Atrial natriuretic peptides stimulate renal gluconeogenesis

Regulation of hepatic glycolysis and gluconeogenesis by atrial natriuretic peptide

Recently we reported the presence of both the guanylyl cyclase-linked (116 kDa) and the ANF-C (66 kDa) atrial natriuretic peptide receptors in the rat liver. Since ANF 103-125 (atriopeptin II) stimulates cGMP production in livers and because cGMP has previously been shown to mimic the actions of cAMP in regulating hepatic carbohydrate metabolism, studies were performed to investigate the effects of atriopeptin II on hepatic glycolysis and gluconeogenesis. Additionally, employing analogs of atrial natriuretic hormone [des-(Q116, S117, G118, L119, G120) ANF 102-121 (C-ANF) and des-(C105,121) ANF 104-126 (analog I)] which bind only the ANF-C receptors, the role of the ANF-C receptors in the hepatic actions of atriopeptin II was evaluated. In perfused livers of fed rats atriopeptin II, but not C-ANF and analog I, inhibited hepatic glycolysis and stimulated glucose production. Moreover, analog I did not alter the ability of atriopeptin II to inhibit hepatic glycolysis. Atriopeptin II, but not C-ANF and analog I, also stimulated cGMP production in perfused rat livers. Furthermore, while atriopeptin II inhibited the activity ratio of pyruvate kinase by 30%, C-ANF did not alter hepatic pyruvate kinase activity. Finally, in rat hepatocytes, atriopeptin II stimulated the synthesis of [14C]glucose from [2-14C]pyruvate by 50% and this effect of atriopeptin II was mimicked by the exogenously supplied cGMP analog, 8-bromo cGMP. Thus atriopeptin II increases hepatic gluconeogenesis and inhibits glycolysis, in part by inhibiting pyruvate kinase activity, and the effects of atriopeptin II are mediated via activation of guanylyl cyclase-linked ANF receptors which elevate cGMP production.

Biosynthesis of guanidinoacetic acid in isolated renal tubules

Guanidinoacetic acid, a precursor of creatine, is an essential substrate for muscle energy metabolism. Since guanidinoacetic acid has been reported to be synthesized from arginine and glycine by glycine amidinotransferase (transamidinase) in kidney homogenates or slices, the purpose of this study was to provide evidence of guanidinoacetic acid synthesis in isolated tubules from rat kidneys, and to clarify the mechanism regulating it. Isolated rat tubules were incubated with various substrates. Guanidinoacetic acid was separated by high performance liquid chromatography and measured fluorometrically. Results obtained were as follows: (1) Guanidinoacetic acid was synthesized from arginine or canavanine and glycine in isolated rat tubules. (2) D,L-Norvaline, ornithine and methionine suppressed guanidinoacetic acid synthesis. (3) Creatine suppressed guanidinoacetic acid synthesis, i.e. creatine was a negative feedback inhibitor of guanidinoacetic acid synthesis in this in vitro system. (4) Guanidinoacetic acid was not synthesized from hydroxyurea, citrulline, argininosuccinic acid or canaline. These data demonstrate that guanidinoacetic acid is synthesized only from arginine or canavanine and glycine, and that the guanidine cycle may not function fully in the rat renal tubule.

Cisplatin-induced alterations in renal structure, ammoniagenesis and gluconeogenesis of rats

Cisplatin [cis-diamminedichloroplatinum (II): CDDP] is a widely used cancer chemotherapeutic agent which has been shown to cause dose-related acute renal failure. The kidney damage is histologically characterized by widespread tubular necrosis, predominantly found in the third segments (S3) of the proximal tubules. To identify the intranephron targets of CDDP more precisely, we examined alterations in ammoniagenesis (AMG) and gluconeogenesis (GLG) using rat kidney slices (for AMG and GLG), tubule suspensions (for GLG), and microdissected nephron segments (for AMG). Ultramicroassay of AMG was carried out using the enzymatic cycling method, and GLG was measured by the HK/G6PHD method. GLG obtained from kidney slices and tubule suspensions on day 3 and day 7 following CDDP treatment did not change significantly from levels in control rats. In contrast, AMG increased on day 3 in the first and third kidney slices cut from the surface inward and decreased significantly on day 7 in the third and fourth slices. Microdissected nephron segments examined on day 7 showed decreased AMG in the second segments (S2; 20.3 +/- 7.7 pmol/mm/15 min vs. 78.7 +/- 9.7 for control, P less than 0.005) and the third segments (S3; 26.3 +/- 14.4 pmol/mm/15 min vs. 79.2 +/- 7.8 for control, P less than 0.005) of the proximal tubules. Additionally, we observed morphological changes under light microscopy to examine the relationship between metabolism and morphology. On day 3 following the CDDP treatment, typical acute tubular necrosis was seen primarily localized in the outer stripe of the outer medulla, while on day 7 the lesion appeared to be recovering. Our data imply a prominent dissociation between renal metabolic and morphologic changes induced by CDDP.

Immunocytochemical localization, binding, and effects of atrial natriuretic peptide in rat adipocytes

The metabolic effects of atrial natriuretic peptide (ANP) have not been widely investigated. Since adipocyte cells represent a model system extensively used to examine the metabolic actions of many peptide hormones, we sought to establish whether ANP could bind to adipocyte membranes, alter cyclic nucleotide metabolism, and affect spontaneous or hormone-stimulated lipolysis. Using in vitro autoradiographic techniques, radiolabelled ANP was found to bind specifically to mammary gland fat cells. Additionally, endogenous ANP-like immunoreactivity could be localized in the plasma membrane compartment and cytoplasmic matrix of fat cells, but not in fat vacuoles. [125I]ANP bound to single high affinity sites (Kd = 0.72 nM) in fat cell membranes. The binding was rapid (equilibrium within 1 min at 25 degrees C) and specific. The atrial peptide was capable of stimulating a time- and concentration-dependent increase in cGMP accumulation in isolated adipocytes, but had no effect on spontaneous or stimulated [-)-isoproterenol, ACTH, forskolin) cAMP formation. ANP did not alter the increase in glycerol production stimulated by l-epinephrine in isolated fat cells. While i.v. infusion of ANP stimulated a marked increase in circulating levels of cGMP, the atrial peptide did not alter plasma triglyceride levels. These data demonstrate the presence of specific ANP binding sites on adipocyte membranes and internalization of ANP-associated immunoreactivity. These receptors are biochemically functional given the ability of ANP to augment cGMP formation. The peptide, however, does not exert an action on adipocyte lipolysis. Adipocytes, therefore, represent an ANP target tissue in which the physiological action of the peptide is yet to be defined.

Atrial natriuretic peptide inhibits renal glutamine extraction

The effect of administered atrial natriuretic peptide, ANP, on renal glutamine extraction, oxygen consumption and ammoniagenesis was determined in the intact functioning kidney of nonacidotic and chronically acidotic rats. Chronic acidosis shifted the metabolic fuel dependency towards glutamine, reflected by a 3.7 fold increase in extraction. Bolus injection of ANP, 1 microgram/100g BW, results in increased GFR, massive diuresis and an acid urine. Glutamine uptake fell in both groups, reversing from uptake to release in nonacidotic animals and dropping nearly 50 percent in acidotic rats. In contrast, oxygen consumption fell only 20 percent. Inhibition of glutamine extraction appears to be an indirect effect of ANP dependent upon the elevated GFR and elimination of glutamine uptake from the blood, but not from the filtrate. Efficacy of ANP, unlike classical diuretics, was not affected by the prevailing acid base condition nor a large shift in the fuel utilized.

Development of a solid-phase extraction technique for alpha-human atrial natriuretic peptide in human plasma

A reliable extraction method was developed for alpha-human atrial natriuretic peptide (alpha-hANP) using Bond Elut C8 columns in tandem. This involved activation of the columns using methanol followed by a water wash to remove the excess methanol. Plasma (1 ml) was then added and a vacuum applied until all was drawn through. Excess protein and other endogenous compounds were removed by washing the columns with water and elution of the alpha-hANP was achieved with 0.75 ml acetonitrile-water-trifluoroacetic acid (80:19.8:0.2, v/v/v). Samples were evaporated under nitrogen and reconstituted in radioimmunoassay buffer ready for analysis. The recovery of alpha-hANP from plasma using this method was found to be 90% +/- 0.6% [mean +/- standard error of the mean (S.E.M.); coefficient of variation (C.V.) = 1.5%] which will allow more precise measurement of the peptide than is presently available. With this high precision of analysis available, having a limit of detection of 0.4 fmol/ml and a range of 0 to 32 fmol/ml, a low-dose infusion of alpha-hANP was conducted and the changes in plasma concentration were followed.

Analogue-specific action in vitro of atrial natriuretic factor on human red blood cell Ca2+-ATPase activity

Specific atrial natriuretic factor (ANF) analogues have been found to have inhibitory activity in vitro in a calmodulin-dependent, human red blood cell membrane Ca2+-adenosine triphosphatase (ATPase) model. Studied at 10(-8) to 10(-6) M concentrations, atriopeptin I (residues 127-147 of rat prepro-ANF sequence) and atriopeptin III (residues 127-150) progressively inhibited Ca2+-ATPase activity by up to 20% (p less than 0.001). This degree of inhibition was consistent with activities of other (calmodulin-independent) enzyme inhibitors in this model. Therefore, the C-terminal Phe-Arg-Tyr sequence (residues 148-150) is unnecessary for atriopeptin action on Ca2+-ATPase. Human and rat atrial peptides with amino acids 123-150 were inactive, indicating that the 123-126 sequence (Ser-Leu-Arg-Arg) must be cleaved to activate atriopeptins in this system. Human ANF fragment 129-150 also had no effect on Ca2+-ATPase, defining the importance of residues 127-128 (Ser-Ser) proximal to the disulfide bridge (joining 129 to 145). The addition of purified calmodulin to red blood cell membranes in the presence of inhibitory ANF did not restore Ca2+-ATPase activity to normal levels, indicating that the ANF effect on this enzyme is calmodulin-independent. Atriopeptin I and atriopeptin III had no effect on red blood cell Na+, K+-ATPase activity in vitro. Thus, the structure-activity relationships of ANF analogues in this novel human cell membrane model are highly specific. Although the inhibitory action of ANF analogues on Ca2+-ATPase, a calcium pump-associated enzyme, may be unique to the red blood cell, the calcium dependence of the gluconeogenic effects of ANF in the kidney would be supported by inhibition of this ATPase.

Comparative effect of ANF and various diuretics on isolated nephron segments

The effect of the synthetic form of ANF 0.1 to 10 microgram/ml (peptide 101-126), a diuretic and natriuretic peptide isolated from rat heart atria, on the metabolism of dog and rat kidney tubules was studied in vitro and compared to that of furosemide (0.1 to 1 mM), hydrochlorothiazide (0.5 mM) or amiloride (0.1 mM). In order to pinpoint eventual site(s) of ANF action along the nephron, proximal tubules, thick ascending limbs and papillary collecting ducts were isolated from dog kidneys as well as proximal tubules from rat kidneys. The substrate uptake (O2, lactate, glutamine, glucose) and production of metabolites (glutamate, ammonium, alanine, glucose) by these nephron segments were measured in absence or presence of the diuretic agents or the vehicle for ANF (acetate 1 mM). The total ATP turnover and the contribution of identified metabolic pathways for this turnover was calculated. It was expected that a molecule with diuretic properties reducing the permeability of cell membranes to NaCl would secondarily reduce the Na-K-ATPase activity, and therefore the oxygen and substrate utilization by affected cells. It was shown: that each nephron segment used presented the expected specific metabolic characteristics; that furosemide markedly inhibits the oxidative metabolism of thick ascending limbs; that acetate (the vehicle used for ANF) displaces the oxidation of glutamine and lactate in nephron segments with aerobic metabolism; that ANF had no effect on the metabolism of the studied segments despite the presence of specific c'GMP-generating receptors in the distal nephron. It is concluded that ANF must exert its natriuretic effect by a mechanism different from that of classical diuretics.

Atrial natriuretic peptide inhibits renin release from juxtaglomerular cells by a cGMP-mediated process

We have examined the effect of a synthetic analogue of human alpha-atrial natriuretic peptide (ANP), APII, on renin release in cultured renal juxtaglomerular cells (JGA cells). Using cell cultures containing 80-90% renal juxtaglomerular cells, we found that ANP (10(-13)-10(-9) M) strongly inhibited renin release from the cells in a dose-dependent fashion (ki, 10 pM) to about 10% of control. Inhibition of renin release by ANP was paralleled by an increase in cellular cGMP levels; while in the presence of the cGMP-phosphodiesterase inhibitor M&B 22948 (1 mM), concentrations of ANP lower by a factor of 100 were required to obtain the same effects on renin release and cGMP levels. The guanylate cyclase inhibitor methylene blue (10 microM), on the other hand, shifted the dose-response curves for renin release and cGMP levels to 100-fold higher concentrations of ANP. Neither the influx of 45Ca into the cells nor the intracellular quin-2 signal, which is a measure for changes of intracellular Ca concentration, was in any way altered by ANP. Our results suggest that ANP inhibits renin release from juxtaglomerular cells by a cGMP-dependent process that does not involve changes in intracellular calcium.