Urinary prostaglandin E excretion: effect of chronic alterations in sodium intake and inhibition of prostaglandin synthesis in the rabbit

Influence of renal sympathectomy, sodium depletion and prostaglandin synthetase inhibition on prostaglandin production and [3H]PGE2 binding characteristics in rat kidney

The effects of renal sympathectomy (unilateral, renal microsurgical denervation), sodium depletion (hypovolaemia) and prostaglandin synthetase inhibition on the rate of prostaglandin synthesis and [3H]PGE2 binding characteristics were studied in the rat kidney. The intrarenal rate of prostaglandin synthesis was measured by monitoring the urinary excretion of 6-keto-PGF1 alpha, the stable hydration product of prostacyclin. Dietary sodium restriction was associated with a 99% decrease in urinary sodium excretion (P less than 0.001) and a 17% decrease of urine volume (n.s.). Renal denervation or sodium deprivation changed neither the rate of excretion of 6-keto-PGF1 alpha nor the density or affinity of [3H]PGE2 binding sites as compared to control. However, in sodium-depleted rats, prostaglandin synthesis inhibition, induced by naproxen, decreased the urinary excretion of 6-keto-PGF1 alpha by 40% (P = 0.011) and increased the number of [3H]PGE2 binding sites by almost 30% (P = 0.031) with no change in binding affinities as compared with sodium-depleted controls. In contrast, sulindac was not able to suppress the renal synthesis and excretion of 6-keto-PGF1 alpha, and did not modulate the [3H]PGE2 binding characteristics. The lack of effect on the excretion of 6-keto-PGF1 alpha and on the [3H]PGE2 binding characteristics supports the view that sulindac spares renal prostaglandin synthesis.

Atrial natriuretic peptide in volume expansion-induced natriuresis in man

The role of atrial natriuretic peptide (ANP) in the extracellular volume expansion (ECVE) induced natriuresis was examined in normal man under basal conditions and following dopamine blockage. Hypotonic ECVE was induced by drinking of 20 ml/kg tap water and subsequent intravenous infusion of 2 1 0, 9% saline over a period of 4 hours. This maneuver caused an increase in the plasma concentrations of ANP from 25.8 +/- 3.4 (means +/- SEM) to 59.7 +/- 6.7 fmol/ml. There was a dissociation between ANP response and urinary sodium excretion. A transient rise in glomerular filtration rate (GFR), plasma dopamine and a continuous decrease in plasma renin activity, aldosterone, vasopressin, and noradrenaline were observed. The natriuretic response to ECVE was blunted during dopamine blockade by metoclopramide, but plasma ANP, renin activity, catecholamine and vasopressin levels were not affected. However, plasma aldosterone rose. Our data are compatible with the concept that intrarenal dopamine and raised plasma concentration of ANP contribute to the natriuretic response to ECVE, but these hormonal changes do not completely explain the underlying mechanisms.

Effects of prostaglandin synthesis inhibitors on the renin-angiotensin system and renal function

Equal doses (8 mg/kg) of the nonsteroidal antiinflammatory drugs indomethacin, naproxen, and sulindac and a large dose of sulindac (32 mg/kg) were administered intragastrically to conscious rats after a normal sodium diet, furosemide stimulation, and a low sodium diet for 8 days. Indomethacin, naproxen, and the high dose sulindac (32 mg/kg) decreased urinary prostaglandin E2 excretion significantly under all experimental conditions. Sulindac (8 mg/kg) suppressed prostaglandin E2 excretion after the normal and low sodium diets but not after furosemide stimulation. Indomethacin decreased plasma active renin levels under all three experimental conditions. In rats receiving a normal sodium diet, indomethacin did not affect free water clearance or renal function; however, after furosemide stimulation or a low sodium diet, indomethacin caused a significant reduction of free water clearance and glomerular filtration rate. Naproxen and sulindac (8 mg/kg) did not suppress active renin under any of the experimental conditions. However, naproxen and sulindac caused a significant reduction in free water clearance and glomerular filtration rate after furosemide stimulation and a low sodium diet. Indomethacin, naproxen, and the high dose sulindac suppressed renal prostaglandin E2 excretion under all experimental conditions. Renal prostaglandin E2 does not appear to be necessary for active renin secretion. Indomethacin is the most potent inhibitor of active renin and, therefore, most likely to cause hyporeninemia. Volume depletion appeared to sensitize the kidney to the adverse effects of nonsteroidal antiinflammatory drugs.

The effect of NaCl intake on 9-ketoprostaglandin reductase activity in the rabbit kidney

Renal 9-ketoprostaglandin reductase activity from rabbits fed 0.3 g or 2.5 g NaCl per 100 g chow was measured in both centrifuged homogenates and in purified enzyme fractions. There was no salt related increase in 9-ketoprostaglandin reductase activity. PGA1-glutathione, 9, 10-phenanthrenequinone, and 4-nitrobenzaldehyde were better substrates for the purified 9-ketoprostaglandin reductases than was PGE2. Several carbonyl reductases were isolated which used PGA1-glutathione, 9, 10-phenanthrenequinone, and 4-nitrobenzaldehyde, but not PGE2, as substrates. Although PGA1-glutathione was a more faithful indicator of PGE2-related 9-ketoprostaglandin reductase activity than either 9, 10-phenanthrenequione or 4-nitrobenzaldehyde, it did not always provide an accurate estimate of that activity.

PGE2, 6-O-PGF1 alpha and sodium balance in essential hypertensive patients

Insufficient production of PGE2 has been thought to be a pathogenic factor in hypertensive patients on salt loading. However, the inter-relationship between PGE2 and sodium is still uncertain. Essential hypertensive patients participated in a salt balance study for two consecutive 5-day periods of restriction and repletion. Urinary PGE2 increased on the second or the third day of salt balance, but subsequently declined and remained stable. On the fifth day, PGE2 excretion on salt loading was lower than on salt deprivation. Changes in urinary PGE2 during salt loading paralleled those in urinary sodium excretion. Urinary 6-O-PGF1 alpha did not show any significant change. On salt deprivation PGE2 excretion was correlated with plasma renin activity (PRA). These results suggest the possibility that PGE2 is involved in sodium excretion on salt loading and that on salt depletion PGE2 excretion increases in association with activation of the renin-angiotensin axis.

Effect of dietary sodium intake on the metabolism of prostaglandins in the kidney in hypertensive patients

To investigate the role of renal prostaglandins (PGs) in the renal handling of sodium, urinary excretion of PGE, PGF2 alpha and PGF2 alpha MUM (main urinary metabolite of PGF2 alpha) were measured after various manipulations of dietary sodium intake in 8 hypertensive patients. A low sodium intake increased urinary excretion of PGF2 alpha MUM (p less than 0.05), but failed to change urinary excretion of PGE and PGF2 alpha. In contrast, a high sodium intake increased urinary excretion of PGE (p less than 0.01) and decreased urinary excretion of PGF2 alpha MUM (p less than 0.02). A low sodium intake decreased the ratio of urinary PGE/PGF2 alpha MUM and high sodium increased it (both p less than 0.001). There was a significant positive correlation between urinary excretion of sodium and that of PGE (p less than 0.001). Additional oral administration of potassium chloride did not change urinary excretion of PGs. These results may suggest that dietary sodium intake may be one of the regulators of the metabolism of PGs in the kidney, supporting the hypothesis that renal PGE has a natriuretic action in humans.

Effect of prostaglandin inhibition on glomerular filtration rate in normal and uremic rabbits

Studies were performed to assess the effect of alterations in prostaglandin biosynthesis on glomerular filtration rate in rabbits with normal renal function and after surgical reduction of renal mass. In normal animals, the administration of either of two cyclo-oxygenase inhibitors resulted in a 53% reduction in urine prostaglandin E excretion, but no change in the creatinine clearance. Creatinine clearance rates were normal almost 71% lower in the uremic animals when compared to the animals with normal renal function. Despite the reduction in renal mass, urine prostaglandin E excretion rates in the uremic animals were over twice that seen in normal rabbits. When factored by either glomerular filtration rate or remaining renal mass, urine prostaglandin E excretion rates in uremic rabbits were compared to normal animals were increased more than 9-times and 40 times respectively. Administration of cyclo-oxygenase inhibitors in the uremic animals resulted in a 71% decrease in urine prostaglandin E excretion, unlike and non-uremic animals, a 53% fall in creatinine clearance. These findings suggest that intact renal prostaglandin biosynthesis is a necessary factor in the homeostatic adaptive mechanisms which maintain the glomerular filtration rate in animals with decreased renal mass.

Effect of salt on prostaglandin metabolism in hypertension-prone and -resistant Dahl rats

The effect of high salt intake on vascular and renomedullary prostaglandin (PG) synthesis was compared in Sprague-Dawley and salt-sensitive (S) and -resistant (R) Dahl rats. Animals were given a diet containing either 0.6% or 8% NaCl starting at 5 weeks of age, and were sacrificed 6 weeks later. Systolic blood pressure of S rats increased to 220 +/- 7 mm Hg but was unaffected in R and Sprague-Dawley rats. Prostaglandin synthesis was studied in aortic rings and renomedullary microsomes using 14C-arachidonate as substrate. [3H]PGE2 degradation was measured in the renocortical cytosol. In Sprague-Dawley and R rats, aortic PGI2 synthesis was not affected by high salt intake, while a significant increase compared to animals on 0.6% NaCl (from 608 +/- 84 to 992 +/-108 pmoles/60 min, p less than 0.05) was noted in S rats. Enhancement of PGI2 synthesis in S rats may be secondary to the hypertension. Salt-loading consistently stimulated renomedullary PGE2 synthesis in all three animal groups. S rats, however, had the lowest PG synthesis in renal medullas compared to Sprague-Dawley and R rats when placed on either diet. Thus, even after 6 weeks on high salt, S rats did not reach the levels of PGE2 synthesis seen in R or Sprague-Dawley rats on regular diet. The activity of cortical 15-hydroxyprostaglandin dehydrogenase was increased by salt-loading in S and Sprague-Dawley, but not in R rats. R rats had lower dehydrogenase activity than the other two groups when placed on either diet. The observed differences in PG synthesis and catabolism will tend to maintain the net output of renal PGs highest in R and lowest in S rats. These differences correlate with the reported differences in renal papillary flow between these two rat strains and may be relevant to their susceptibility or resistance to hypertension in response to salt.

Increased renal secretion of norepinephrine and prostaglandin E2 during sodium depletion in the dog

To determine whether vasoactive renal hormones modulate renal blood flow during alterations of sodium balance, simultaneous measurements of arterial and renal venous concentrations of norepinephrine and prostaglandin E2 (PGE2) and of plasma renin activity, as well as renal blood flow and systemic hemodynamics were carried out in 24 sodium-depleted and 28 sodium-replete anesthetized dogs. The mean arterial blood pressure of the sodium depleted dogs was not significantly different from that of the animals fed a normal sodium diet, but cardiac output was significantly lower (3.07 +/- 0.18 vs. 3.77 +/- 0.17 liters/min, mean +/- SEM; P < 0.01). Despite the higher total peripheral vascular resistance in the sodium-depleted dogs (46.1 +/- 2.9 vs. 37.0 +/- 2.1 arbitrary resistance U; P < 0.02), the renal blood flow and renal vascular resistance were not significantly different in the two groups. The arterial plasma renin activity and concentration of norepinephrine were higher in the sodium-depleted animals than in the controls; the arterial concentration of PGE2 was equal in both groups. The renal venous plasma renin activity was higher in the sodium-depleted dogs. Similarly, the renal venous norepinephrine concentration was higher in the sodium-depleted dogs than in the controls (457 +/- 44 vs. 196 +/- 25 pg/ml; P < 0.01); renal venous PGE2 concentration was also higher in the sodium depleted dogs (92 +/- 22 vs. 48 +/- 11 pg/ml; P < 0.01). Administration of indomethacin to five sodium-replete dogs had no effect on renal blood flow. In five sodium-depleted dogs indomethacin lowered renal blood flow from 243 +/- 19 to 189 +/- 30 ml/min (P < 0.05) and PGE2 in renal venous blood from 71 +/- 14 to 15 +/- 2 pg/ml (P < 0.02). The results indicate that moderate chronic sodium depletion, in addition to enhancing the activity of the renin-angiotensin system, also increases the activity of the renal adrenergic nervous system and increases renal PGE2 synthesis. In sodium-depleted dogs, inhibition of prostaglandin synthesis was associated with a significant decrease in renal blood flow. The results suggest that the renal blood flow is maintained during moderate sodium depletion by an effect of the prostaglandins to oppose the vasoconstrictor effects of angiotensin II and the renal sympathetic nervous system.

Effects of indomethacin on furosemide-stimulated urinary PGE2 excretion in man

We studied the effects of furosemide on urinary excretion of PGE2 and sodium and the effects of inhibition of prostaglandin (PG) synthesis with indomethacin or furosemide-induced PGE2 excretion and natriuresis in normal man. Furosemide (20 mg i.v.) increased the urinary excretion of PGE2 from 71.2 +/- 17.2 to 255.9 +/- 41.0 ng/4 h. Sodium excretion increased in parallel. Indomethacin, in a dose sufficient to decrease basal urinary PGE2 excretion by > 90%, significantly decreased both urinary PGE2 and sodium excretion under furosemide without affecting delivery of furosemide into the urine. The urinary excretion of furosemide was 9.4 +/- 0.4 and 9.3 +/- 1.4 mg/24 h with and without indomethacin, respectively. However, the furosemide-induced increment in PGE2 excretion correlated significantly with sodium excretion rate with and without indomethacin. Indomethacin changed the relationship between absolute amounts of furosemide in urine and PGE2 excretion but did not affect the increment in excretion over baseline or the significant correlation of urinary PGE2 with sodium excretion.

Renal prostaglandins and sodium balance in the rabbit: lack of effect of aspirin-like drugs

Urinary prostaglandin E2 (PGE2) and PGF2 excretion was previously found to be inversely related to sodium intake in the rabbit. Renal prostaglandin (PG) synthesis might therefore be involved in the renal handling of sodium. This possibility was tested by studying sodium excretion during inhibition of renal PG synthesis with four different nonsteroidal anti-inflammatory drugs in unanaesthetized, female rabbits. The rabbits n = 5-6) were kept in metabolic cages and chronically maintained on different sodium containing diets. On a medium salt diet (0.4% NaCl), neither treatment with indomethacin (1.5 mg/kg x 2 or 3 mg/kg x 2) nor diclofenac (1.5 mg/kg x 2) for three days changed the urinary excretion of sodium and water although the mean excretion of immunoreactive PGE2 (iPGE2) and iPGF2 were reduced by between 43-78%. On a very low salt diet (0.05% NaCl), two days treatment with aspirin (30 mg/kg x 2), diclofenac (3 mg/kg x 2), indomethacin (3.5 mg/kg x 2) or naproxen (10 mg/kg x 2) did not alter sodium excretion in any significant direction. The mean urinary PGE2 and PGF2 excretion was reduced by 35-63% and 63-85%, respectively. These results do not support a major role of PGs in the chronic regulation of sodium balance in the rabbit. The possible influence of mineralocorticoids on renal PG synthesis was studied by administration of aldosterone (100 microgram/kg x 2) for two days to rabbits on a high salt diet (2% NaCl) and cancrenoate (10 mg/kg x 2), an aldosterone antagonist, to rabbits on the very low salt diet. However, neither drug significantly changed the urinary excretion of PGF2 alpha, indicating that renal PG synthesis is not influenced by mineralocorticoids in the rabbit.

Sodium intake as a determinant of urinary prostaglandin E2 excretion

The effect of salt-loading on urinary prostaglandin D2 (PGE2) was studied in the rat. PGE2 doubled after 4 days of gavage with 40 ml 0.9% saline. Control animals receiving 5% glucose or tap water did not show this increase. When rats were allowed ad lib saline, PGE2 increased after 2 weeks, but returned to normal at 5 weeks. This same phenomenon of increased PGE2 early in the course of salt-loading was also observed in Dahl salt-sensitive and salt-resistant rats fed an 8% salt diet. Inhibition of prostaglandin biosynthesis by meclofenamate or indomethacin did not attenuate the natriuretic effect of saline loading. We conclude: 1) Salt-related changes in PGE2 is a time-dependent phenomenon, and 2) PGE2 is not the natriuretic factor since levels normalize despite continued salt-loading, and natriuresis was unaffected by prostaglandin blockade.

The relationship of urinary prostaglandins and plasma renin to sodium balance and diuresis in normal man

The significance of changes in sodium balance and urinary sodium excretion for renal PG excretion was studied in normal man. In protocol B a strongly negative sodium balance was produced in 5 healthy young subjects by a low sodium diet given for 7 days (lo mmol Na/day) with 80 mg furosemide p.o. added on the last two days. 24 hour urinary PGE2 excretion remained constant, while plasma renin increased. In protocol A the effect of i.v. furosemide (1 mg/kg bwt) on urinary PGE2 and PGF2 alpha excretion rates was examined in 5 healthy young subjects. Rapid but short-lasting increases in PG excretion rates ran in parallel with the changes in urine flow rate. The study suggests that PGE2 is not of importance for the sodium homeostasis in normal man. Renal prostaglandins may play a modifying role for the renal response to loop diuretics but are hardly instrumental for the diuretic effect.