The effect of suppression of prostaglandin synthesis on renal function in rats with intact and reduced renal mass

Regulation of ROMK and channel-inducing factor (CHIF) in acute renal failure due to ischemic reperfusion injury

BACKGROUND

Acute renal failure caused by ischemia followed by reperfusion is often associated with severe hyperkalemia. The present study was undertaken to characterize the effects of renal ischemia and reperfusion on plasma potassium (K) and on the gene expression of channel-inducing factor (CHIF), a putative K channel regulator, and of ROMK, the distal nephron secretory K channel.

METHODS

The following groups of rats were studied: (1) sham operated (sham); (2) after one hour of ischemia by bilateral renal artery clamping (I), and after one hour of ischemia; (3) one hour of reperfusion (I-R 1 h); (4) 24 hours of reperfusion (I-R 24 h); (5) 48 hours of reperfusion (I-R 48 h); and (6) 72 hours reperfusion (I-R 72 h). The expression of CHIF and ROMK was examined by Northern blot hybridization in renal cortex, medulla, and papilla and in the colon. The abundance of ROMK protein was determined in the renal cortex and medulla by immunoblotting.

RESULTS

Maximal plasma creatinine and potassium levels after ischemia and reperfusion were 470 +/- 16 micromol/L, P < 0.0001 versus sham, and 9.65 +/- 0.33 mmol/L, P < 0.0001 versus sham, respectively. The expression of CHIF was significantly down-regulated in the medulla and papilla, with a maximal decrease of 80% at 48 to 72 hours. In contrast, a most significant increase in CHIF mRNA expression (250% of baseline) was noted in the colon after 24 to 48 hours of reperfusion. ROMK expression was reduced in the cortex and was completely abolished in the medulla at 48 to 72 hours of reperfusion. Ischemia and reperfusion injury significantly decreased ROMK protein abundance to 10% of control in the medullary fractions.

CONCLUSIONS

These results suggest that down-regulation of renal CHIF and ROMK may contribute at least partly to the hyperkalemia of acute renal failure after ischemia and reperfusion, while CHIF up-regulation in the colon may act as a compensatory mechanism of maintaining K balance via increased K secretion.

Methylprednisolone dosage effects on peripheral lymphocyte subpopulations and eicosanoid synthesis

Glucocorticoids have a major role in the treatment of glomerular diseases. Despite recent advances in understanding of their mechanism of action, very few studies have addressed the relative advantage of the wide range of different dose regimens employed in clinical practice. We studied the effects of methylprednisolone given intravenously for three consecutive days at the doses of 1 mg/kg (group 1, N = 7; group 2, N = 5), 5 mg/kg (group 3, N = 5) or 15 mg/kg (group 4, N = 6) on total blood peripheral leukocytes and on lymphocyte subsets in patients with glomerular diseases, and investigated whether such effects were a function of the drug concentration in the blood. Since glucocorticoids have an inhibitory effect on the formation of eicosanoids in different cells, we also investigated in the same patients the effect of 1 and 15 mg/kg methylprednisolone on systemic and renal eicosanoid synthesis. Results of pharmacokinetic study showed that the three different doses of methylprednisolone we used resulted in major differences in patient's exposure to the drug, and within the same dose there was a great individual variability. By contrast the three different doses of methylprednisolone induced a comparable drop in the absolute number of lymphocytes six hours after the first injection of methylprednisolone, while 24 hours later blood lymphocyte counts returned to the pre-injection values in all patients. Analysis of lymphocyte subsets showed a selective decrease in the number of circulating CD4+ and CD8+ cells six hours after methylprednisolone which was comparable in the four groups of patients studied. As for the effect of methylprednisolone on systemic and renal eicosanoid synthesis in patients with glomerular diseases, 1 and 15 mg/kg were equally unable to reduce thromboxane A2 (TxA2) and prostaglandin E2 (PGE2) release by circulating polimorphonuclear cells (PMNs). By contrast, methylprednisolone partially inhibited eicosanoid synthesis by PMNs in vitro. Consistent with the data on PMNs, urinary excretion of TxA2 and prostacyclin (PGI2) metabolites were unaltered by the different doses of methylprednisolone. By contrast urinary PGE2 was markedly and significantly reduced in patients given 15 but not 1 mg/kg. We conclude that 1 mg/kg methylprednisolone given to patients with glomerular diseases has the same effect on peripheral total blood leukocyte count and lymphocyte subsets than 5 and 15 mg/kg. The same is true for eicosanoid synthesis by PMNs. Renal synthesis of PGE2 is inhibited by 15 mg/kg but not by 1 mg/kg.(ABSTRACT TRUNCATED AT 400 WORDS)

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.

Effect of indomethacin in vivo and PGE2 in vitro on MTAL Na-K-ATPase of the rat kidney

We have previously shown that prostaglandin synthesis inhibition in rats which reduces urinary excretion of PGE2 and sodium, is associated with increased Na-K-ATPase activity in renal medulla. To further characterize this interaction studies were performed in isolated segments of medullary thick ascending limb of Henle's loop (MTAL) in rats. The effect of pretreatment with indomethacin in vivo and incubation with PGE2 in vitro on MTAL Na-K-ATPase activity was studied. Pretreatment of rats with indomethacin increased Na-K-ATPase of the MTAL from 37.2 +/- 2.0 x 10(-11) mol/mm/min in controls to 62.7 +/- 2.2 (p less than 0.001) while Mg-ATPase was only slightly decreased. Incubation of MTAL Na-K-ATPase from indomethacin pretreated rats with increasing concentration of PGE2 in vitro dose dependently inhibited MTAL Na-K-ATPase activity with no effect on Mg-ATPase. Baseline Na-K-ATPase was 62.7 +/- 2.2 in MTAL from indomethacin pretreated rats and decreased to 36.9 +/- 1.4 (p less than 0.001) with 1 microM of PGE2, to 26.5 +/- 2.3 (p less than 0.001) with 10 microM PGE2 and to 22.0 +/- 1.0 (p less than 0.001) with 100 microM PGE2. 100 microM PGE2 in the incubation medium inhibited MTAL Na-K-ATPase of intact rats from 37.2 +/- 2 to 21.3 +/- 1.2 (p less than 0.001) and completely abolished the indomethacin induced increase in MTAL Na-K-ATPase. The results of this study show stimulation of MTAL Na-K-ATPase by pretreatment with indomethacin in vivo and, direct inhibition of MTAL Na-K-ATPase by PGE2 in vitro.(ABSTRACT TRUNCATED AT 250 WORDS)

Acute renal failure with selective medullary injury in the rat

Since human acute renal failure (ARF) is frequently the result of multiple rather than single insults, we used a combination of treatments to induce ARF in rats. Uninephrectomized, salt-depleted rats injected with indomethacin developed ARF after administration of radiocontrast. After 24 h, the plasma creatine rose from 103 +/- 3 to 211 +/- 22 mumol/liter (mean +/- SE) and the creatinine clearance dropped from 0.7 +/- 0.1 to 0.2 +/- 0.04 ml/min (P less than 0.001). Severe injury was confined to the outer medulla and comprised necrosis of medullary thick ascending limbs (mTALs), tubular collapse, and casts. Other nephron segments were free of damage except for the proximal convoluted tubules which showed vacuole formation originating from lateral limiting membranes that resembled changes reported in human contrast nephropathy. Cell damage to mTALs included mitochondrial swelling, nuclear pyknosis, and cytoplasmic disruption with superimposed calcification; these changes were most severe in the deepest areas of the outer medulla, away from vasa recta in zones remote from oxygen supply. The fraction of mTALs with severe damage was 30 +/- 7% (range 2-68) and the extent of injury was correlated with a rise in plasma creatinine (r = 0.8, P less than 0.001). Thus, the nature of mTAL injury was similar to the selective lesions observed in isolated kidneys perfused with cell-free medium and was shown to derive from an imbalance between high oxygen demand by actively transporting mTALs and the meager oxygen supply to the renal medulla. Combined multiple renal insults in the rat produce ARF that resembles the clinical syndrome of contrast nephropathy and is characterized by selective mTAL injury conditioned by medullary hypoxia.

Renal effects of nonsteroidal anti-inflammatory drugs in chronic glomerular disease

In contrast to a variety of clinical conditions characterized by ineffective circulatory volume, chronic glomerular disease is not usually associated with increased circulating levels of vasoconstrictor hormones. However, a reduction in glomerular prostacyclin production can possibly account for the prostaglandin dependence of renal function in these patients by virtue of the enhanced constrictor effects of angiotensin II on glomerular arterioles and mesangium. The reduction of renal function induced by a nonselective cyclo-oxygenase inhibitor is inversely related to the basal prostacyclin production. Increased renal synthesis of vasoconstrictor thromboxane A2, as noted in patients with systemic lupus erythematosus, might contribute to the cyclo-oxygenase dependence of renal function. Aspirin as well as a variety of structurally unrelated nonsteroidal anti-inflammatory drugs reduce renal function in systemic lupus erythematosus patients. However, the functional consequences of renal cyclo-oxygenase inhibition are partially attenuated in patients with lupus nephritis vis-à-vis other forms of chronic glomerular disease because of concomitant suppression of enhanced glomerular thromboxane A2 production. Animal data consistent with a prevailing functional significance of vasodilator prostaglandins have also been reported. Short-term administration of sulindac at the recommended dosage is relatively safe in patients with chronic glomerular disease because of selective sparing of glomerular cyclo-oxygenase activity. The long-term consequences of selective versus nonselective cyclo-oxygenase inhibition remain to be established in humans. The beneficial effects of a combination of aspirin and dipyridamole in slowing the deterioration of renal function in patients with membranoproliferative glomerulonephritis does provide a rationale for exploring the effects of low-dose aspirin (i.e., 0.5 to 1.0 mg/kg per day), which most effectively suppresses platelet thromboxane A2 production without interfering with renal prostacyclin synthesis.