Renal effect of prostaglandin synthetase inhibition in rats: micropuncture studies

Effect of inhibitors of prostaglandin synthesis on the furosemide action in the loop Henle of rat kidney

Inhibitors of prostaglandin synthesis such as indomethacin and meclofenamate may attenuate the diuretic response to furosemide. The purpose of the present study was to clarify whether an inhibition of furosemide's action in the loop of Henle is involved in this interaction. In a first set of experiments, the effect of indomethacin and meclofenamate on the diuretic response to furosemide was re-evaluated in anaesthetized rats. Single loops of Henle of rat kidneys were perfused in vivo in a second group of rats. Furosemide was added to the perfusion fluid and the effect of indomethacin (5 mg/kg i.v.) and meclofenamate (5 mg/kg i.v.) on furosemide's action in the loops was tested. In the absence of furosemide, indomethacin and meclofenamate did not significantly affect urine flow and sodium chloride excretion, or the loop's sodium and chloride reabsorption. Both prostaglandin inhibitors, however, significantly attenuated the diuretic effect of furosemide (1 mg/kg i.v.) and the inhibitory action of the diuretic, at a concentration of 10(-4) M, on the loop's sodium and chloride transport. The results identify the loop of Henle as a tubular site of interaction between furosemide and prostaglandin inhibitors. The interaction may be related to a furosemide-induced stimulation of renal prostaglandins.

Additive renal effects of indomethacin and dipyridamole in man

The effect of dipyridamole alone and in combination with indomethacin was studied in 11 water loaded rheumatic patients with normal kidney function. The adenosine uptake inhibitor dipyridamole and the prostaglandin synthesis inhibitor indomethacin decreased diuresis by 25-50% when given alone and by approximately 80% when combined. These effects were not associated with changes in renal PAH-clearance, but may be partially related to a fall in GFR. Sodium excretion was decreased in parallel with the fall in GFR. In addition, both indomethacin and dipyridamole reduced the free water clearance. Theophylline at therapeutic plasma levels reversed the reduction in GFR and transiently counteracted the inhibition of tubular water and sodium excretion induced by combined dipyridamole and indomethacin treatment. The results suggest that endogenous adenosine and prostaglandins are both involved in the control of renal function in man, and that drugs that affect these two autocoids may interact to cause adverse effects in man.

Prostaglandins inhibit renal ammoniagenesis in the rat

We describe the inhibitory effect of prostaglandins (PGs) on in vivo rat renal ammonia synthesis. The influence of systemic pH upon urinary PG excretion and ammoniagenesis was also investigated. Finally, PG production by incubated rat renal cortical slices was suppressed to investigate the PG-ammonia interplay in the absence of changes in renal blood flow, glomerular filtration rate, ambient electrolyte concentrations or extrarenal hormonal factors. In vivo ammonia synthesis doubled and PG excretion fell by 44% in normal rats, after intravenous administration of 1 mg/kg of meclofenamate. Higher doses of meclofenamate further augmented ammonia production and further reduced PG excretion. PG depletion was also associated with an increase in fractional excretion of ammonia (FENH3) that was independent of changes in urine flow rate or pH. Acute metabolic acidosis (AMA) increased total ammonia synthesis but also stimulated PG production. Administration of meclofenamate to rats with mild AMA markedly reduced urinary PG excretion, further augmented ammonia synthesis, and significantly increased the FENH3. Inhibition of stimulated PG synthesis during severe AMA did not increase ammoniagenesis or FENH3. Acute metabolic alkalosis did not alter production of PGs or ammonia, but reduced the FENH3 by 42%. Meclofenamate nearly normalized the FENH3 but stimulated synthesis to a lesser degree than was seen in nonalkalotic rats that received meclofenamate. Inhibition of PG synthesis in incubated rat renal cortical slices also stimulated ammoniagenesis. Conversely, stimulation of PG synthesis decreased ammonia production and acidification of the incubation medium increased prostaglandin F2 alpha production. Thus, in vitro findings support the in vivo results. We conclude that PGs inhibit ammonia synthesis in normal rats and in those undergoing mild AMA. Severe acidosis overrides this inhibitory effect of PGs, whereas metabolic alkalosis suppresses the stimulatory effect of PG synthesis inhibition.

Effects of cyclooxygenase inhibitors on plasma and urinary kallikrein

In vitro studies were performed to investigate the direct effects of the cyclooxygenase inhibitors (COI), meclofenamate, imidazol, acetylsalicylic acid (ASA), indomethacin, and acetaminophen on the plasma kallikrein system and on urinary kallikrein excretion. Biological (guinea pig ileum) and colorimetric (synthetic substrate) methods were used. Results showed that all COIs except ASA affected both the activation of pre-kallikrein in plasma and the direct activity of plasma kallikrein, but none of the COIs tested was able to alter the urinary kallikrein excretion. Additionally, in Wistar rats we studied the effects of chronic administration of ASA, meclofenamate, and indomethacin on the plasma kallikrein system and urinary kallikrein excretion. In contrast to the in vitro studies, administration of these COIs reduced the amount of total 24-hour urinary kallikrein excretion. Moreover, the pre-plasma kallikrein and total plasma kallikrein levels were diminished in all experimental groups. However, only ASA and meclofenamate increased the free-plasma kallikrein levels despite the fact the three COIs used reduced the high molecular weight kininogen. Thus, in vitro data suggest an important and direct effect of meclofenamate, imidazol, indomethacin, and acetaminophen on the plasma kallikrein system, without any effect on urinary secretion. ASA was the only COI that showed no direct effect in these experiments. Chronic COI administration in Wistar rats suggests that ASA, meclofenamate, and indomethacin affect both the plasma kallikrein-kinin system and kallikrein excretion. When these drugs are used to evaluate the interactions between prostaglandins and the kallikrein-kinin system, their possible effects through both mechanisms, directly or via prostaglandin inhibition, should be considered.

Prostaglandin synthesis and renal function in man

1. Experiments were performed to determine the changes in renal function which occur following prostaglandin synthetase inhibition in healthy conscious humans. It was hoped that such experiments could provide information on the mechanism by which renal prostaglandin synthesis influences urinary excretion. 2. In water-diuretic male subjects (receiving a slow saline infusion) the renal excretion of sodium and water was reduced following I.V. acetylsalicylic acid (1 g) administration, while the effective renal plasma flow (p-aminohippurate clearance), and glomerular filtration rate (inulin clearance) remained unaltered. 3. In normally hydrated female subjects on an unrestricted diet, the mean urinary prostaglandin E output was 8.5 ng/hr. The renal excretion of sodium, water and urinary prostaglandin E were significantly reduced (P less than 0.05) following oral acetylsalicylic acid (1.2 g) administration. 4. In normally hydrated female subjects on an unrestricted diet the renal excretion of sodium and water was reduced following oral paracetamol (1.5 g) administration. 5. It is concluded that following renal prostaglandin synthetase inhibition in conscious humans, the excretion of sodium and water can be reduced without measurable changes in the glomerular filtration rate or effective renal plasma flow. It is suggested that in conscious healthy humans, the kidney may continually synthesize prostaglandin which might help to maintain sodium and water excretion by a direct action on the renal tubule without influencing renal blood flow. The relevance of this hypothesis to the intrarenal location of prostaglandin synthetase is discussed.

Renal function in conscious rats after indomethacin. Evidence for a tubular action of endogenous prostaglandins

1. Conscious rats, with implanted carotid arterial cannulae, received a saline infusion (5.8 ml./hr) via a tail vein for a 6 hr period. The urinary excretion of water, sodium, potassium, urea and the osmolal output were monitored, together with the systemic blood pressure. Glomerular filtration rate (inulin clearance) and effective renal plasma flow (p-aminohippurate clearance) were also measured. Four hours after the start of the infusion, indomethacin (10 mg/kg body weight) in buffered saline, or buffered saline alone, was administered via the tail vein. 2. Following indomethacin administration, urine flow, sodium output and osmolal output were markedly reduced (P less than 0.01). However, there were no measurable changes in the systemic blood pressure, glomerular filtration rate, or effective renal plasma flow. 3. It is concluded that the changes in urinary excretion observed after indomethacin are not dependent on changes in effective renal plasma flow or glomerular filtration, and it is suggested that indomethacin inhibits the synthesis of endogenous prostaglandins which directly influence renal tubular function.