PGE2 mediates the effect of pentagastrin on intestinal adenylate cyclase and Na-K-ATPase activities

Gastrin decreases Na+,K+-ATPase activity via a PI 3-kinase- and PKC-dependent pathway in human renal proximal tubule cells

The natriuretic effect of gastrin suggests a role in the coordinated regulation of sodium balance by the gastrointestinal tract and the kidney. The renal molecular targets and signal transduction pathways for such an effect of gastrin are largely unknown. Recently, we reported that gastrin induces NHE3 phosphorylation and internalization via phosphatidylinositol (PI) 3-kinase and PKCα. In this study, we show that gastrin induced the phosphorylation of human Na(+),K(+)-ATPase at serine 16, resulting in its endocytosis via Rab5 and Rab7 endosomes. The gastrin-stimulated phosphorylation of Na(+),K(+)-ATPase was dependent on PI 3-kinase because the phosphorylation was blocked by the PI 3-kinase inhibitor wortmannin. The phosphorylation of Na(+),K(+)-ATPase was also blocked by chelerythrine, a pan-PKC inhibitor, Gö-6976, a conventional PKC (cPKC) inhibitor, and BAPTA-AM, an intracellular calcium chelator, suggesting the importance of cPKC and intracellular calcium in the gastrin signaling pathway. The gastrin-mediated phosphorylation of Na(+),K(+)-ATPase was also inhibited by U-73122, a phospholipase C (PLC) inhibitor. These results suggest that gastrin regulates sodium hydrogen exchanger and pump in renal proximal tubule cells at the apical and basolateral membranes.

The importance of the gastrorenal axis in the control of body sodium homeostasis

NEW FINDINGS

What is the topic of this review? Sensing the amount of ingested sodium is one mechanism by which sodium balance is regulated. This review describes the role of gastrin in the cross-talk between the stomach and the kidney following the ingestion of sodium. What advances does it highlight? Neural mechanisms and several gut hormones, including cholecystokinin and uroguanylin, have been suggested to mediate the natriuresis after an oral sodium load. It is proposed that gastrin produced by G-cells via its receptor, cholecystokinin B receptor, interacts with renal D1 -like dopamine receptors to increase renal sodium excretion. Hypertension develops with chronically increased sodium intake when sodium that accumulates in the body can no longer be sequestered, extracellular fluid volume is expanded, and compensatory neural, hormonal and pressure-natriuresis mechanisms fail. Sensing the amount of ingested sodium, by the stomach, is one mechanism by which sodium balance is regulated. The natriuresis following the ingestion of a certain amount of sodium may be due to an enterokine, gastrin, secreted by G-cells in the stomach and duodenum and released into the circulation. Circulating gastrin levels are 10- to 20-fold higher than those for cholecystokinin. Of all the gut hormones circulating in the plasma, gastrin is the one that is reabsorbed to the greatest extent by renal tubules. Gastrin, via its receptor, the cholecystokinin type B receptor (CCKBR), is natriuretic in mammals, including humans, by inhibition of renal sodium transport. Germline deletion of gastrin (Gast) or Cckbr gene in mice causes salt-sensitive hypertension. Selective silencing of Gast in the stomach and duodenum impairs the ability to excrete an oral sodium load and also increases blood pressure. Thus, the gastrorenal axis, mediated by gastrin, can complement pronatriuretic hormones, such as dopamine, to increase sodium excretion after an oral sodium load. These studies in mice may be translatable to humans because the chromosomal loci of CCKBR and GAST are linked to human essential hypertension. Understanding the role of genes in the regulation of renal function and blood pressure may lead to the tailoring of antihypertensive treatment based on genetic make-up.

The Synergistic Roles of Cholecystokinin B and Dopamine D5 Receptors on the Regulation of Renal Sodium Excretion

Renal dopamine D₁-like receptors (D₁R and D₅R) and the gastrin receptor (CCK_BR) are involved in the maintenance of sodium homeostasis. The D₁R has been found to interact synergistically with CCK_BR in renal proximal tubule (RPT) cells to promote natriuresis and diuresis. D₅R, which has a higher affinity for dopamine than D₁R, has some constitutive activity. Hence, we sought to investigate the interaction between D₅R and CCK_BR in the regulation of renal sodium excretion. In present study, we found D₅R and CCK_BR increase each other’s expression in a concentration- and time-dependent manner in the HK-2 cell, the specificity of which was verified in HEK293 cells heterologously expressing both human D₅R and CCK_BR and in RPT cells from a male normotensive human. The specificity of D₅R in the D₅R and CCK_BR interaction was verified further using a selective D₅R antagonist, LE-PM436. Also, D₅R and CCK_BR colocalize and co-immunoprecipitate in BALB/c mouse RPTs and human RPT cells. CCK_BR protein expression in plasma membrane-enriched fractions of renal cortex (PMFs) is greater in D₅R^-/- mice than D₅R^+/+ littermates and D₅R protein expression in PMFs is also greater in CCK_BR^-/- mice than CCK_BR^+/+ littermates. High salt diet, relative to normal salt diet, increased the expression of CCK_BR and D₅R proteins in PMFs. Disruption of CCK_BR in mice caused hypertension and decreased sodium excretion. The natriuresis in salt-loaded BALB/c mice was decreased by YF476, a CCK_BR antagonist and Sch23390, a D₁R/D₅R antagonist. Furthermore, the natriuresis caused by gastrin was blocked by Sch23390 while the natriuresis caused by fenoldopam, a D₁R/D₅R agonist, was blocked by YF476. Taken together, our findings indicate that CCK_BR and D₅R synergistically interact in the kidney, which may contribute to the maintenance of normal sodium balance following an increase in sodium intake.

Gastrin induces sodium-hydrogen exchanger 3 phosphorylation and mTOR activation via a phosphoinositide 3-kinase-/protein kinase C-dependent but AKT-independent pathway in renal proximal tubule cells derived from a normotensive male human

Gastrin is natriuretic, but its renal molecular targets and signal transduction pathways are not fully known. In this study, we confirmed the existence of CCKBR (a gastrin receptor) in male human renal proximal tubule cells and discovered that gastrin induced S6 phosphorylation, a downstream component of the phosphatidylinositol 3 kinase (PI3 kinase)-mammalian target of rapamycin pathway. Gastrin also increased the phosphorylation of sodium-hydrogen exchanger 3 (NHE3) at serine 552, caused its internalization, and decreased its expression at the cell surface and NHE activity. The phosphorylation of NHE3 and S6 was dependent on PI3 kinases because it was blocked by 2 different PI3-kinase inhibitors, wortmannin and LY294,002. The phosphorylation of NHE3 and S6 was not affected by the protein kinase A inhibitor H-89 but was blocked by a pan-PKC (chelerythrine) and a conventional PKC (cPKC) inhibitor (Gö6976) (10 μM) and an intracellular calcium chelator, 1,2-bis-(o-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid, tetra(acetoxymethyl)-ester, suggesting the importance of cPKC and intracellular calcium in the gastrin signaling pathway. The cPKC involved was probably PKCα because it was phosphorylated by gastrin. The gastrin-mediated phosphorylation of NHE3, S6, and PKCα was via phospholipase C because it was blocked by a phospholipase C inhibitor, U73122 (10 μM). The phosphorylation (activation) of AKT, which is usually upstream of mammalian target of rapamycin in the classic PI3 kinase-AKT-p70S6K signaling pathway, was not affected, suggesting that the gastrin-induced phosphorylation of NHE3 and S6 is dependent on both PI3 kinase and PKCα but not AKT.

CCKB/gastrin receptors mediate changes in sodium and potassium absorption in the isolated perfused rat kidney

BACKGROUND

To evaluate the function of cholecystokinin B (CCKB)/gastrin receptors in the rat kidney, we identified the receptors by Northern blot and localized the receptors by immunohistochemistry. The functional effects of gastrin were studied under standardized in vitro conditions using the isolated perfused kidney.

METHODS

Rat kidneys were mounted in an organ bath by attaching the renal artery to a perfusion system. A catheter was inserted into the renal vein and the ureter to collect samples that were analyzed for the concentrations of electrolytes. After a preperfusion period, gastrin-17-I was given via the renal artery (10-8 to 10-6 mol/L). Subsequently, hemodynamic parameters (for example, perfusate flow) and changes in sodium and potassium absorption were determined. All data were subjected to a nonparametric analysis of variance and, in case of significant results, to subsequent paired comparisons by the a posteriori Wilcoxon test.

RESULTS

Northern blot analysis detected CCKB receptor transcripts in total RNA isolated from kidneys. Immunohistochemistry localized CCKB receptors on tubules and collecting duct cells. Compared with controls, gastrin (10-6 mol/L) caused a decrease in the fractional sodium reabsorption (basal 80%, 10 minutes after application of gastrin 71%, after 20 minutes 62%, P < 0.05). This effect was inhibited by the CCKB receptor antagonist L-365,260. Gastrin decreased urinary potassium excretion at 10-8 and 10-6 mol/L [maximal decrease at 10-6 mol/L from baseline values (100%) to 49% after 10 minutes and to 69% after 20 minutes, P < 0.05, N = 6]. This effect was also abolished by the CCKB receptor antagonist L-365,260. Gastrin (10-6 mol/L) reduced perfusate flow by 31% (P < 0.05).

CONCLUSIONS

CCKB receptors are expressed in the rat kidney on tubules and collecting ducts. These receptors mediate changes in renal potassium and sodium absorption. In addition, gastrin causes a decrease in perfusate flow, indicating that CCKB receptors might also modulate vascular resistance in the kidney.

Mechanisms of sodium pump regulation

The Na(+)-K(+)-ATPase, or sodium pump, is the membrane-bound enzyme that maintains the Na(+) and K(+) gradients across the plasma membrane of animal cells. Because of its importance in many basic and specialized cellular functions, this enzyme must be able to adapt to changing cellular and physiological stimuli. This review presents an overview of the many mechanisms in place to regulate sodium pump activity in a tissue-specific manner. These mechanisms include regulation by substrates, membrane-associated components such as cytoskeletal elements and the gamma-subunit, and circulating endogenous inhibitors as well as a variety of hormones, including corticosteroids, peptide hormones, and catecholamines. In addition, the review considers the effects of a range of specific intracellular signaling pathways involved in the regulation of pump activity and subcellular distribution, with particular consideration given to the effects of protein kinases and phosphatases.

Renal sodium handling and stimulation of medullary Na-K-ATPase during blockade of prostaglandin synthesis

The effect of suppression of prostaglandin synthesis on renal sodium handling and microsomal Na-K ATPase was studied in control and indomethacin treated intact rats maintained on a normal sodium diet (series A) and chronically salt loaded (series B). Indomethacin administration resulted in a decreased GFR and a significantly depressed urinary excretion and an increased fractional reabsorption of sodium in animals fed the normal sodium diet or chronically salt loaded. In rats maintained on a normal Na diet, the activity of the renal medullary Na-K ATPase after indomethacin was 206.3 +/- 6.4 ug Pi/mg protein, i.e. significantly higher as compared with the enzyme activity in the medullary renal fraction from control animals in which it averaged 148 +/- 7.79 ug Pi/mg protein (p less than 0.001). While after chronic salt load a similar increment in the activity of renal medullary Na-K ATPase was observed, no additional stimulation was elicited by subsequent indomethacin administration. The addition of exogenous PGE2, 0.1 mM to microsomal fractions obtained from kidneys of normal rats, was associated with a moderate suppression of the medullary Na-K-ATPase activity, from a basal level of 170 +/- 16 to 151.3 +/- 13 umol Pi/mg protein/hr (p less than 0.005). In isolated segments of medullary thick ascending limb of Henle's loop (MTAL) addition of PGE2 to the incubation medium resulted in a significant inhibition of Na-K ATPase from 37.2 +/- 2 to 21.25 +/- 1.17 x 10(-11) mol/mm/min (p less than 0.0001). These findings suggest that the increased renal Na reabsorption after inhibition of PG synthesis might be related, at least partly, to stimulation of medullary Na-K ATPase. In parallel, the reported natriuretic effect of prostaglandins might imply a direct inhibitory effect of these mediators on renal Na-K ATPase.

Prostaglandins and the gastrointestinal mucosa: are they important in its function, disease, or treatment?

In 1971 interest in the role of prostaglandins in the gastrointestinal tract was stimulated by the publication of two hypotheses--that aspirin damaged the gastric mucosa by inhibiting prostaglandin synthesis (1) and that cholera toxin caused diarrhea by stimulating it (2). Subsequent research into the gastrointestinal actions of prostaglandins has been considerable and now impinges on clinical practice. This paper reviews the involvement of prostaglandins and related compounds in mucosal protection, in ulcer healing, in diarrhea, and in gastrointestinal inflammation, with particular reference to the growing body of human data.

Gastrointestinal effects of long-term colchicine therapy in patients with recurrent polyserositis (familial mediterranean fever)

Twelve patients with recurrent polyserositis (RP, familial Mediterranean fever) on colchicine prophylaxis (1.0-2.0 mg daily) for three years or more were evaluated for the presence of gastrointestinal effects possibly attributable to the drug. Two patients had bulky stools, two others had transient diarrhea, and one had heartburn. Serum vitamin B12, calcium, and carotene levels were normal in all cases, and D-xylose absorption was normal in 11 of the 12. Three patients had mild steatorrhea (7.5, 7.9, and 9.9 g daily). Jejunal biopsies from these and a fourth patient with bulky stools but normal fecal fat excretion showed no abnormal histological changes. However, (Na + K)-ATPase activity was significantly decreased in all four cases. Colchicine had to be discontinued in only one of the 12 cases. It is concluded that mild steatorrhea and enzyme inhibition may occur in patients on long-term colchicine prophylaxis and that careful periodic observations for this and other adverse effects is imperative in such patients.