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.     

Effects of indomethacin on renal inner medullary plasma flow

The effects of the prostaglandin system on renal hemodynamics were studied by treating rats with a single intraperitoneal dose of indomethacin, an inhibitor of prostaglandin synthesis. Medullary plasma flow was significantly reduced 30–45 minutes after indomethacin, but was elevated 3–6 hours after indomethacin. These changes in medullary plasma flow correlated well with circulating levels of prostaglandins A and E. Total renal blood flow decreased following indomethacin treatment, but returned to normal levels within an hour. These results indicate that the inhibition of prostaglandin synthesis following a single intraperitoneal dose of indomethacin is short-lived and is followed by a significant elevation in prostaglandin synthesis. It is likely that prostaglandin levels play an important role in the control of renal medullary plasma flow.     

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 nomral 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<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, 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<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 × 10⁻¹¹ mol/mm/min (p<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.     

Salicylate nephropathy in the Gunn rat: potential role of prostaglandins

We examined the potential role of prostaglandins in the development of analgesic nephropathy in the Gunn strain of rat. The homozygous Gunn rats have unconjugated hyperbilirubinemia due to the absence of glucuronyl transferase, leading to marked bilirubin deposition in renal medulla and papilla. These rats are also highly susceptible to develop papillary necrosis with analgesic administration. We used homozygous (jj) and phenotypically normal heterozygous (jJ) animals. Four groups of rats (n = 7) were studied: jj and jJ rats treated either with aspirin 300 mg/kg every other day or sham-treated. After one week, slices of cortex, outer and inner medulla from one kidney were incubated in buffer and prostaglandin synthesis was determined by radioimmunoassay. The other kidney was examined histologically. A marked corticomedullary gradient of prostaglandin synthesis was observed in all groups. PGE2 synthesis was significantly higher in outer medulla, but not cortex or inner medulla, of jj (38 +/- 6 ng/mg prot) than jJ rats (15 +/- 3) (p less than 0.01). Aspirin treatment reduced PGE2 synthesis in all regions, but outer medullary PGE2 remained higher in jj (18 +/- 3) than jJ rats (9 +/- 2) (p less than 0.05). PGF2 alpha was also significantly higher in the outer medulla of jj rats with and without aspirin administration (p less than 0.05). The changes in renal prostaglandin synthesis were accompanied by evidence of renal damage in aspirin-treated jj but not jJ rats as evidenced by: increased incidence and severity of hematuria (p less than 0.01); increased serum creatinine (p less than 0.05); and increase in outer medullary histopathologic lesions (p less than 0.005 compared to either sham-treated jj or aspirin-treated jJ). These results suggest that enhanced prostaglandin synthesis contributes to maintenance of renal function and morphological integrity, and that inhibition of prostaglandin synthesis may lead to pathological renal medullary lesions and deterioration of renal function.     

Dissociation between renal medullary PGE2-synthesis and urine PGE2-excretion. Antagonism by bumetanide of chlorazanil induced urine PGE2-excretion in rats

Normal conscious female Sprague-Dawley rats were treated with chlorazanil (3 mg/kg i.p.), and urine was collected for 3 hours. Urine prostaglandin E₂-excretion increased from 25±3 to 271±32 ng/kg/3 h. The enhancement of urine PGE₂-excretion was inhibited by pretreatment with bumetanide (75 mg/kg p.o.). In separate experiments the papillary quantity of PGE₂ was determined in freshly homogenized tissue. The basal level (14±2 ng PGE₂/papilla) was increased by chlorazanil to 51±11 ng PGE₂/papilla and 24±7 ng PGE₂/papilla at one and two hours respectively after drug administration. The capacity of chlorazanil to increase medullary PGE₂ accumulation was unaffected by bumetanide dissociated the medullary PGE₂ level from the excretion of PGE₂ in urine, when the former was elevated by chlorazanil.     

Modulation of prostaglandin of the renal vascular action of arginne vasopressin

To determine whether the renal vascular effect of arginine vasopressin (AVP) is modulated by renal prostaglandin E₂ (PGE₂) were determined during the infusion of AVP in dogs during control conditions and after the administration of the inhibitor of prostaglandin synthesis, indomethacin. During control conditions, intrarenal administration for 10 min of a dose of AVP calculated to increase arterial renal plasma AVP concentration by 75 pg/ml produced a slight renal vasodilation (p<0.01) and an increase in renal venous plasma concentration of PGE₂. Renal venous PGE₂ concentration during control and AVP infusion averaged 33 ± 7 and 52 ± 12 pg/ml (p<0.05), respectively. After administration of indomethacin, the same dose of AVP induced renal vasoconstriction (p<0.05) and failed to enhance renal venous PGE₂ concentration (9 ± 1 to 8 ± 1 pg/ml). Intrarenal administration of 20 ng/kg. min of AVP for 10 min induced a marked renal vasoconstriction (p<0.01) and increased renal venous plasma PGE₂. Renal venous PGE₂ during control and AVP infusion averaged 31 ± 10 and 121 ± 31 pg/ml (p<0.01), respectively. Administration of the same dose of AVP following indomethacin produced a significantly greater and longer lasting renal vasoconstriction (p<0.01) and failed to increase renal venous plasma PGE₂ (10 ± 1 to 9 ± 1 pg/ml). These results indicate that a concentration of AVP comparable to that observed in several pathophysiological conditions induces a slight renal vasodilation which is mediated by renal prostaglandins. The results also indicate that higher doses of AVP induce renal vasoconstriction and that prostaglandin synthesis induced by AVP attenautes the renal vasoconstriction produced by this peptide.     

The effect of sodium intake on renal prostaglandin production

The effect of chronic alterations in dietary sodium intake on renal arachidonic acid (AA) metabolism was studied in male Wistar rats who were maintained for 14 days on a diet consisting of sodium-deficient food and either deionized water (low salt intake, LSI), 1% saline (normal salt intake, NSI), or 2% saline (high salt intake, HSI). 24 h Urinary Sodium (UNaV) and plasma renin activity (PRA) measurements were shown to validate the dietary protocol. Microsomal preparations from the cortices and medullae were incubated with radiolabeled exogenous AA, and endogenous urinary prostaglandin (PG) levels were assayed by RIA to quantify renal PG synthesis. Cortical PGF2 alpha and PGE2 synthesis was found to be the greatest following LSI. In contrast, medullary PGF2 alpha was shown to be the least following LSI and to increase with increased sodium intake. Likewise, urinary PGF2 alpha levels significantly increased with increasing sodium intake. Changes in urinary PGE2 levels showed the same trend as PGF2 alpha but did not achieve statistical significance. These data show that dietary sodium differentially affects renal cortical and medullary PG synthesis and may reflect physiological differences in the regulation of cyclooxygenase in these zones. These data further suggest that the major source of urinary PGs is the renal medulla since the relationship of urinary levels to sodium intake mimics that described for the synthesis of PGs by the medullary tissue.     

Salicylate nephropathy in the Gunn rat: Potential role of prostaglandins

We examined the potential role of prostaglandins in the development of analgesic nephropathy in the Gunn strain of rat. The homozygous Gunn rats have unconjugated hyperbilirubinemia due to the abscence of glucuronyl transferase, leading to marked bilirubin deposition in renal medulla and papilla. These rats are also highly susceptible to develop papillary necrosis with analgesic administration.We used homozygous (jj) and phenotypically normal heterozygous )jJ) animals. Four groups of rats (n = 7) were studied: jj and jJ rats treated either with aspirin 300 mg/kg every other day or sham-treated. After one week, slices of cortex, outer and inner medulla from one kidney wre incubated in buffer and prostaglandin synthesis was determined by radioimmunoassay. The other kidney was examined histologically.A marked corticomedullary gradient of prostaglandin synthesis was observed in all groups, PGE2 synthesis was significantly higher in outer medulla, but not cortex or inner medulla, of jj (38 ± 6 mg/mg prot) than jJ rats (15 ± 3) (p<0.01). Aspirin treatment reduced PGE2 synthesis in all regions, but outer medullary PGE2 remained higher in jj (18 ± 3) than jJ rats (9 ± 2) (p<0.05). PGE2α was also significantly higher in the outer medulla of jj rats with and without aspirin administration (p<0.05). The changes in renal prostaglandin synthesis were accompanied by evidence of renal damage in aspirin-treated jj but not jJ rats as evidenced by: increased incidence and severity of hematuria (p<0.01); increased serum creatinine (p<0.05); and increase in outer medullary histopathologic lesions (p<0.005 compared to either sham-treated jj or aspirin-treated jJ).These results suggest that enhanced protaglandin synthesis contributes to maintenance of renal function and morphological integrity, and that inhibition of protaglandin synthesis may lead to pathological renal medullary lesions and deterioration of renal function.     

The effect of prostaglandin synthetase inhibitors on renal function: studies in normal rats during sodium or water diuresis and in rats with chronic renal insufficiency

The effect of acute infusion of the prostaglandin synthetase inhibitors - meclofenamate or indomethacin - was examined in awake rats. Studies were performed in normal rats undergoing either sodium or water diuresis and in salt-replete rats with chronic renal insufficiency. Prostaglandin synthetase inhibitors had no effect on renal plasma flow, glomerular filtration rate or fractional excretion of sodium in any of the groups. Absolute urinary excretion rates for sodium and potassium decreased only in the normal, salt-replete rats. In contrast, prostaglandin synthetase inhibitors consistently decreased urinary flow and osmolar clearance under all experimental conditions studied. In the normal, salt-replete rats the fall in urine flow was preceded by an increase in urinary excretion of cyclic AMP. These results show that inhibitors of prostaglandin synthesis enhance the ability of the kidney to reabsorb water. This effect may be secondary to increased cyclic AMP generation and to increased urea recirculation resulting in higher urea accumulation in the renal medulla.     

Distinctive effect of angiotensin II on prostaglandin production in dog renal and femoral arteries

The effect of angiotensin II (Ang II) on prostaglandin (PG) production in dog renal and femoral vasculature was examined in vivo and in vitro. In pentobarbital anesthetized dogs, the reduction of blood flow induced by intra-arterial infusion of Ang II was potentiated by pre-treatment with indomethacin (5 mg/kg) in the renal but not the femoral vasculature. Isolated renal and femoral arterial strips were incubated and the release of PGE2 and PGI2 (as 6-keto-PGF1 alpha) into the medium was measured by radioimmunoassay. Basal PGE2 and PGI2 production by renal and femoral arterial strips was approximately the same. PGI2 production was predominant for both strips. Ang II stimulated PG production in renal but not femoral arteries. In the renal artery, Ang II-induced PG production was inhibited by indomethacin (10(-6) M), mepacrine (10(-4) M) and saralasin (10(-6) M). These results suggest that Ang II stimulates PG production by the renal artery per se and the Ang II receptor is linked to phospholipase A2 in the renal but not the femoral artery.     

The role of alpha-adrenergic receptors in the vasoconstrictor response induced by indomethacin in the kidney.

The effect of indomethacin, an inhibitor of prostaglandin (PG) synthesis, was studied on the renal circulation, Na+ and water excretion in anaesthesized dogs during alpha-receptor inhibition. Indomethacin decreased cortical blood flow (CBFcontr, 454 +/- 142; CBFindo, 332 +/- 51 ml per min per 100 g; p less than 0.02) as well as medullary blood flow (OMBFcontr, 339 +/- 95; OMBFindo, 183 +/- 46 ml per min per 100 g; p less than 0.001), salt and water excretion, further it caused a shift in the intrarenal blood flow distribution toward the cortex. Alpha-blockade prevented the indomethacin-induced vasoconstriction in the cortex (CBF alpha inhibition + indo, 455 +/- 76 ml per min per 100 g) but not in the medullar (OMBF alpha inhibition + indo, 259 +/- 102 ml per min per 100 g, p less than 0.05). Alpha-blockade failed to prevent the indomethacin-induced antidiuresis, antinatriuresis and the intrarenal blood flow redistribution. GFR remained unaffected in all three series of studies. Our experimental findings are in line with the presumption that alpha-receptors are involved in the renal circulatory changes caused by indomethacin, probably as a result of an enhanced NE release during the inhibition of PG production. A NE--PG feed back mechanism is suggested in the regulation of renal circulation. The reduction of salt and water output induced by indomethacin appears to be independent of the alterations in renal haemodynamics, and seems rather to be the result of enhanced Na+ reabsorption, predominantly at the distal segment of the nephron, in the absence of PG, and/or a direct action of indomethacin.     

The effect of prostaglandin synthetase inhibitors on renal function: Studies in normal rats during sodium or water diuresis and in rats with chronic renal insufficiency

The effect of acute infusion of the prostaglandin synthetase inhibitors — meclofenamate or indomethacin — was examined in awake rats. Studies were performed in normal rats undergoing either sodium or water diuresis and in salt-replete rats with chronic renal insufficiency. Prostaglandin synthetase inhibitors had no effect on renal plasma flow, glomerular filtration rate or fractional excretion of sodium in any of the groups. Absolute urinary excretion rates for sodium and potassium decreased only in the normal, salt-replete rats. In contrast, prostaglandin synthetase inhibitors consistently decreased urinary flow and osmolar clearance under all experimental conditions studied. In the normal, salt-replete rats the fall in urine flow was preceded by an increase in urinary excretion of cyclic AMP. These results show that inhibitors of prostaglandin synthesis enhance the ability of the kidney to reabsorb water. This effect may be secondary to increased cyclic AMP generation and to increased urea recirculation resulting in higher urea accumulation in the renal medulla.     

Application of high performance liquid chromatography and gas chromatography-mass spectrometry to analysis of prostaglandin E1 in biological media.

A method for the quantitation of prostaglandin (PG) E₁ in biological samples of gas chromatography—mass spectrometry has been developed. PGE₁ was separated from PGE₂, 13,14-dihydro-PGE₂, and other potentially interfering prostaglandins by reversed phase high performance liquid chromatography. After conversion of PGE₁ to PGB₁ by treatment with methanolic KOH, PGB₁ was derivatized to the methyl ester trimethylsilyl ether and analyzed by selected ion monitoring using hexadeutero-PGE₁ as an internal standard. Measurable levels of PGE₁ were found in human and rat urine and in incubates of rat and rabbit renal papilla. PGE₁ excretion and production by renal slices was blocked by treatment with indomethacin. A complete mass spectrum of derivatized PGE₁ was obtained from PGE₁ generated by rabbit renal papillary slices.     

Marked reduction in intramembranous particle clusters in the terminal portion of inner medullary collecting ducts of antidiuretic rats

Summary Antidiuretic hormone (ADH) induces an aggregation of intramembranous particles (IMP) into discrete clusters in the luminal plasma membrane of rat renal papillary collecting duct cells (Harmanci et al. 1978). The correlation between an elevated dose of ADH, increased urine osmolality, and greater IMP cluster frequency has led to speculation that the water permeability of the luminal plasma membrane is reflected by the IMP cluster density (Harmanci et al. 1980). The present study indirectly evaluated this water permeability by quantitating collecting duct IMP cluster frequency from freeze fracture replicas in two regions of the renal papilla, at its base and at its tip, in antidiuretic and in water diuretic rats. During antidiuresis there was a high frequency of IMP clusters (189/100 m² of luminal plasma membrane) in cells from the papilla base but not at the papilla tip (9.0/100 m²). During water diuresis there were few IMP clusters in either cells from the papilla base (5.9/100 m²) or at the papilla tip (1.4/100 m²). Most significantly these results suggest that the water permeability of the terminal portion of the inner medullary collecting duct of antidiuretic rats is significantly lower than that of the collecting duct epithelium higher in the papilla.Preliminary findings of this study were presented at the Second International Congress of Cell Biology, West Berlin 1980     

Enhanced erythropoietin and prostaglandin E production in the dog following renal artery constriction.

Renal artery constriction (RAC) to 30% of normal flow for 12 hr in the unilaterally nephrectomized dog produced a marked increase in both erythropoietin titers and prostaglandin E (PGE) levels in the blood. In dogs pretreated prior to RAC with indomethacin, a potent inhibitor of prostaglandin synthetase, there was no significant increase in either PGE or erythropoietin levels as compared to zero-time control values. These data suggested an involvement of renal PGE in the generation of erythropoietin following RAC.     

Impaired prostaglandin release from the kidneys of salt-loaded and hypertensive rats

Prostaglandin (PG) release by the isolated perfused kidney of the rat has been stimulated by nor-adrenaline infusion and measured by bioassay. There was no basal output of PGE₂-like activity, but stimulated release reached mean concentrations of 9.1 ng/g kidney/ml perfusate in kidneys from female albino rats drinking water and 2.9 ng/g/ml in those from animals given 1.5% NaCl to drink. Kidneys from uninephrectomised animals with mock-clipped renal arteries released 7.3 ng PG/g/ml and those from rats with uninephrectomy and constricted renal arteries 3.3 ng/g/ml.     

Immunohistochemically demonstrated increase of prostaglandin F2-alpha in neurons after reoxygenation in anoxic rats

Immunohistochemical staining for prostaglandin F2-alpha (PG F2 alpha) was conducted to identify PG F2 alpha synthesizing or binding sites in anoxic rat brains. Anoxia was produced in 22 rats to lower the arterial oxygen tension (PaO2) to 21 +/- 4 mmHg by ventilation with a 95% nitrogen and 5% carbon dioxide gas mixture. In 8 animals anoxia was continued for 30 sec, and in 14 rats for 3 min. Prior to decapitation, 5 animals in the 30-sec anoxia group and 8 rats in the 3-min anoxia group were reoxygenated for 5 min, while the remaining 9 were not. Five-min reoxygenation returned the PaO2 to 106 +/- 7. Three non-reoxygenated and 3 reoxygenated rats, all pretreated with indomethacin, and 5 normal rats served as controls. The brains were snap-frozen. The cryosections were stained by the indirect immunofluorescence method. PG F2 alpha was noted mainly in pial vessels in all normal rats. All reoxygenated rats showed a positive reaction not only in blood vessels, but also in neurons, particularly hippocampal neurons and Purkinje cells. The staining of the above neurons was noted to be less in non-reoxygenated rats. The stronger staining was observed in rats reoxygenated after 3-min anoxia than 30-sec anoxia. The indomethacin-pretreated rats showed almost no increase in staining intensity. The above results indicate that reoxygenation after anoxia results in an increase of PG F2 alpha in neurons of both cerebrum and cerebellum.     

Effect of furegrelate on renal plasma flow after angiotensin II infusion

We had previously shown that selective thromboxane synthetase inhibition with furegrelate increases urinary excretion of 6-ketoPGF1 alpha, the hydrolysis product of prostacyclin after stimulation of renal prostaglandin synthesis with furosemide. The present study assessed the functional significance of this "redirection" of prostaglandin formation using a more physiologic stimulus, angiotensin II. Sprague-Dawley rats (n = 27) were fitted with a transabdominal bladder cannula. Five days later they were given angiotensin II (10 mg.kg-1.min-1) by intravenous infusion. After 30 min, an infusion of furegrelate, 2 mg/kg, then 2 mg.kg-1.h-1, (n = 9); indomethacin, 2 mg/kg, then 2 mg.kg-1.h-1 (n = 9); or vehicle, 250 microL, then 0.018 mL/min (n = 9) was begun for 60 min. Clearance of [14C]para-aminohippuric acid was taken as a measure of renal plasma flow. Angiotensin II raised the mean arterial pressure in all groups. Administration of furegrelate or indomethacin did not change mean arterial pressure or heart rate. Angiotensin II reduced [14C]p-aminohippuric acid clearance by about 32% (1.42 +/- 0.18 to 0.97 +/- 0.07 mL.min-1.100 g-1, p less than 0.05). Furegrelate attenuated this renal vasoconstriction (0.97 +/- 0.07 to 1.38 +/- 0.17 mL.min-1.100 g-1, p less than 0.05), while indomethacin increased it by a further 32% (1.78 +/- 0.12 to 1.20 +/- 0.12 mL.min-1.100 g-1, p less than 0.05). Vehicle alone had no effect. Furegrelate reduced serum thromboxane B2 by 90% (6.52 +/- 0.030 to 0.7 +/- 0.21 ng/100 microL, p less than 0.05), while indomethacin reduced it by 73% (5.9 +/- 0.99 to 1.4 +/- 0.20 ng/100 microL, p less than 0.05). We conclude that furegrelate attenuates the renal vasoconstriction of angiotensin II, presumably by enhancing the formation of vasodilator prostaglandins.     

Testing the "redirection hypothesis" of prostaglandin metabolism in the kidney

Furosemide increases the synthesis of two major renal eicosanoids, prostacyclin (PGI2) and thromboxane A2 (TXA2), by stimulating the release of arachidonic acid which in turn is metabolized to PGG2/PGH2, then to PGI2 and TXA2. PGI2 may mediate, in part, the early increment in plasma renin activity (PRA) after furosemide. We hypothesized that thromboxane synthetase inhibition should direct prostaglandin endoperoxide metabolism toward PGI2, thereby enhancing the effects of furosemide on renin release. Furosemide (2.0 mg . kg-1 i.v.) was injected into Sprague-Dawley rats pretreated either with vehicle or with U-63,557A (a thromboxane synthetase inhibitor, 2 mg/kg-1 followed by 2 mg/kg-1 X hr-1). Urinary 6ketoPGF1 alpha and thromboxane B2 (TXB2), reflecting renal synthesis of PGI2 and TXA2, as well as PRA and serum TXB2, were measured. Serum TXB2 was reduced by 96% after U-63,557A. U-63,557A did not affect the basal PRA. Furosemide increased PRA in both vehicle and U63,557A treated rats. However, the PRA-increment at 10, 20 and 40 min following furosemide administration was greater in U-63,557A-treated rats than in vehicle-treated rats and urine 6ketoPGF1 alpha excretion rates were increased. These effects of thromboxane synthesis inhibition are consistent with a redirection of renal PG synthesis toward PGI2 and further suggest that such redirection can be physiologically relevant.     

Adenosine as a natural prostaglandin antagonist in vascular smooth muscle

Adenosine has actions on smooth muscle similar to those of prostaglandin (PG) antagonists. Like some PG antagonists it is a phosphodiesterase inhibitor and seems to interfere with calcium effects. It has agonist/antagonist interactions with theophylline, a PG antagonist. In rat mesenteric vascular smooth muscle adenosine blocked responses to noradrenaline which depend on release of intracellular calcium but not those to potassium ions which depend on calcium entry from extracellular fluid. Partial inhibition of endogenous PG synthesis by indomethacin enhanced the adenosine effect. In preparations in which vascular reactivity had been abolished by indomethacin and then partly restored by 1 or 5 ng/ml PGE2, adenosine also inhibited responses to noradrenaline: the curve for the 5 ng/ml PGE2 concentration was to the right of and parallel to the 1 ng/ml curve consistent with a competitive interaction between adenosine and PGE2. Similar interactions between adenosine and PGE2 were shown in human lymphocytes in which activation also depends on calcium release. These findings suggest how calcium-dependent metabolic responses may be controlled and indicate further reasons for caution in the interpretation of cyclic AMP experiments.