Induction of cytochrome P450 isozymes in rat renal microsomes by cyclosporin A

Cocaine-induced kidney toxicity: an in vitro study using primary cultured human proximal tubular epithelial cells

Renal failure resulting from cocaine abuse has been well documented, although the underlying mechanisms remain to be investigated. In the present study, primary cultured human proximal tubular epithelial cells (HPTECs) of the kidney were used to investigate its ability to metabolize cocaine, as well as the cytotoxicity induced by cocaine and its metabolites benzoylecgonine (BE), ecgonine methyl ester (EME) and norcocaine (NCOC). Gas chromatography/ion trap-mass spectrometry (GC/IT-MS) analysis of HPTECs exposed to cocaine (1 mM) for 72 h confirmed its metabolism into EME and NCOC, but not BE. EME levels increased along the exposure time to cocaine, while NCOC concentration diminished after reaching a maximum at 6 h, indicating a possible secondary metabolism for this metabolite. Cocaine promoted a concentration-dependent loss of cell viability, whereas BE and EME were found to be non-toxic to HPTECs at the tested conditions. In contrast, NCOC revealed to have higher intrinsic nephrotoxicity than the parent compound. Moreover, cocaine-induced cell death was partially reversed in the presence of ketoconazole (KTZ), a potent CYP3A inhibitor, supporting the hypothesis that NCOC may play a role in cocaine-induced nephrotoxicity. Cocaine-induced cytotoxicity was found to involve intracellular glutathione depletion at low concentrations and to induce mitochondrial damage at higher concentrations. Under the present experimental conditions, HPTECs death pathway followed an apoptotic pattern, which was evident for concentrations as low as 0.1 mM.

Cyclosporine A exposure during pregnancy in mice: effects on reproductive performance in mothers and offspring

BACKGROUND

Pregnancies after organ transplantation and under immunosuppressive treatment are associated with slightly elevated risks for obstetric and post-natal complications but can usually be managed well. However, little is known about the effects of intrauterine exposure (IUE) to immunosuppressants in the growing and adult offspring. One major issue is the potentially negative effects of immunosuppressive medication on reproduction. This study investigates the effect of exposure during pregnancy to the most commonly used immunosuppressant in organ transplantation, cyclosporine A (CsA), on the reproductive outcome in mothers and offspring.

METHODS

Female C57CBA-F1 mice received 0, 10, 20 or 30 mg/kg bodyweight of CsA daily by subcutaneous mini-osmotic pumps during mating and pregnancy. Blood concentrations of CsA, implantation rates, resorption rates and fetal weights were analysed. In addition, female and male mice exposed to CsA in utero were mated to unexposed partners and pregnancy outcomes were analysed.

RESULTS

Direct maternal exposure to CsA at high doses reduced implantation rates and fetal survival. IUE to CsA reduced adolescent growth but did not affect fertility, although a reduction in birthweight was seen in offspring of females exposed to CsA in utero.

CONCLUSIONS

CsA exposure during pregnancy correlates with impaired reproductive outcome, but offspring fertility is not affected. The cause of reduction in adolescent weight gain and low birthweight in offspring of females exposed to CsA in utero need further investigation.

Cyclosporin A-induced oxidative stress is not the consequence of an increase in mitochondrial membrane potential

Cyclosporin A induces closure of the mitochondrial permeability transition pore. We aimed to investigate whether this closure results in concomitant increases in mitochondrial membrane potential (DeltaPsim) and the production of reactive oxygen species. Fluorescent probes were used to assess DeltaPsim (JC-1, 5,5',6,6'-tetrachloro-1,1',3,3'-tetraethyl-benzimidazolyl-carbocyanine iodide), reactive oxygen species [DCF, 5- (and 6)-chloromethyl-2',7'-dichlorodihydrofluorescein diacetate, acetyl ester] and [Ca2+][Fluo-3, glycine N-[4-[6-[(acetyloxy)methoxy]-2,7-dichloro-3-oxo-3H-xanthen-9-yl]-2-[2-[2-[bis[2-[(acetyloxy)methoxy]-2-oxyethyl]amino]-5-methylphenoxy]ethoxy]phenyl]-N-[2-[(acetyloxy)methoxy]-2-oxyethyl]-(acetyloxy)methyl ester] in human kidney cells (HK-2 cells) and in a line of human small cell carcinoma cells (GLC4 cells), because these do not express cyclosporin A-sensitive P-glycoprotein. We used transfected GLC4 cells expressing P-glycoprotein as control for GLC4 cells. NIM811 (N-methyl-4-isoleucine-cyclosporin) and PSC833 (SDZ-PSC833) were applied as selective mitochondrial permeability transition pore and P-glycoprotein blockers, respectively. To study the effect of cyclosporin A on mitochondrial function, we isolated mitochondria from fresh pig livers. Cyclosporin A and PSC833 induced a more than two-fold increase in JC-1 fluorescence in HK-2 cells, whereas NIM811 had no effect. None of the three substances induced a significant increase in JC-1 fluorescence in GLC4 cells. Despite this, cyclosporin A, NIM811 and PSC833 induced a 1.5-fold increase in DCF fluorescence (P<0.05) and a two-fold increase in Fluo-3 fluorescence (P<0.05). Studies in isolated mitochondria showed that blockage of mitochondrial permeability transition pores by cyclosporin A affected neither DeltaPsim, ATP synthesis, nor respiration rate. The mitochondrial permeability transition pore blockers cyclosporin A and NIM811, but also the non-mitochondrial permeability transition pore blocker PSC833, induced comparable degrees of reactive oxygen species production and cytosolic [Ca2+]. Neither mitochondria, effects on P-glycoprotein nor inhibition of calcineurin therefore play a role in cyclosporin A-induced oxidative stress and disturbed Ca2+ homeostasis.

Roles of Lipid Mediators in Kidney Injury

Small lipids such as eicosanoids exert diverse and complex functions. In addition to their role in regulating normal kidney function, these lipids also play important roles in the pathogenesis of kidney diseases. Increased glomerular cyclooxygenase (COX)1 or COX2 expression has been reported in patients with nephritis and in animal models of nephritis. COX inhibitors have shown beneficial effects on lupus nephritis and passive Heymann nephritis, but not anti-Thy1.1-induced nephritis. 5-Lipoxygenase-derived leukotrienes are involved in inflammatory glomerular injury. Lipoxygenase product 12-hydroxyeicosatetraenoic acid may mediate angiotensin II and transforming growth factor beta-induced mesangial cell abnormality in diabetic nephropathy. P450 arachidonic acid mono-oxygenase-derived 20-hydroxyeicosatetraenoic acid and epoxyeicosatrienoic acids are involved in several forms of kidney injury, including renal injury in metabolic syndrome. Ceramide also has been shown to be an important signaling molecule that is involved in the pathogenesis of acute kidney injury caused by ischemia/reperfusion and toxic insults. Those pathways should provide fruitful targets for intervention in the pharmacologic treatment of renal disease.

Role of 20-hydroxyeicosatetraenoic acid (20-HETE) in vascular system

Cytochrome P450s (P450) metabolize arachidonic acid (AA) to hydroxyeicosatetraenoic acids (HETEs) and epoxyeicosatrienoic acids (EETs). Among these eicosanoids, 20-HETE is formed in a tissue and cell-specific fashion and plays an important role in the regulation of vascular tone in the brain, kidney, heart and splanchnic beds. 20-HETE is a potent vasoconstrictor produced in vascular smooth muscle (VSM) cells. It depolarizes VSM by blocking the open-state probability of Ca2+-activated K+-channels. Inhibitors of the formation of 20-HETE block the myogenic response of renal and cerebral arterioles in vitro and autoregulation of renal and cerebral blood flow in vivo. The formation of 20-HETE in vascular smooth muscle is stimulated by angiotensin II, endothelin and norepinephrine and is inhibited by nitric oxide (NO). 20-HETE also stimulates mitogenic and angiogenic responses in vitro and in vivo. Changes in the production of 20-HETE have been observed in ischemic cerebrovascular diseases, cardiac ischemia-reperfusion injury, kidney diseases, hypertension, diabetes, uremia, toxemia of pregnancy. The physiological and pathophysiological role of 20-HETE in the regulation of vascular tone are being revealed by the use of newly developed inhibitors of the synthesis of 20-HETE and 20-HETE analogs. The present review summarizes recent findings implicating a critical role for 20-HETE in altering cardiovascular function in a variety of pathological conditions.

Protective effect of 20-hydroxyeicosatetraenoic acid (20-HETE) on glomerular protein permeability barrier

BACKGROUND

Proteinuria is a significant problem in medicine today, although glomerular events underlying it are unknown. Products of cytochrome P450 (CYP450) pathway of arachidonic acid metabolism are increasingly recognized as playing major roles in renal function. We used in vitro albumin permeability (P(alb)) as a measure of injury and puromycin aminonucleoside (PAN) as an injurious agent to test the hypothesis that 20-hydroxyeicosatetraenoic acid (20-HETE) protects the glomerular filtration barrier from increased P(alb).

METHODS

We determined P(alb) in the following experimental groups: (1) isolated rat glomeruli incubated with PAN (5 microg/mL) for 5, 15, 30 or 60 minutes; (2) isolated glomeruli preincubated with 20-HETE (1.0 nmol/L to 100 nmol/L) for 15 minutes followed by additional incubation with PAN (5 microg/mL) for 15 minutes; (3) isolated glomeruli from rats treated with the CYP450 4A inducer clofibrate, and incubated with PAN (5 microg/mL) for 15 minutes; and (4) appropriate controls for each group. CYP450 4A levels were measured in glomeruli isolated from rats treated with clofibrate or vehicle.

RESULTS

PAN increased P(alb) of isolated glomeruli as early as 5 minutes (P(alb) 0.33 +/- 0.21, P < 0.05 vs. control). Maximal effect occurred at 30 minutes (P(alb) 0.75 +/- 0.16, P < 0.001 vs. control). Inclusion of 20-HETE (100 nmol/L) blocked the increased P(alb) caused by PAN (P(alb) 0.05 +/- 0.13). Likewise, glomeruli isolated from rats treated with clofibrate were protected from PAN-induced increase in P(alb) (P(alb) 0.19 +/- 0.03). Treatment with clofibrate significantly increased glomerular CYP450 4A expression.

CONCLUSION

PAN directly and immediately affects the glomerular permeability barrier. Furthermore, exogenous 20-HETE or clofibrate treatment protects glomeruli from increased P(alb) caused by PAN. Relative lack of 20-HETE may be a general characteristic of proteinuric states. Conversely, measures used to treat and/or prevent proteinuria may act to restore or increase glomerular 20-HETE levels.

Increased Excretion of Urinary 20-HETE in Rats With Cyclosporine-Induced Nephrotoxicity

The present study examined the contribution of 20-hydroxy-5,8,11,14-eicosatetraenoic acid (20-HETE) in cyclosporine A (CsA)-induced renal nephrotoxicity. Treatment of rats with CsA (50 mg/kg) for 9 days induced renal damage as indicated by marked increase in urine flow (from 9.0 +/- 0.3 ml/day to 46.6 +/- 7.1 ml/day) and a 3 - 5-fold rise in blood urea nitrogen (BUN) levels. The urinary excretion of 20-HETE increased from 164 +/- 5 ng/day (N = 5) to 2432 +/- 290 ng/day (N = 5, P<0.01) after 9 days of CsA treatment. The increase in the urinary excretion of 20-HETE in the CsA treated rats was highly correlated with the increase in BUN levels (r = 0.819, P<0.001) and urine volume (r = 0.832, P<0.001). Immunohistochemical examination of kidney revealed that expression of cytochrome P450 4A (CYP4A) protein was markedly enhanced in the proximal tubules of CsA-treated rats. These results indicate that CsA-induced nephrotoxicity in rats is associated with a marked elevation in the renal production of 20-HETE and that 20-HETE may contribute to the pathophysiological condition of CsA-induced nephrotoxicity.

Regulation of hepatobiliary excretion of sinomenine by P-glycoprotein in Sprague-Dawley rats

Sinomenine, an herbal ingredient isolated from Sinomenium acutum, is used for the amelioration of arthritis. Using microdialysis and a specially constructed hepato-duodenal shunt probe, the present study investigated the pharmacokinetics of sinomenine in rat blood and bile and the effects of P-glycoprotein modulation and cytochrome P450 inhibition. The results indicated that the pharmacokinetics of sinomenine in rat blood appeared to be dose dependent in the 3 to 30 mg/kg range. The disposition of sinomenine in the bile exhibited a slow elimination phase, reaching a peak concentration in 20-40 min following intravenous administration. The area under the concentration versus time curves (AUC's) for sinomenine in the bile were significantly greater than those in the blood at dosages of 3, 10, and 30 mg/kg with the blood-to-bile distribution ratios (k = AUC(bile) / AUC(blood)) being 3.85 +/- 0.29 and 3.52 +/- 0.28 at 10 and 30 mg/kg, respectively, indicating active hepatobiliary excretion. Coadministration with 20 mg/kg of cyclosporin A 10 min prior to sinomenine administration resulted in a significant reduction of the bile AUC's for the dosages of 10 and 30 mg/kg., resulting in the bile/blood distribution ratio being significantly reduced to 0.47 +/- 0.05 and 0.49 +/- 0.05, respectively. On the other hand, proadifen treatment increased both the blood and bile AUC's, resulting in insignificant effects on the blood-to-bile distribution ratios. In conclusion, our results indicated that sinomenine underwent active hepatobiliary elimination which may be regulated by the P-glycoprotein and that P-450 was likely involved in its metabolism.

Contribution of CYP4A8 to the Formation of 20-Hydroxyeicosatetraenoic Acid from Arachidonic Acid in Rat Kidney

20-Hydroxyeicosatetraenoic acid (20-HETE) has been shown to be an arachidonic acid metabolite of the cytochrome P450 (CYP) enzymes belonging to the CYP4A subfamily and is a predominant regulator of renal vascular tone and tubular ion reabsorption in rat kidney. CYP4A8 is one of the CYP4A enzymes expressed in rat kidney, but its contribution to 20-HETE formation has not been assessed. In order to clarify that the role of CYP4A8, we have developed bacterial expression systems for the expression of recombinant CYP4A8 (rCYP4A8). We also produced an antibody against rCYP4A8 which was used for immunoinhibition and immunohistochemical studies. In a reconstituted system, rCYP4A8 sufficiently catalyzed 20-HETE formation as well as prostaglandin A(1) omega-hydroxylation, a marker activity for CYP4A8. In addition, anti-rCYP4A8 sera significantly inhibited prostaglandin A(1) omega-hydroxylation and strongly inhibited arachidonic acid omega-hydroxylation in rat kidney microsomes. These observations suggested for the first time that CYP4A8 also contributed to 20-HETE formation in rat kidney. Furthermore, immunohistochemstry suggested that CYP4A8 is present in preglomerular arteries, where 20-HETE has been established to be a vasoconstrictor.

HET0016, a potent and selective inhibitor of 20-HETE synthesizing enzyme

The present study examined the inhibitory effects of N-hydroxy-N'-(4-butyl-2-methylphenyl)-formamidine (HET0016) on the renal metabolism of arachidonic acid by cytochrome P450 (CYP) enzymes. HET0016 exhibited a high degree of selectivity in inhibiting the formation of 20-hydroxy-5,8,11,14-eicosatetraenoic acid (20-HETE) in rat renal microsomes. The IC(50) value averaged 35+/-4 nM, whereas the IC(50) value for inhibition of the formation of epoxyeicosatrienoic acids by HET0016 averaged 2800+/-300 nM. In human renal microsomes, HET0016 potently inhibited the formation of 20-HETE with an IC(50) value of 8.9+/-2.7 nM. Higher concentrations of HET0016 also inhibited the CYP2C9, CYP2D6 and CYP3A4-catalysed substrates oxidation with IC(50) values of 3300, 83,900 and 71,000 nM. The IC(50) value for HET0016 on cyclo-oxygenase activity was 2300 nM. These results indicate that HET0016 is a potent and selective inhibitor of CYP enzymes responsible for the formation of 20-HETE in man and rat.

The effect of immunosuppressive agents (FK-506, rapamycin) on renal P450 systems in rat models

It is well known that cyclosporin, rapamycin and FK-506 (tacrolimus) are metabolized by the liver microsomal cytochrome P450 enzyme system. Although there have been reports of interaction between these drugs and the renal P450 enzyme system, differences among these immunosuppressants has not been comprehensively demonstrated. We have studied the individual capacities of these immunosuppressants to induce renal microsomal P450 enzymes similar to CYP2B4 and CYP4A2 by examining renal function in treated rats, and have correlated the results by means of biochemical, immunological and immunohistochemical assays of renal P450 enzymes. Cyclosporin caused impairment of renal function with an increase in renal-specific P450 content, but FK-506 and rapamycin did not. Laurate omega- and (omega-1)-hydroxylase activity increased in rats treated with rapamycin but decreased in those treated with FK-506. Prostaglandin A1 (PGA1) omega-hydroxylase activity increased in rats treated with FK-506 but was reduced by treatment with cyclosporin. Aminopyrine N-demethylase activity increased in rats treated with cyclosporin or FK-506, but not in those treated with rapamycin. Western-blot analysis revealed significant induction of P450, (similar to CYP2B4 of the rabbit P450 isozyme) in kidneys from rats treated with cyclosporin but not in those from rats receiving FK-506 or rapamycin. Histochemical studies clearly demonstrated a form of P450 such as CYP4A2 in the proximal tubules of rats treated with cyclosporin, but not in those of rats treated with FK-506 or rapamycin. These results show that although cyclosporin has a strong effect on renal P450 systems and induces such a system in kidney cortex (microsomal P450), FK-506 and rapamycin have no substantial effect on the induction of renal P450. These findings might clarify the nephrotoxicity induced by these immunosuppressive drugs.

Peroxisome proliferator-activated receptor alpha controls the hepatic CYP4A induction adaptive response to starvation and diabetes

The hepatic CYP4A enzymes are important fatty acid and prostaglandin omega-hydroxylases that are highly inducible by fibric acid hypolipidemic agents and other peroxisome proliferators. Induction of the CYP4A enzymes by peroxisome proliferators is mediated through the nuclear peroxisome proliferator-activated receptor alpha (PPARalpha). Fatty acids have recently been identified as endogenous ligands of PPARalpha, and this receptor has been implicated in the regulation of lipid homeostasis. In the present report we characterized the induction of the hepatic CYP4A genes in rats during the altered lipid metabolism associated with starvation and diabetes. The mRNA levels of CYP4A1, CYP4A2, and CYP4A3 were induced 7-17-fold in the livers of fasted animals and 3-8-fold in the livers of diabetic animals. This was accompanied by corresponding changes in CYP4A protein levels and arachidonic and lauric acid omega-hydroxylase activity. Interestingly, feeding animals after the fasting period caused as much as an 80% suppression of CYP4A mRNA levels, whereas CYP4A protein levels and functional activity returned to control values. A second PPARalpha-responsive gene, acyl-CoA oxidase, was also induced in rat liver by diabetes and fasting. By using PPARalpha-deficient mice, we unambiguously demonstrated that PPARalpha is strictly required for hepatic CYP4A induction by starvation and diabetes. Similarly, induction of hepatic thiolase and bifunctional enzyme also required expression of PPARalpha. This represents the first evidence for the pathophysiologically induced activation of a nuclear receptor.

Alteration of cytochrome P-450 isozymes by captopril and idrapril in hepatic and renal microsomes of normotensive and spontaneously hypertensive rats

To examine the effects of angiotensin-converting enzyme (ACE)inhibitors such as captopril and idrapril on the P-450 system, these compounds were administered 100 mg/kg i.p. for 4 days to spontaneously hypertensive (SHRs) and normotensive Wistar-Kyoto (WKY) and Sprague-Dawley (SD) rats; thereafter, the principal hepatic and renal microsomal monooxygenase activities were determined. In all the rat strains used, both captopril and idrapril decreased only the P-450 2C11, (as determined by immunoblotting) and its linked activities such as 16alpha-, 2alpha- and 17-testosterone hydroxylases. These changes were accompanied by a significant decrease of blood testosterone levels both in normotensive and, more markedly, in hypertensive rats and by a reduction of systolic blood pressure, but only in SHRs. Only in SHRs as well, the renal immunodetectable P-450 4A content and the P-450 4A-dependent activities, such as the (omega)-lauric acid hydroxylase, diminished after captopril or idrapril treatment. These data suggest that the decrease of increased blood pressure in hypertensive SHRs by the ACE inhibitors may be linked to the downregulation of the circulating testosterone level, the renal P-450 4A expression, and the related formation of the potent vasoconstrictor (omega)-hydroxy arachidonic acid.

The site of general anaesthesia and cytochrome P450 oxygenases: similarities defined by straight chain and cyclic alcohols

1. General anaesthetics disrupt normal cell receptivity and responsiveness while sparing vital respiratory processes. Ultimate elucidation of the molecular basis of general anaesthesia presumes the identification of one or more subcellular components with appropriate sensitivity to the entire array of anaesthetics. 2. Previously, we showed the universal cellular enzymes, cytochrome P450 mono-oxygenases, to be sensitive at relevant concentrations to all anaesthetics tested. The potential significance of P450 inhibition by anaesthetics resides in the contribution of this enzyme family, in conjunction with that of cyclo-oxygenases and lipoxygenases, to the generation from arachidonic acid of lipid second messengers, the eicosanoids. 3. We have shown that P450 enzymes model the site of general anaesthesia in the tadpole with respect to (a) an absolute sensitivity to increasing chain-length series of flexible, straight chain primary and secondary alcohols and straight chain diols, (b) an absolute sensitivity to increasing molecular weight series of rigid cyclic alkanols and cyclic alkanemethanols, (c) the points of abrupt change and of reversal (cut-off) in the linear relationship between increasing anaesthetic potency with increasing carbon chain length, and (d) non-differentiation between secondary alkanol enantiomers. These findings reveal the P450 enzyme family as the most relevant biomolecular counterpart of the site of general anaesthesia, thus far identified.

Comparison of effects of xenobiotics on extrahepatic and hepatic microsomal drug-metabolizing enzymes in mice

The content of microsomal protein is the same in both kidneys and small intestine, corresponding to 57% of the control value expressed as 100% in the untreated liver. The contents of P450 and cytochrome b(5), and the activity of NADPH-cytochrome c reductase in the kidney were higher than those in the small intestine, which were 17%, 22% and 41% of controls, respectively, in the former and 5%, 11% and 22% of controls in the latter. As compared with similar measurements made in the liver, the activities of substrate-metabolizing enzymes in these extrahepatic organs were very low. The activities of renal aniline hydroxylase, aminopyrine N-demethylase, 7-ethoxycoumarin O-deethylase, 7-methoxycoumarin O-demethylase and benzo(a)pyrene hydroxylase were 6%, 5%, 3%, 0.6% and 0.2% of controls, respectively. The activities of these enzymes in the small intestine were lower than those in the kidney or below the limits of detection. These results suggested that isoforms or their contents of P450 responsible for these substrate biotransformations are different among liver, kidneys and small intestine. Meantime, this study showed similar significant inductions by phenobarbital and rifampin of small intestinal and hepatic microsomal drug-metabolizing enzymes. In contrast, neither phenobarbital nor rifampin was capable of increasing renal microsomal enzymes, with the exception of benzo(a)pyrene hydroxylase which was induced by rifampin. These findings indicated that both liver and small intestine, but not kidneys contain the same phenobarbital- and rifampin-inducible P450 isoforms, cytochrome b(5) and NADPH-cytochrome c reductase. In addition, CCl(4) could be bioactivated by CYP2E1 to free radicals in the kidney which caused destruction of microsomal enzymes. In mice pretreated with phenobarbital, CCl(4) also attenuated the increase in content of P450 in the small intestine, which appeared to be a result of induction by phenobarbital of CYP2E1.

Hydroxyethyl cyclosporin A induces and decreases P4503A and P-glycoprotein levels in rat liver

1. A new immunosuppressant SDZ IMM 125 (IMM), the hydroxyethyl derivative of D-serine8-cyclosporin (cyclosporin A, CSA), induced or decreased the liver P450s of rat, in particular the 3A proteins, depending on the dose and duration of exposure. Doses of 20 mg/kg/day for 2 weeks and 10 mg/kg/day for 26 weeks induced the rat liver 3A levels 2- and 1.8-fold respectively, whereas 52 weeks of 24 mg/kg/day decreased the 3A levels by 22%. High doses of IMM, 100 mg/kg/day for 26 weeks, significantly decreased the 3A levels by 56%. 2. Changes in the rate of IMM biotransformation paralleled the changes in the levels of liver 3A indicating that liver 3A levels could influence the clearance of IMM. 3. Both IMM and CSA affected liver and kidney P-glycoprotein (Pgp) levels. The increases measured after short-term treatment (20 mg/kg/day for 2 weeks) in the liver (1.8-fold) and kidney (1.3-fold) were less pronounced in the long-term studies in which liver Pgp levels were increased 1.4-fold (48 mg/kg/day for 52 weeks). At higher doses (100 mg/kg/day for 26 weeks) Pgp levels were significantly decreased. The modulation of Pgp levels by IMM did not parallel the changes in 3A levels, indicating that Pgp regulation is most likely due to a direct effect of the cyclosporin rather than a co-regulation mechanism linked to 3A or P4501A modulation. 4. Increased arachidonic metabolism to the 19- and 20-HETE metabolites, a possible mechanism of the cyclosporin-induced renal hypertension, occurred in the liver microsomes and not the kidney S9 fraction of the 2-week study, and only at very high doses (100 mg/kg/day) in the longer studies (26 weeks).