electrogenic Na+ transport in renal epithelial cells is mediated by a calcium-dependent signal transduction mechanism. The present study was performed to determine whether the insulin-induced increase in intracellular Ca2+ (Cai2+) was mediated by hydrolysis of phosphatidylinositol and release of inositol trisphosphate. Experiments were conducted with cultured A6 cells, derived from Xenopus Laevis, grown on permeable supports. Addition of insulin resulted in 2 to 3 fold increases in inositol trisphosphate and a 50% increase in 1,2 diacylglycerol within 10s, which corresponded to the time-course, previously reported, of insulin stimulated increases in Na+ transport and Cai2+. Further studies showed that aldosterone, previously shown to stimulate an increase in 1,4,5-inositol trisphosphate at onset of the rise in Na+ transport, also increased DAG levels during the initial phase of stimulation of Na+ transport. These studies provide the first evidence that a biological response induced by insulin is mediated by hydrolysis of phosphatidylinositol 4,5-bisphosphate (PIP2) which results in two products, inositol trisphosphate which causes the release of Ca2+ from intracellular stores and 1,2 diacylglycerol. In addition this study provides further support for the proposal that a common signal transduction mechanism mediates electrogenic Na+ transport by multiple agonists.

Vasopressin-stimulated electrogenic sodium transport in A6 cells is linked to a Ca(2+)-mobilizing signal mechanism

Vasopressin is known to activate two types of cell surface receptors; V2, coupled to adenylate cyclase, and V1, linked to a Ca(2+)-dependent transduction system. We investigated whether arginine vasopressin (AVP) stimulation of electrogenic sodium transport in A6 cells, derived from Xenopus laevis, is mediated by activation of either one or both types of AVP-specific receptors. AVP caused a rapid increase in electrogenic sodium transport, reflected by the transepithelial potential difference (VT) and equivalent short circuit current (Ieq) measurements. AVP also rapidly increased intracellular Ca2+ (Ca2+i) and total inositol trisphosphate. The increase in Ieq was dependent on the rise in (Ca2+i), because 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (BAPTA) dose-dependently inhibited the Ieq response. There was no evidence, however, that activation of adenylate cyclase mediated AVP-stimulated Ieq; transport was not inhibited after AVP-induced activation of adenylate cyclase was abolished by 2',5'-dideoxyadenosine or when cAMP-dependent protein kinase (PKA) activity was abolished by the specific PKA inhibitor IP20. Further studies showed that although both forskolin and 8-(4-chlorophenylthio)-cAMP stimulated Ieq, this occurred by mechanisms independent of PKA activation. These results indicate that AVP-stimulated Na+ transport is mediated by a V1 receptor and a Ca(2+)-dependent mechanism.