Prostaglandins and natriuresis: the effect of renal prostaglandins on PAH uptake by kidney cortex

Paradoxical increase of renal blood flow in anesthetized hypertensive dog treated with indomethacin

To evaluate the effect of prostaglandin inhibition on the renal blood flow of the ischemic kidney, we administered indomethacin to 10 anesthetized dogs with renal artery stenosis and contralateral nephrectomy. Following the operation to produce renal ischemia, there was an increase of blood pressure associated with an increase of renin and the prostaglandins F1 (PGF1), and E (PGE). The administration of indomethacin to the intact, normotensive animals caused the anticipated decrease of prostaglandin E, renin, and renal blood flow. However, in the hypertensive dogs, indomethacin caused a paradoxical 45 per cent increase in the renal blood flow, despite a 44 per cent decrease of prostaglandin E. PGF1, PGE, renin, and erythropoietin exhibited the anticipated decreased levels. The study suggests that prostaglandins may not be the sole important factor in the regulation of renal blood flow in the presence of ischemia. Other important factors likely include the renin-sensitive angiotensin, the adrenergic, and the kallikrein-kinin systems.

Enhancement of urine prostaglandin excretion by chlorazanil in dogs

In conscious dogs chlorazanil (2.5 mg/kg intravenously) markedly enhanced (5--10 fold) urinary excretion of prostaglandin E2 and F2alpha. The effect came to a peak at 15--30 min. following the administration. Chlorazanil did not modify renal blood flow or inulin clearance in non-pretreated or indomethacin pretreated dogs and the plasma renin activity remained unchanged. A marginal natriuretic and antikaliuretic activity by chlorazanil was similarly observed in non-pretreated dogs and dogs pretreated with indomethacin. The possible effects of the prostaglandins released by chlorazanil remained obscure. In vitro chlorazanil (10(-4) M) exhibited a moderate inhibition of 15-OH-prostaglandin dehydrogenase but an enhanced excretion of 15-keto-13, 14-dihydro PGE2alpha in vivo suggested that chlorazanil increased renal prostaglandin activity by increased prostaglandin synthesis, probably due to increased precursor availability. This was presumably mediated by some as yet unknown factor since chlorazanil ( 4 x 10(-5)M) failed to affect the release of precursor and prostaglandins from isolated Krebs-Henseleit perfused rabbit kidneys.

A bilateral antidiuresis to renal artery infusion of prostaglandin E1 in dogs treated with phenylbutazone

1. In acute experiments, high levels of endogenous prostaglandins, provoked by operative stress, could obscure or alter the actions of infused prostaglandins on the kidney. For this reason we decided to compare the effects of infusing prostaglandin E(1) into the renal artery of the dog before and after the administration of phenylbutazone, a prostaglandin synthetase inhibitor.2. Infusion of prostaglandin E(1) into the left renal artery of the pre-phenylbutazone treated dog undergoing a mannitol diuresis increased renal plasma flow, glomerular filtration rate and the excretion of salt and water. The findings are in general agreement with those reported by others.3. Following phenylbutazone administration the vascular and saluretic actions of prostaglandin E(1) were unchanged but a reduced diuretic effect was observed. The response to a low dose of prostaglandin E(1) (0.05 mug/min) was reduced from 1.46 +/- 0.15 to 0.96 +/- 0.16 ml./min (P < 0.001) and the response to a high dose (0.5 mug/min) from 1.82 +/- 0.19 to 0.99 +/- 0.31 ml./min (P < 0.002).4. A significantly less dilute urine was excreted during prostaglandin infusion in the dog after phenylbutazone treatment than before. The reduction in the diuretic response was of the same order as the decrease in the free water clearance response, while the increase in osmolar clearance was unchanged.5. In water-loaded dogs treated with phenylbutazone, infusion of prostaglandin E(1) into the left renal artery had a biphasic effect on urine output from the left kidney. An initial diuretic response to a low dose of prostaglandin E(1) disappeared with the infusion of higher doses, and antidiuresis developed in the immediate post-infusion period.6. As prostaglandin was infused into the left kidney progressive antidiuresis was seen in the non-infused right kidney.7. It is concluded that endogenous prostaglandins do not obscure or alter the vascular and saluretic actions of intrarenal prostaglandin E(1). The findings question the proposed link between the vascular and saluretic actions of this compound.8. It is suggested that the reduced diuretic effect of prostaglandin E(1) in series no. 1, and the antidiuresis in the water-loaded dogs, are caused by the release of endogenous ADH. It is further suggested that phenylbutazone unmasks this release by removing the endogenous prostaglandins. If these deductions are correct, the findings support the anti-ADH role assigned to endogenous prostaglandins by Anderson, Berl, McDonald & Schrier (1975).

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

Prostaglandin F2alpha is metabolized in 100 000 X g supernatants of rabbit kidney by a 15-hydroxydehydrogenase and a delta13-reductase to F-series metabolites, and byt a 9-hydroxydehydrogenase to E-series compounds. The reactions were measured by radiochemical and biological assays. The 15-hydroxydehydrogenase and 9-hydroxydehydrogenase are localised specifically in renal cortex, have optimal activity at alkaline ph and are effective over a wide range of substrate concentrations. The 9-hydroxydehydrogenase oxidises the 9-alpha hydroxyl group of prostaglandin F2alpha and F1alpha but not the 9-beta hydroxyl group of prostaglandin F2beta. This enzyme is also found in rabbit stomach and ileum, but not in 8 other organs examined; the 15-hydroxydehydrogenase has a wider distribution. Unlike other 9-hydroxydehydrogenase enzymes described to date, the enzyme prepared from rabbit kidney converts prostaglandin F2alpha directly to E2 in substantial amounts; this conversion may be of importance in renal homeostasis.

Pathways of prostaglandin F2alpha metabolism in mammalian kidneys

1 High-speed cytoplasmic supernatants of rat, rabbit, pig and guinea-pig kidneys were prepared and the metabolism of 10 mug/ml prostaglandin F(2alpha) labelled with [(3)H(1)-9beta]-prostaglandin F(2alpha) studied by thin layer radiochromatography and bioassay.2 The metabolism of prostaglandin F(2alpha) measured by radiochromatography parallels biological inactivation in all species except the rabbit.3 Kidneys metabolize prostaglandin F(2alpha) by two divergent pathways, yielding a mixture of prostaglandin E and F metabolites.4 15-Hydroxyprostaglandin dehydrogenase and prostaglandin Delta-13 reductase are present in all species in characteristic proportions. Thus prostaglandin F(2alpha) is metabolized sequentially to 15-keto prostaglandin F(2alpha) and 13,14-dihydro-15-keto prostaglandin F(2alpha). The rate and profile of formation of these metabolites is species-dependent.5 13,14-Dihydro-15-keto prostaglandin F(2alpha) is the principal prostaglandin F series metabolite in all species.6 Pig and guinea-pig kidney contain an unidentified enzyme which converts 13,14-dihydro-15-keto prostaglandin F(2alpha) to 13,14-dihydro prostaglandin F(2alpha).7 Rat kidney contains a high concentration of a prostaglandin 9-hydroxy dehydrogenase which converts 13,14-dihydro-15-keto prostaglandin F(2alpha) to 13,14-dihydro-15-keto prostaglandin E(2).8 Rabbit kidney contains a novel 9-hydroxydehydrogenase which oxidises prostaglandin F(2alpha) directly to E(2), thus producing a compound with more potent renal actions. The possible implications of this enzyme for kidney homeostasis are discussed.

Renal prostaglandins and the regulation of blood pressure and sodium and water homeostasis

In addition to its well known prohypertensive role in various states of experimental and human hypertension, the kidney has also been shown to exert an antihypertensive "endocrine" function. According to this hypothesis, certain forms of experimental and human hypertension might not solely be the result of an excess in the activity of such renal pressor systems as the renin-angiotensin system and the sympathetic nervous system, but might also result from an absolute or relative deficiency of intra-renal vasodilator antihypertensive factors which might allow pressor systems to act unopposed to produce peripheral arteriolar vasoconstriction and sustained hypertension. At least four factors have been characterized in the kidney of various animal species and man which might be responsible for such an antihypertensive function. These are (1) the renomedullary prostaglandins (PGs), (2) the renomedullary antihypertensive neutral lipid, (3) antirenin phospholipid and (4) the renal kinins. This review is restricted to an examination of the possibility that the vasodepressor renomedullary prostaglandins (PGA and/or PGE) may, at least in part, mediate the so-called antihypertensive function of the kidney and participate in the regulation of renal blood flow and natriuresis by physiologic antagonism of various renal vasoconstrictor stimuli such as the renal renin-angiotensin and the sympathetic nervous systems.

Radioimmunoassay of prostaglandin A. Intrarenal PGA2 as a factor mediating saline-induced natriuresis

A highly specific, sensitive (25 pg) radioimmunoassay for prostaglandin A is described; the assay is characterized by high reproducibility (coefficient of variation 1-2%) and low cross-reactivity with respect to PGE 1 and PGE 2 (10%), PGB 2 (15%), and PGF 2α (0.01%). High specificity for PGA was obtained by chromatographic separation of the prostaglandins. Since it has been suggested that PGA is a natriuretic "hormone," the concentration of PGA in the renal cortex, outer medulla, and papilla was determined in rabbits on low-sodium and high-sodium diets in the presence and the absence of the prostaglandin synthetase inhibitor indomethacin. In rabbits on the low-sodium diet (12 mEq/24 hours). PGA2 was 15 ± 4 pg/mg in the cortex, 502 ± 67 pg/mg in the outer medulla, and 3740 ± 640 pg/mg in the papilla. In rabbits on saline loading (56 mEq/24 hours), cortical and outer medullary PGA2 rose to 32 ± 6 pg/mg and 1275 ± 290 pg/mg, respectively, but papillary PGA decreased to 1842 ± 480 pg/mg. PGA concentration in the outer medulla and papilla was markedly inhibited by indomethacin (50 mg/day), and this inhibition was associated with sodium retention. The results indicated that sodium induced papillary synthesis of PGA and intrarenal circulation of the prostaglandin to the outer medulla and the cortex possibly via the long veins of the outer medulla occurred. The results are compatible with PGA 2 involvement in salineinduced natriuresis.

The prostaglandins

Discovery ‘Prostaglandin’ was discovered in the 1930s by Professor U. S. von Euler of Stockholm during a search for adrenaline in animal tissues (Euler 1934, 1936). Developments were slow in this field at first, partly because of the lack of chemical methods for the purification of lipid-soluble substances. Isolation of two prosta­glandins (PGE1and PGF1α) was achieved in 1959 by Bergström & Sjövall (1960a, b). They and their colleagues (Bergström, Ryhage, Samuelsson & Sjövall 1963) showed that prostaglandins are a family of closely-related substances, all derivatives of the parent compound, prostanoic acid. Chemical structure Prostanoic acid is a fully saturated, twenty carbon monocarboxylic acid con­taining a cyclopentane ring (figure 1). This compound is not a naturally occurring substance but forms the basis for the nomenclature of the prostaglandins. How­ever, in the laboratory, trivial names and abbreviations are more frequently used.

The effect of dialysates and ultrafiltrates of plasma of saline-loaded dogs on toad bladder sodium transport

In order to obtain direct evidence for the existence of a natriuretic hormone, dialysates and ultrafiltrates of plasma of dogs expanded with saline were tested for effects on sodium transport by the toad urinary bladder. Dialysate was obtained by dialysis of blood in vivo in a clinical dialyzer and by dialysis in vitro of small volumes of blood using a miniature model of the clinical dialyzer. Ultrafiltrates were prepared using selective molecular filters which permit passage of substances on the basis of molecular weight and three dimensional configuration. Dialysates and ultrafiltrates of hydropenic dogs caused a change in toad bladder potential difference of + 1% and in short circuit current of - 5%. In contrast, dialysates and ultrafiltrates from expanded dogs caused a change in potential difference of - 23% and in short circuit current of - 32%, a highly significant difference. Onset of reduction of short circuit current occurred within 3-5 min, reaching a maximum in 10-20 min. The effect was rapidly reversible, was specific for the serosal surface of the bladder, and could not be explained on the basis of nonspecific alterations in ionic composition or by dilutional effects. Ultrafiltrates of jugular vein plasma caused significantly more reduction of short circuit current than ultrafiltrates of femoral vein plasma. The data indicate the presence in plasma of saline-loaded dogs of a dialyzable inhibitor of toad bladder sodium transport. Ultrafiltrate studies using membranes of appropriate selectivity suggest the factor has a molecular weight of less than 3000.