Effect of cytochrome P450 arachidonate metabolites on ion transport in rabbit kidney loop of Henle.

In the medullary segment of the thick ascending limb of the loop of Henle (mTALH), arachidonic acid (AA) is metabolized by a cytochrome P450-dependent monooxygenase to products that affect ion transport. The linkage between changes in ion transport and AA metabolism in isolated cells of the mTALH was examined. AA produced a concentration-dependent inhibition of 86Rb uptake--an effect that was prevented by selective blockade of cytochrome P450 monooxygenases. Inhibition by cytochrome P450 blockade of the effect of AA on 86Rb uptake could be circumvented by addition of the principal products of AA metabolism in the mTALH.

Treatment with tin prevents the development of hypertension in spontaneously hypertensive rats.

Cytochrome P-450-dependent metabolites of arachidonic acid (AA) increased in the kidneys of young, spontaneously hypertensive rats (SHRs) during the period of rapid elevation of blood pressure (BP) but not in adult SHRs or in Wistar Kyoto rats (WKYs) with normal BP. Treatment of SHRs and WKYs with stannous chloride (SnCl2), which selectively depletes renal cytochrome P-450, restored BP to normal, coincident with a natriuresis, in young but not in adult SHRs and did not affect either BP or sodium excretion in WKYs. Depletion of renal cytochrome P-450 was associated with decreased generation of these AA metabolites only in young SHRs. The antihypertensive effect of SnCl2 in young SHRs was greatly reduced by prevention of its cytochrome P-450-depleting action.

Eicosanoid excretion in hepatic cirrhosis. Predominance of 20-HETE.

The cytochrome P450 system transforms AA to hydroxyeicosatetraenoic acid (HETE) metabolites that are vasoactive and affect transport in several nephron segments. A principal product of this system, 20-HETE, participates in key mechanisms that regulate the renal circulation and extracellular fluid volume. We hypothesized that excess production of 20-HETE, which constricts the renal vasculature, contributes to the renal functional disturbances in patients with hepatic cirrhosis, particularly the depression of renal hemodynamics. The development of a precise and sensitive gas chromatographic/mass spectrometric method makes it possible to measure 20-HETE and the subterminal HETEs (16-,17-,18-, and 19-HETEs) in biological fluids. As 20-HETE was excreted as the glucuronide conjugate, measurement of 20-HETE required treatment of urine with glucuronidase. We measured HETEs in the urine of patients with cirrhosis, and compared these values to those of normal subjects. Urinary excretion rate of 20-HETE was highest in patients with ascites; 12.5+/-3.2 ng/min vs. 5.0+/-1.5 and 1.6+/-0.2 ng/min in cirrhotic patients without ascites and in normal subjects, respectively. Excretion of 16-, 17-, and 18-HETEs was not increased. In patients with cirrhosis, the excretory rate of 20-HETE was several-fold higher than those of prostaglandins and thromboxane, whereas in normal subjects 20-HETE and prostaglandins were excreted at similar rates. Of the eicosanoids, only increased excretion of 20-HETE in subjects with cirrhosis was correlated (r = -0.61; P < 0.01) with reduction of renal plasma flow (RPF).

Renal cytochrome P450 omega-hydroxylase and epoxygenase activity are differentially modified by nitric oxide and sodium chloride.

Renal function is perturbed by inhibition of nitric oxide synthase (NOS). To probe the basis of this effect, we characterized the effects of nitric oxide (NO), a known suppressor of cytochrome P450 (CYP) enzymes, on metabolism of arachidonic acid (AA), the expression of omega-hydroxylase, and the efflux of 20-hydroxyeicosatetraenoic acid (20-HETE) from the isolated kidney. The capacity to convert [(14)C]AA to HETEs and epoxides (EETs) was greater in cortical microsomes than in medullary microsomes. Sodium nitroprusside (10-100 microM), an NO donor, inhibited renal microsomal conversion of [(14)C]AA to HETEs and EETs in a dose-dependent manner. 8-bromo cGMP (100 microM), the cell-permeable analogue of cGMP, did not affect conversion of [(14)C]AA. Inhibition of NOS with N(omega)-nitro-L-arginine-methyl ester (L-NAME) significantly increased conversion of [(14)C]AA to HETE and greatly increased the expression of omega-hydroxylase protein, but this treatment had only a modest effect on epoxygenase activity. L-NAME induced a 4-fold increase in renal efflux of 20-HETE, as did L-nitroarginine. Oral treatment with 2% sodium chloride (NaCl) for 7 days increased renal epoxygenase activity, both in the cortex and the medulla. In contrast, cortical omega-hydroxylase activity was reduced by treatment with 2% NaCl. Coadministration of L-NAME and 2% NaCl decreased conversion of [(14)C]AA to HETEs without affecting epoxygenase activity. Thus, inhibition of NOS increased omega-hydroxylase activity, CYP4A expression, and renal efflux of 20-HETE, whereas 2% NaCl stimulated epoxygenase activity.

Some determinants of the effects of VAL-5-angiotensin II amide on glomerular filtration rate and sodium excretion in dogs.

In 12 dogs anesthetized with chloralose, angiotensin (angiotensin II amide) given intravenously increased the glomerular filtration rate (GFR) of an ischemic kidney while simultaneously having little effect on the GFR of the contralateral kidney. In the ischemic kidney, in 14 of 30 observations, increments of GFR greater than 100% of mean control GFR (9 ml/min) occurred in response to angiotensin. The magnitude of the increase in GFR produced by angiotensin was independent of dose (range 0.005-0.050 mug/kg per min), the degree of accompanying pressor response, and alterations in renal blood flow (RBF) (electromagnetic flow-meter). In the ischemic kidney, increments of GFR could be produced by sub-pressor doses of angiotensin. Dissociations between increments of GFR and sodium excretion occurred. Equivalent increments of GFR in the ischemic kidney in dogs receiving either 5% glucose in water or 10% mannitol in 0.3% saline were associated with natriuresis only in the latter group: a) as an initial response of the contralateral kidney to renal arterial constriction (RAC) in spite of a concomitant reduction in RBF and an unchanged GFR; b) in the ischemic kidney on giving angiotensin. The natriuresis produced by angiotensin was independent of the magnitude of elevations in blood pressure, altered filtration fraction, and was associated with a further reduction in RBF. After release of RAC in the dogs receiving mannitol, an antinatriuresis was again observed in response to angiotensin. The presence of unilateral renal ischemia allowed the demonstration of a differential action of angiotensin on the GFR of an ischemic and nonischemic kidney. The natriuresis in response to angiotensin requires, in addition to mannitol, the participation of undefined factors invoked by unilateral renal ischemia.

Stimulation of rat erythrocyte P2X7 receptor induces the release of epoxyeicosatrienoic acids.

BACKGROUND AND PURPOSE: Red blood cells (RBCs) are reservoirs of vasodilatory, antiaggregatory, and antiinflammatory lipid mediators-epoxyeicosatrienoic acids (EETs). This study addresses the formation and release of erythrocyte-derived EETs in response to ATP receptor stimulation that may represent an important mechanism regarding circulatory regulation. EXPERIMENTAL APPROACH: Erythrocyte EET formation and release were investigated by incubating rat RBCs in physiological salt solution with agents that effected ATP release via P2 receptor stimulation of phospholipase A2 and epoxygenase-like activities with activation of the ATP secretory mechanism. EETs were analyzed by gas and liquid chromatography-mass spectrometry. KEY RESULTS: EETs were released from rat RBCs: 14,15-, 11,12-, 8,9- and 5,6-EETs in a ratio of 1.2:1.0:0.9:0.8. EETs were produced by epoxidation of arachidonic acid catalyzed by hemoglobin. Spontaneous release of EETs, 0.66+/-0.14 ng per 10(9) RBCs, was dose-dependently increased by an ATP analog, BzATP, and inhibited by P2X(7) receptor antagonists. 5 microM ATP increased release of EETs over 20% to 0.83+/-0.15 ng per 10(9) RBCs; 10 microM BzATP tripled the amount of EET release to 1.87+/-0.20 ng per 10(9) RBCs. EET release by ATP or BzATP was not associated with hemolysis. Carbenoxolone, a gap junction inhibitor that inhibits ATP release, and glibenclamide, an inhibitor of the cystic fibrosis transmembrane conductance regulator (CFTR), which is required for ATP release, inhibited the spontaneous and stimulated EET release from RBCs. CONCLUSIONS AND IMPLICATIONS: EETs are produced and released from RBCs via a mechanism that is mediated by ATP stimulation of P2X(7) receptors coupled to ATP transporters, pannexin-1 and CFTR.

Vascular prostaglandin synthesis: the early days.

Prostacyclin (PGI2) and thromboxane (TxA2) labile cyclooxygenase (COX) products via PGH2 were identified in biological fluids by the ingenious application of the principle of parallel pharmacological assays developed by John Vane. Either organ perfusates or circulating blood superfuse bioassay tissues arranged in a cascade. Tissues were selected based on specificity of responses to targeted eicosanoids. Additionally, PGI2 inhibited platelet aggregation, a finding that led to discovery of its critical anti-thrombotic activity at the blood-endothelial interface. The biological activities of PGI2 and TxA2 were the fingerprints for tracking their isolation and ultimate chemical identification. These studies were responsible for opening the modern era of vascular biology that has facilitated the development of a rational approach to the treatment of diabetic and hypertensive complications involving the arterial circulation.

Missing links: Cytochrome P450 arachidonate products A new class of lipid mediators.

Three major enzyme systems-cyclooxygenases, lipoxygenases, and cytochrome P450 monooxygenases (CYP)-generate biological mediators from arachidonic acid (AA). A distinct profile of arachidonate products characterizes each renal tubular segment and each section of the renal vascular tree from arteries to microvessels to veins. The most recent discoveries on renal mechanisms subserved by arachidonate metabolites relate to CYP pathways of AA metabolism. These arachidonate products have prominent effects on blood vessels and ion transport, including modulation and mediation of the actions of vasoactive hormones. The diverse properties of these AA metabolites and the wide distribution of the CYP system make them prime candidates for participation in regulatory mechanisms that affect the circulation and transporting epithelia. CYP-AA products are as important to renal and circulatory control as are nitric oxide and prostaglandins.

Arachidonate metabolites and kinins in blood pressure regulation.

Arachidonic acid (AA) can be metabolized to an array of products affecting biological mechanisms such as those governing vascular reactivity and transport function. Metabolism of AA by cyclooxygenase in the nephron is discretely localized and is overshadowed in some nephron segments by a considerable capacity to generate P-450 AA metabolites. The synthesis of renal P-450 AA products is increased in hypertension. AA metabolites participate in fluid and electrolyte homeostasis and regulation of tissue blood flow and act as modulators of pressor systems. In addition, eicosanoids either augment or mediate the vasodilator-diuretic actions of the kallikrein-kinin system.

Decreased vascular responsiveness produced by angiotensin-converting enzyme inhibitors in the rat isolated kidney.

The effects of three angiotensin converting enzyme (ACE) inhibitors - captopril, MK-421 diacid, and teprotide - on renal vascular responses to graded (50, 100, 200 ng) injections of norepinephrine (NE) were examined in rat isolated perfused kidneys, having a mean basal perfusion pressure of 78 +/- 10 mm Hg. The minimum dose of captopril (0.05 microgram/ml, low dose) that abolished the vasoconstrictor responses to 100 and 200 ng angiotensin I did not affect NE-induced renal vasoconstriction, whereas a dose 100 times greater (high-dose captopril, 5 micrograms/ml) reduced the vasoconstrictor action of NE. MK-421 diacid also at high dose (1 microgram/ml), caused similar reduction in renal vasoconstrictor responses to NE. In contrast, a high dose of teprotide (50 micrograms/ml) did not affect renal vascular responsiveness to NE. The threshold dose of NE that released prostaglandins, measured by bioassay, was 50 ng. Indomethacin (1 microgram/ml) prevented NE-induced release of prostaglandins but did not affect the ability of captopril to attenuate NE-induced vasoconstriction. We conclude that captopril and MK-421 diacid decreases vascular reactivity in the rat isolated kidney by a mechanism independent of ACE inhibition and unrelated to a prostaglandin-dependent vascular mechanism. Moreover, the presence of mercapto function in the ACE inhibitor is not essential since captopril, which has a sulfhydryl group, and MK-421 diacid, which lacks this group, have similar effects on renal vascular responsiveness.

Aspirin enhances the antihypertensive effect of captopril in spontaneously hypertensive rats.

Activation of renal or vascular prostaglandin mechanisms (or both) has been proposed to contribute to the antihypertensive action of captopril. In conscious spontaneously hypertensive rats (SHR) studied in the established phase of hypertension, the blood pressure-lowering effect of captopril, 30 mg/kg/12 hr p.o. given for 7 days, was greatly enhanced by the addition of aspirin, 200 mg/kg/day s.c. Systolic blood pressure decreased from 185 +/- 6 and 182 +/- 4 to 135 +/- 3 mm Hg in rats treated, respectively, with captopril and aspirin or captopril alone, and was unaltered by either vehicle or aspirin alone. Water intake was inconsistently affected by captopril but was increased (p less than 0.01) by aspirin and was even higher after captopril-aspirin treatment (p less than 0.01). Urine volume was elevated in all 3 drug-treated groups, increasing threefold after captopril-aspirin treatment. Excretion of sodium and potassium was unchanged by any treatment regimen. In the vehicle group, prostaglandin F2 alpha excretion, measured by radioimmunoassay, ranged between 65 and 93 ng/8 hr and was twofold to fourfold higher than that of prostaglandin E2. Prostaglandin F2 alpha was unaffected during captopril treatment, whereas prostaglandin E2 excretion decreased to 12 +/- 2 ng/8 hr (p less than 0.01) by Day 7. Long-term aspirin treatment, either with or without captopril, did not cause sustained inhibition of renal prostaglandin excretion, although a transient effect occurred within the first four hours of administration. These results indicate 1) aspirin potentiates the blood pressure-lowering effect of captopril in SHR, an effect that is associated with a threefold increase in urine flow.(ABSTRACT TRUNCATED AT 250 WORDS)

Identification of arachidonate P-450 metabolites in human platelet phospholipids.

Phospholipase A2 (Naja mocambique) catalyzed release of epoxyeicosatrienoic acids (EETs) and 20-hydroxyeicosatetraenoic acid (20-HETE) from phospholipids of isolated human platelets. The amount of EETs released by phospholipase A2 measured by gas chromatography/mass spectrometry (GC/MS) was 4.3 +/- 0.9 pmol/10(6) platelets. No EETs were detected when phospholipase A2 was omitted from the incubations. The relative abundance of EET isomers (14,15-EET, 11,12-EET, 8,9-EET, and 5,6-EET) from human platelets was 5.4:4.5:3.7:1, respectively, as established by a new method based on particle-beam liquid chromatography/mass spectrometry (LC/MS). Fractionation of platelet phospholipids by normal-phase high-performance liquid chromatography followed by hydrolysis and GC/MS analyses indicated that the amount of EETs was highest in fractions containing phosphatidylinositol and phosphatidylserine (142 and 61 pmol/nmol of phosphorus, respectively) while low in phosphatidylcholine and phosphatidylethanolamine (19 and 11 pmol/nmol of phosphorus, respectively). The majority of EETs associated with phosphatidylcholine was found in fractions containing 1-O-alkylphosphatidylcholine. Human platelet phospholipids also released 20-HETE on phospholipase treatment (9.7 +/- 1.6 fmol/10(5) cells) and at least three other HETEs, one of which was tentatively identified as 16-HETE. Activation of human platelets by thrombin or platelet-activating factor released 5 to 7 fmol EET/10(6) cells. Receptor-mediated hydrolysis of phospholipids containing EETs and 20-HETE may play a role in stimulus-response coupling in platelets.

Prostaglandin-related renin release from rabbit renal cortical slices.

The possibility that 6-keto-prostaglandin E1 (6-keto-PGE1) affects renin release was studied using rabbit renal cortical slices, a preparation that eliminates hemodynamic, neural, and blood-borne factors that might influence renin release. The medium used for incubating the slices was collected for renin assay at the end of each of four successive 20-minute periods. Test agents were added only once, at the beginning of Period 3 (experimental period). Between Periods 3 an 4 (recovery period), the medium was aspirated and the slices rinsed with Krebs solution before replacing the medium. Renin release did not change in vehicle-treated slices. Unlike the PGI2-induced changes, the effects of 6-keto-PGE1 on renin release were sustained in Period 4. Indomethacin potentiated renin stimulation induced by 10 microM concentrations of PGI2 and 6-keto-PGE1 in Period 3 and by 6-keto-PGE1 in Period 4. Using platelet antiaggregatory activity as an index of stability, we found that PGI2 was largely inactivated within 10 minutes under the conditions used for incubating the slices (pH 7.4, 37 degrees C), while 6-keto-PGE1 was stable. The results lend further support to the concept that 6-keto PgE1 is capable of releasing renin through a direct action.

6-keto PGE1: a possible metabolite of prostacyclin having platelet antiaggregatory effects.

Hepatic metabolism of prostacyclin (PGI2) results in the formation of several biologically inactive lipids and one stable product that has the same chromatographic and biological properties as authentic 6-keto PGE1. Both prostaglandins, 6-keto PGE1 and PGI2, have comparable potency in their antiaggregatory and disaggregatory effects on platelets. They contract the superfused rat stomach strip but differ in their effects on the bovine coronary artery, which is contracted by 6-keto PGE1 but relaxed by PGI2. Further, 6-keto PGE1 is considerably more stable than PGI2. Thus, 6-keto PGE1 could account for some of the prolonged effects occasionally seen with PGI2.

Chronic treatment with tin normalizes blood pressure in spontaneously hypertensive rats.

We have reported that short-term treatment of spontaneously hypertensive rats (SHR) and normotensive Wistar-Kyoto (WKY) rats with stannous chloride (SnCl2), which selectively depletes renal cytochrome P450, restores blood pressure to normal in young but not in adult SHR, and is without effect on blood pressure of either young or adult WKY rats. We report in the present study that chronic treatment with SnCl2, begun at age 5 weeks, prevented the development of hypertension in SHR over a period of 15 weeks at which time they were killed. Suspension of SnCl2 treatment after 8 weeks (i.e., at age 13 weeks) did not result in return of blood pressure to hypertensive levels in SHR. Age-matched WKY rats were not affected by tin treatment. These findings provide additional evidence that administration of tin, which stimulates heme oxygenase, thereby producing depletion of cytochrome P450, restores blood pressure to normal levels in SHR.

Endothelial mechanism in the vascular action of hydralazine.

Relaxation of precontracted isolated chains of aortic rings with intact endothelium and in those with the endothelium removed was studied in response to various antihypertensive vasodilator drugs. Of the drugs tested--nitroprusside, nitroglycerin, prazosin, minoxidil, diazoxide and hydralazine--only the vascular relaxant effects of hydralazine were found to be dependent, in part, on the presence of intact endothelium. The endothelial component of the hydralazine response represented a major contribution to the net relaxant effect on the vascular smooth muscle, particularly at lower concentrations, 90 nM to 1 microM, which are also clinically relevant.

Renal arachidonic acid metabolism. The third pathway.

Cells were isolated from the outer medulla of the rabbit kidney, primarily from the thick ascending limb of Henle's loop (mTALH). These mTALH cells are heavily invested with a cytochrome P450-linked monooxygenase that represents the third pathway by which arachidonic acid is metabolized. After cell separation, approximately 80% of the cells proved to be mTALH in origin, based on electron microscopic criteria and immunofluorescent localization of Tamm-Horsfall protein, a specific marker for mTALH cells. The specific activity of alkaline phosphatase, a marker for proximal tubular cells, decreased threefold after separation of mTALH cells from outer medullary cells, associated with a fourfold increase in the capacity of the separated mTALH cells to metabolize arachidonic acid. Incubation of mTALH cells with 14C-arachidonic acid resulted in formation of oxygenated metabolites, identified as two peaks (P1 and P2), which accounted for 30 to 40% of the recovered radioactivity. Formation of prostaglandin E2 and F2 alpha accounted for only 3 to 5%. The chromatographic retention times of P1 and P2 were different from products of lipoxygenases. An inhibitor of cytochrome P450-dependent enzymes, SKF-525A (50 microM), reduced product formation by mTALH cells by more than 70%, while induction of cytochrome P450 increased product formation. Formation of P1 and P2 by cell-free homogenates of mTALH was totally dependent on the presence of nicotinamide adenine dinucleotide phosphate, reduced form (NADPH), which suggests a NADPH-dependent cytochrome P450-linked monooxygenase pathway. Vasopressin and calcitonin (10(-10) M to 10(-7) M) stimulated release of arachidonic acid metabolites from mTALH cells.(ABSTRACT TRUNCATED AT 250 WORDS)

The antihypertensive effect of captopril in essential hypertension: relationship to prostaglandins and the kallikrein-kinin system.

Two groups, each with nine essential hypertensive patients, were maintained on 10 mmol sodium daily over 14-17 days and treated in this sequence: placebo; captopril (25 or 50 mg given thrice daily) or indomethacin (50 mg given thrice daily) alone; captopril plus indomethacin, and (4) captopril alone. The initial fall in mean blood pressure induced by captopril (118 +/- 1 to 102 +/- 1 mmHg) was unaffected by the addition of indomethacin. However, if indomethacin treatment preceded captopril, the antihypertensive effect was attenuated (116 +/- 4 to 109 +/- 4), and was associated with significant reductions in urinary prostaglandin and kinin excretion. Addition of captopril to indomethacin returned kinin excretion to placebo levels but did not affect indomethacin-induced reduction in prostaglandin excretion. Captopril alone stimulated plasma renin activity (PRA) fivefold; aldosterone excretion was lowered by 25% and further reduced by indomethacin. Thus, when captopril and indomethacin are administered together, the order of administration is critical to the antihypertensive effect of captopril.

Tumor necrosis factor-alpha-angiotensin interactions and regulation of blood pressure.

OBJECTIVES: To compare the levels of tumor necrosis factor-alpha (TNF) produced by medullary thick ascending limb tubules (MTAL) obtained from normotensive and angiotensin II (Ang II)-dependent hypertensive rats and determine whether TNF participates in a mechanism that opposes elevation of blood pressure by Ang II. DESIGN: We have previously demonstrated that in-vitro administration of Ang II increases production of TNF and prostaglandin E2 (PGE2) by the MTAL. We hypothesize that production of TNF and PGE2 by the MTAL is elevated in in-vivo models of Ang II-dependent hypertension and acts to modulate the pressor effects of Ang II. Thus, inhibition of TNF should disclose whether this cytokine acts to modulate Ang II-induced hypertension. METHODS: MTAL tubules obtained from normotensive and Ang II-dependent hypertensive rats were isolated by enzymatic digestion and sieving. Tubules were cultured in the absence of exogenous Ang II. TNF and PGE2 levels were measured by enzyme-linked immunosorbent assay. Anti-TNF antiserum was administered intravenously to normotensive and Ang II-dependent hypertensive rats and their mean arterial pressures were measured. RESULTS: Production of TNF and PGE2 was significantly greater in MTAL tubules isolated from Ang II hypertensive rats than it was in those from normotensive controls. Administration of anti-TNF antiserum exacerbated the Ang II-mediated increase in mean arterial pressure. CONCLUSIONS: The higher levels of production of TNF and PGE2 by MTAL tubules isolated from Ang II hypertensive rats compared with those of normotensive controls are consistent with results of in-vitro experiments showing that administration of Ang II increases production of TNF and PGE2 by the MTAL. TNF and PGE2 participate in a counter-regulatory mechanism that opposes the pressor actions of Ang II.

Renal vascular responsiveness to arachidonic acid in experimental diabetes.

1. Isolated perfused kidneys from diabetic rats (duration 4-6 and 20-24 weeks) were more sensitive to the vasoconstrictor effects of arachidonic acid than kidneys from age-matched control rats. Sensitivity diminished with age in both control and diabetic groups. 2. The enhanced vasoconstrictor effect of arachidonic acid in diabetic rat kidneys was associated with increased conversion to prostaglandins. 3. The renal vasoconstrictor response to arachidonic acid in both groups was reduced by thromboxane A2/prostaglandin H2 receptor antagonism but not by inhibition of thromboxane synthase. 4. Diabetic rat kidneys were also more sensitive to the vasoconstrictor effects of the endoperoxide analogue, U46619, while vasoconstrictor responses to phenylephrine were not markedly different from those of control rat kidneys. 5. In conclusion, prostaglandin endoperoxides appear to mediate arachidonic acid-induced vasoconstriction in diabetic and control rat kidneys. The enhanced renal vasoconstrictor response to arachidonic acid in diabetic rats results from increased sensitivity to endoperoxides and increased formation of endoperoxides from arachidonic acid.