Prostaglandin metabolism in rabbit kidney. Identification and properties of a novel prostaglandin 9-hydroxydehydrogenase

Effect of prostanoids on renin release from rabbit afferent arterioles with and without macula densa

There is evidence that cyclooxygenase products of arachidonic acid participate in the control of renin release. In this study we tested the hypothesis that prostaglandin (PG) I2 and/or its metabolite(s), which are synthesized in the afferent arteriole (AF), stimulate renin release by acting directly on the AF while PGE2 stimulates renin release indirectly via the macula densa. AF alone and AF with macula densa attached (AF-MD) were microdissected from rabbit kidneys and incubated in vitro. The renin release rate from a single AF (or an AF-MD) was calculated and expressed as ng AI.hr-1. AF-1/hr (where AI is angiotensin I). When arachidonic acid (0.12 mM) or PGI2 (10 microM) was added to AF, renin release increased significantly (P less than 0.0001) from 1.04 +/- 0.21 to 3.12 +/- 0.86 (x +/- SEM, N = 7), and from 0.45 +/- 0.14 to 1.48 +/- 0.53 (N = 9), respectively. During the recovery period, renin release increased even further, reaching 9.53 +/- 1.76 and 4.50 +/- 1.24, respectively. A PGI2 synthetase inhibitor, 9, 11-azoprosta-5,13-dienoic acid blocked the effect of arachidonic acid. To examine whether the increases in renin release during the recovery period were due to metabolite(s) of PGI2, we tested the effect of both 6-keto-PGE1 (an active metabolite of PGI2) and carba-PGI2 (a synthetic analog that is metabolized differently from PGI2). Six-keto-PGE1 and carba-PGI2 increased renin release only during the experimental period with no further increase during the recovery period.(ABSTRACT TRUNCATED AT 250 WORDS)

Histochemical localization of prostaglandin dehydrogenase activity for PGF-2 alpha in some bovine tissues

Prostaglandin dehydrogenase activity is histochemically detected in various bovine tissues (kidney, liver, lung, parotid and naso-labial glands) using as substrate prostaglandin F-2 alpha. Kidney, liver and lung showed the highest intensity of the reaction, but parotid and naso-labial glands also displayed enzymatic activity at the level of the ductal cells.

The use of tritiated prostaglandins in metabolism studies. I: Evaluation of the kinetic isotope effect in the prostaglandin dehydrogenase reactions

Although numerous data exist concerning tritium kinetic isotope effect in enzymic reactions, little is related to the metabolism of tritiated prostaglandins. The present study reports an evaluation of the kinetic isotope effect which occurs during the oxidation of 15-hydroxyl group of tritium-labeled prostaglandins E2 and F2 alpha by the 15-hydroxyprostaglandin dehydrogenase and during the oxidation of 9-hydroxyl group of tritium-labeled prostaglandin F2 alpha by the 9-hydroxyprostaglandin dehydrogenase. The large kinetic isotope effect tends to limit the validity of the dehydrogenase assay using tritium-labeled prostaglandins as substrate. However these assays can be considered to be an indication of relative enzyme activity.

Stimulatory cholinergic effect on the release of antiaggregatory activity into the circulation of cat and man and its modification by beta-adrenergic antagonists

The release of PGI2-like activity into the circulation in response to cholinergic agonists and modification of this response by beta-adrenergic antagonists was investigated in anaesthetized cats and healthy humans. Antiaggregatory activity in the arterial blood was continuously assayed by measuring platelet aggregation on blood superfused collagen strip. In some of the human experiments, after the administration of the drugs, the conversion of [14C]-arachidonate to [14C]-prostaglandins in the pulmonary vascular bed was studied. Cholinergic agonists stimulated the release of PGI2-like activity into the circulation, which effect was potentiated in cats by beta-adrenergic antagonists. In humans the latter agents did not stimulate the conversion of [14C]-arachidonate to prostaglandins in the pulmonary circulation and, moreover, inhibited the stimulatory cholinergic effect. The results suggest that an interplay between cholinergic and beta-adrenergic mediators may be involved, although in a different way in cats and in humans, in the release of PGI2-like activity into the systemic circulation.

Enzymatic inactivation of 6-keto-prostaglandin E1 in vitro: Comparison with prostaglandin E1

The inactivation of 6-keto PGE1, a biologically active and stable metabolite of prostacyclin, was studied in 100,000 g cytosolic supernatants by bioassay on rat stomach strip (contraction) and human platelets (inhibition of ADP-induced aggregation). PGE1 was used as a reference compound. Both PGs were inactivated in supernatants from colon, kidney and liver of rat, rabbit and guinea-pig. Inactivation was time- and NAD+ -dependent and was generally greater for PGE1 than 6-keto-PGE1. The enzyme responsible for 6-keto-PGE1 inactivation in cytosolic supernatants is distinct from prostaglandin 15-hydroxydehydrogenase and 9-keto reductase, is not inhibitable by sulphasalazine-like drugs and its activity is recoverable after precipitation by ammonium sulphate. We conclude that 6-keto-PGE1 can be inactivated by enzymes with wide tissue distribution, but further studies are needed for identification of these novel enzymes and the products formed as well as to assess their significance in the intact animal.

Formation of 6-keto prostaglandin E1 in mammalian kidneys

1 The metabolism of prostacyclin (PGI2) and 6-keto prostaglandin F1 alpha (6-keto PGF1 alpha) was studied in cell-free homogenates of rat, rabbit and guinea-pig kidney. 2 Rabbit kidney converted both PGI2 and 6-keto PGF1 alpha to a stable metabolite with chromatographic and biological activity identical to that of authentic 6-keto PGE1. Activity was found in the kidney cortex but not medulla, was inhibited by NAD+ or NADP+ (5 mM) and showed an optimum temperature requirement of 37 degrees C. 3 Guinea-pig kidney converted PGI2 but not 6-keto PGF1 alpha to a labile, biologically active metabolite which was not 6-keto pge1. 4 No conversion of prostacyclin or 6-keto PGF1 alpha to biologically active metabolites occurred in cell-free homogenates of rat kidney, liver and colon or guinea-pig liver and colon. 5 6-keto PGE1 rapidly lost spasmogenic activity on the rat stomach strip following incubation with rabbit or guinea-pig kidney supernatant in the absence of added cofactors. No loss of activity occurred on incubation with rat kidney. 6 Rutin (50 microM) potently inhibited synthesis of 6-keto PGE1 from added PGI2 by rabbit kidney cortex. This reaction was potentiated by a similar concentration of sulphasalazine, carbenoxolone, imidazole, papaverine or indomethacin. 7 The relevance of these findings for the possible physiological and pathological roles of 6-keto PGE1 in the kidney is discussed.

The effects of unsaturated fatty acids on the synthesis of arachidonic acid in rat kidney cells

Cultured rat kidney cells absorbed exogenous linoleic acid (cic, cis-18:2n-6) and esterified it mostly into glycerophospholipids. As the concentration of 18:2 was increased (5-200 microM) the quantity absorbed increased linearly and the amount esterified in the triacylglycerol increased. The cells possessed active acyl delta 6-desaturase and elongase which facilely converted 18:2n-6 to 20:4n-6. At low intracellular concentrations of 18:2n-6 other unsaturated fatty acids, i.e., gamma-linolenic (18:3n-6), alpha-linolenic (18:3n-3), dihomo-gamma-linolenic (20:3n-6), and especially trans, trans-linoleic acid (trans, trans-18:2n- -6) at concentrations ranging from 25 to 200 microM depressed delta 6-desaturase activity. However, suppression of 20:4 synthesis even by trans, trans-18:2 was readily overcome by increasing the concentration of available cis, cis-18:2n-6.

Metabolism of prostaglandin E2 in the isolated perfused kidney of the rabbit

In the Krebs-perfused rabbit isolated kidney, [3H]PGE2 (5 microCi, 165 Ci/mmole) was infused intra-artially for 5 min; venous and urinary effluents were collected every 2 min for 20 min. Efflux of radioactive material peaked at 8 min and declined thereafter. The kidney retained 35% of the infused 3H. Samples were extracted for acidic lipids; PGE2, PGF2 alpha and metabolites were separated by TLC and quantified by a radiometric method. Efflux of [3H]PGF2 alpha into urinary and venous outflows increased progressively over the first 12 min and then plateaued for the remaining 4 min. By 12 min, conversion of [3H]PGE2 to [3H]PGF2 alpha was 70 and 80% as determined by radiolabeled products recovered in the urinary and venous effluents respectively. Estimates of total conversion of [3H]PGE2 to [3H]PGE2 alpha were 62 and 52% of the radiolabeled material exiting in the urinary and venous effluents respectively. The 15-keto and 13,14-dihydro-15-keto metabolites of [3H]PGF2 alpha appeared in the urine but were not found in the venous outflow. We conclude that PGE-9-ketoreductase (PGE-9KRD) activity is high in the rabbit isolated perfused kidney. Further, the extent of conversion of PGE2 to PGF2 alpha and metabolism of newly formed PGF2 alpha may differ within the vascular and tubular compartments of the kidney. PGE-9KRD activity may be important in the regulation of renal vascular tone, compliance of veins, and salt and water balance.

Stimulation of renin release by 6-oxo-prostaglandin E1 and prostacyclin

1 Renin release induced by 6-oxo-prostaglandin E1 (6-oxo-PGE1) was compared to release in response to prostacyclin (PGI2) and 6-oxo-PGF1 alpha in slices of rabbit renal cortex. 2 Krebs-Ringer medium bathing slices of renal cortex was collected for renin assay after four successive 20 min intervals (periods I-IV). Renin release did not increase during periods I to IV in untreated slices. Agonists were added, only once, at the beginning of period III. Between periods III and IV, the incubation solution was aspirated and replaced with fresh medium. 3 PGI2 increased renin release during period III while 6-oxo-PGE1 stimulated release during periods III and IV. 6-oxo-PGE1 stimulated renin release (24%-74%) in concentrations ranging from 1-33 microM while PGI2 stimulated release at 10 microM (60%) but not at 5 microM. 6-oxo-PGF1 alpha, 10 microM, did not release renin during period III (period III, 9%), but caused a small rise in period IV (29%). 4 6-oxo-PGE1, unlike PGI2, was stable under the incubation conditions (pH 7.4, 37 degrees C) as indicated by recovery of undiminished platelet anti-aggregatory material after 20 min. 5 In the rabbit kidney, activity of 9-hydroxyprostaglandin dehydrogenase was greatest in the cortex and negligible in the papilla, corresponding to the zonal distribution of renin. 6 The prominent and sustained in vitro renin releasing effect of 6-oxo-PGE1, as well as the cortical localization of enzyme activity capable of generating this stable prostacyclin metabolite, suggest that formation of 6-oxo-PGE1 may contribute to PGI2-induced renin release and may explain the delayed stimulation caused by 6-oxo-PGF1 alpha.

NAD+-dependent 15-hydroxyprostaglandin dehydrogenase from porcine kidney. I. Purification and partial characterization

The cytoplasmic NAD+-dependent 15-hydroxyprostaglandin dehydrogenase (11 alpha, 15-dihydroxy-9-oxoprost-13-enoate:NAD+ 15-oxidoreductase, EC 1.1.1.141) from porcine kidney was purified to a specific activity of 1.2 unit per mg protein by a series of chromatographic techniques including affinity chromatography. The native molecular weight of the enzyme was estimated to be 45 000. Substrate specificity studies indicated that the enzyme was NAD+-specific and was able to catabolize readily various prostaglandins, with the exception of prostaglandin B and thromboxane B. The enzyme was inhibited by sulfhydryl inhibitors, diuretic drugs and various fatty acids.

NAD+-dependent 15-hgydroxyprostaglandin dehydrogenase from porcine kidney. II. Kinetic studies

The kinetic mechanism of porcine renal NAD+-dependent 15-hydroxyprostaglandin dehydrogenase (11 alpha, 15-dihydroxy-9-oxoprost-13-enoate:NAD+ 15-oxidoreductase, EC 1.1.1.141) was investigated. Initial velocity studies gave intersecting double reciprocal plots that conform to a sequential mechanism. Product inhibition studies indicated that 15-keto-prostaglandin E2 exhibited linear non-competitive inhibtion with respect to either prostaglandin E2 or NAD+, and NADH yielded linear competitive inhibition with respect to NAD+. Dead-end inhibition studies showed that adenosine-5'-diphosphoribose inhibited the enzyme competitively with respect to NAD+ as expected, but inhibited the enzyme non-competitively with respect to prostaglandin, E2. Alternate substrate studies indicated that a mixture of 3-acetyl-NAD+ and NAD+ gave a concave upward double reciprocal plot, while a mixture of prostaglandin E2 and prostaglandin F2 alpha yielded a linear plot. These results are consistent with an ordered Bi-Bi mechanism where NAD+ is added first, followed by prostaglandin E2, and 15-keto-prostaglandin E2 is released, followed by NADH.

Effects of sulphasalazine and its metabolites on prostaglandin synthesis, inactivation and actions on smooth muscle

1 We have investigated the effects of sulphasalazine and of its principal colonic metabolites (5-aminosalicylic acid and sulphapyridine) on prostaglandin inactivation, synthesis and actions on gastrointestinal smooth muscle.2 Sulphasalazine inhibits prostaglandin F(2alpha) breakdown in 100,000 g supernatants in all organs so far tested from 7 species with an ID(50) of approx. 50 muM; it has a selective action on prostaglandin 15-hydroxydehydrogenase and does not inhibit prostaglandin Delta-13 reductase, prostaglandin 9-hydroxydehydrogenase or ;enzyme X' at millimolar concentrations. Enzyme activities were measured radiochemically or by bioassay.3 Sulphapyridine and 5-aminosalicylic acid do not inhibit prostaglandin inactivation in vitro (4 species tested). A methyl analogue of sulphasalazine is a more potent inhibitor than the parent compound. Rabbit colon prostaglandin F(2alpha) metabolism in vitro was inhibited by the following drugs with ID(50) values (muM) of: diphloretin phosphate 20, sulphasalazine 50, indomethacin 220, frusemide 1000 and aspirin 10,000. A similar rank order of potencies was obtained with rabbit kidney.4 Sulphasalazine at 50 to 100 muM inhibited inactivation of prostaglandin E(2) in the perfused rat and guinea-pig lung by 3 to 40% (rat) and 32 to 100% (guinea-pig) when measured by superfusion cascade bioassay and of prostaglandin F(2alpha) by 43.6 +/- 6.5% in rat lung perfused with 50 muM sulphasalazine and assayed radiochemically.5 Prostaglandins E(1) and E(2) were 97.0 +/- 8.2% and 92.3 +/- 6.8% inactivated in the lungs after intravenous injection in the anaesthetized rat as measured by reference to their vasodepressor potencies when injected intra-arterially. Prostaglandin A(2) was not similarly inactivated. Pulmonary inactivation was prevented in the presence of an intravenous infusion of 16.3 mug kg(-1) min(-1) sulphasalazine and partially inhibited at a lower infusion rate.6 Prostaglandin biosynthesis from arachidonic acid was measured in microsomal preparations from four sources by bioassay and radiochemical methods. Indomethacin was a potent inhibitor (ID(50) 0.8 to 4.1 muM) but sulphasalazine and its methyl analogue were very weak inhibitors (ID(50) 1500 to > 5000 muM), 5-aminosalicylic acid was weaker still and sulphapyridine inactive.7 Sulphasalazine at 50 muM did not affect the actions of prostaglandins on five smooth muscle preparations; at 500 muM there was a rapidly reversible and probably non-specific antagonism of responses to low doses of prostaglandins.8 The specificity and selectivity of the interaction of sulphasalazine and its metabolites with the formation, breakdown and actions of prostaglandins are discussed.

Kidney lipids: changes caused by dietary 9-trans, 12-trans-octadecadienoate

Trans,trans-linoleate at 50 and 100% of dietary fat decreased kidney size and altered its composition. Trans,trans-linoleate as the sole source of dietary fat impaired growth and caused more severe symptoms of essential fatty acid deficiency than was observed with hydrogenated coconut oil (HCO). The concentration of renal cholesterol, phospholipids (PL), triglycerides (TG) and cholesteryl esters (CE) were also decreased. Linoleic (18:2), homo-gamma-linolenic acid (20:3n6) and arachidonic acid (20:4n6) were significantly depressed in lipid classes, especially in PL and CE, by dietary trans,trans-linoleate. The increase in eicosatrienoate (20:3n9), especially in PL and CE of kidneys of rats fed HCO (essential fatty acid deficient), was slight in rats fed 100% trans,trans-linoleate, indicating that the trans,trans acid probably inhibited acyl elongation and desaturation.

Experimental hyperthyroidism in rats suppresses in vitro prostaglandin metabolism in lung and kidney

Metabolism of prostaglandin (PG) F2alpha and PGE2 was depressed 40--62% in 100,000 g cytoplasmic supernatants of lungs and kidneys prepared from rats made hyperthyroid by 18 daily L(-) thyroxine injections (200microgram, s--c). These hyperthyroid rats had elevated serum thyroxine levels, cardiac hypertrophy and thyroid atrophy. There were no differences in soluble protein concentrations, NAD+ utilisation by endogenous enzymes and substrates, or in the NAD+ dependence of 15-hydroxyprostaglandin dehydrogenase (15-PGDH) between the supernatants prepared from hyperthyroid rats and saline-injected controls. Thyroxine did not inhibit PG metabolism in vitro up to 260 micrometer. These results suggest that thyroxine specifically decreases intracellular levels of PG-metabolising enzymes, especially of the rate-limiting 15-PGDH. Metabolism of PGF2alpha and PGE2 by 15-PGDH was faster in smaller rats and declined with increasing animal weight. These studies imply that some of the clinical features of hyperthyroidism in man might be caused by deficiencies in PG metabolism.