Glomerular filtration rate and salt and water reabsorption during pregnancy in the conscious rat

Progression of renal damage and tubular regeneration in pregnant and non-pregnant adult female rats inoculated with a sublethal dose of Shiga toxin 2

BACKGROUND

Shiga toxin-producing Escherichia coli is the main cause of post-diarrheal hemolytic uremic syndrome (HUS) which produces acute kidney injury mainly in children, although it can also affect adults. The kidneys are the organs most affected by Shiga toxin type 2 (Stx2) in patients with HUS. However, previous studies in pregnant rats showed that a sublethal dose of Stx2 causes severe damage in the uteroplacental unit and induces abortion, whereas produces mild to moderate renal damage. The aim of the present work was to study the progression of renal injury caused by a sublethal dose of Stx2, as well as renal recovery, in pregnant and non-pregnant rats, and to investigate whether pregnancy physiology may affect renal damage progression mediated by Stx2.

METHODS

Renal function and histopathology was evaluated in pregnant rats intraperitoneally injected with a sublethal dose of Stx2 (0.5 ng/g bwt) at the early stage of gestation (day 8 of gestation), and results in these rats were compared over time with those observed in non-pregnant female rats injected with the same Stx2 dose. Hence, progression of cell proliferation and dedifferentiation in renal tubular epithelia was also investigated.

RESULTS

The sublethal dose of Stx2 induced abortion in pregnant rats as well as a significant more extended functional and histological renal injury in non-pregnant rats than in pregnant rats. Stx2 also caused decreased ability to concentrate urine in non-pregnant rats compared to their controls. However, renal water handling in pregnant rats was not altered by Stx2, and was significantly different than in non-pregnant rats. The greatest renal injury in both pregnant and non-pregnant rats was observed at 4 days post-Stx2 injection, and coincided with a significant increase in tubular epithelial proliferation. Expression of mesenchymal marker vimentin in tubular epithelia was consistent with the level of tubular damage, being higher in non-pregnant rats than in pregnant rats. Recovery from Stx2-induced kidney injury was faster in pregnant rats than in non-pregnant rats.

CONCLUSIONS

Adaptive mechanisms developed during pregnancy such as changes in water handle and renal hemodynamic may contribute to lessen the Stx2-induced renal injury, perhaps at the expense of fetal loss.

Prediction of methotrexate CNS distribution in different species - influence of disease conditions

Children and adults with malignant diseases have a high risk of prevalence of the tumor in the central nervous system (CNS). As prophylaxis treatment methotrexate is often given. In order to monitor methotrexate exposure in the CNS, cerebrospinal fluid (CSF) concentrations are often measured. However, the question is in how far we can rely on CSF concentrations of methotrexate as appropriate surrogate for brain target site concentrations, especially under disease conditions. In this study, we have investigated the spatial distribution of unbound methotrexate in healthy rat brain by parallel microdialysis, with or without inhibition of Mrp/Oat/Oatp-mediated active transport processes by a co-administration of probenecid. Specifically, we have focused on the relationship between brain extracellular fluid (brainECF) and CSF concentrations. The data were used to develop a systems-based pharmacokinetic (SBPK) brain distribution model for methotrexate. This model was subsequently applied on literature data on methotrexate brain distribution in other healthy and diseased rats (brainECF), healthy dogs (CSF) and diseased children (CSF) and adults (brainECF and CSF). Important differences between brainECF and CSF kinetics were found, but we have found that inhibition of Mrp/Oat/Oatp-mediated active transport processes does not significantly influence the relationship between brainECF and CSF fluid methotrexate concentrations. It is concluded that in parallel obtained data on unbound brainECF, CSF and plasma concentrations, under dynamic conditions, combined with advanced mathematical modeling is a most valid approach to develop SBPK models that allow for revealing the mechanisms underlying the relationship between brainECF and CSF concentrations in health and disease.

Evaluation of placental and lactational pharmacokinetics of PFOA and PFOS in the pregnant, lactating, fetal and neonatal rat using a physiologically based pharmacokinetic model

Perfluoroalkyl carboxylates and sulfonates (PFAAs) have many consumer and industrial applications. Developmental toxicity studies in animals have raised concern about potential developmental effects of PFOA and PFOS in humans. We have developed PBPK models for PFAAs in the rat to help define a relationship between external dose, internal tissue concentrations, and observed adverse effects, and to understand how physiological changes that occur during gestation and lactation affect tissue distribution of PFAAs in the mother, fetus, and neonate. The models developed here expand upon a PBPK model for PFAAs in the adult female rat, and are consistent with available PK data. These models, along with the adult rat PFAA models, published in the companion paper, will help address concerns about possible health effects due to PFAA exposure in the fetus and neonate and will be useful in comparing PK across life stages.

Maternal plasma hypertonicity is accentuated in the postterm rat

OBJECTIVE

In humans and rats, pregnancy-associated maternal plasma volume expansion and plasma hypotonicity may facilitate maternal-to-fetal water transfer. Although reduced amniotic fluid volume occurs commonly in postterm pregnancy, the mechanisms are unknown. We previously demonstrated a reversal of pregnancy-induced maternal plasma hypotonicity that occurs in the near term (20 days) pregnant rats. We sought to determine whether the relative maternal plasma hypertonicity continues in the postterm period.

STUDY DESIGN

Rat gestation (normal, 21 days) was prolonged with subcutaneous progesterone injection. Pregnant rats at gestation, 18 days, 21 days, and 24 days and nonpregnant rats were studied. Maternal and fetal hematocrit levels, plasma osmolality, electrolyte levels, and amniotic fluid volume were determined. In addition, maternal and fetal tissues were analyzed for water and electrolyte content.

RESULTS

Compared with term (21days), postterm pregnant rats (24 days) had a significant increase in maternal and fetal plasma osmolality (293.7+/-1.4 mOsm/kg vs 302.8+/-3.7 mOsm/kg and 301.0+/-2.0 mOsm/kg vs 310.3+/-3.2 mOsm/kg, respectively; P<.01) and sodium and chloride concentrations. Conversely, both maternal and fetal hematocrit levels decreased significantly in the postterm period. Postterm rats demonstrated an increased fetal mortality rate (24%) and a significantly reduced amniotic fluid volume (4.2+/-0.6 mL vs 6.6+/-0.6 mL, P<.01).

CONCLUSION

These results indicate that the near-term reversal of maternal plasma hypotonicity that has been observed previously is further accentuated in the postterm pregnancy. This continued hypertonicity may induce a fetal-to-maternal water flow and contribute to postterm oligohydramnios and increased fetal morbidity and mortality rates.

Effects of maternal uninephrectomy on the development of fetal rat kidney: numerical and volumetric changes of the glomerulus and formation of the anionic site in the glomerular basement membrane

Developmental processes of the fetal rat kidney from uninephrectomized mothers were studied. The uninephrectomy was performed on day 5 of gestation. Glomerular number and volume in the fetal kidneys on days 18, 20, and 22 of gestation were morphometrically determined. To investigate the anionic site formation in the glomerular basement membrane, distribution of cationized ferritin (CF) in the fetal glomerulus was examined electron microscopically after CF injection. Blood urea nitrogen (BUN) concentration in the pregnant rats was also determined on various days after uninephrectomy. On fetal days 20 and 22, the glomerular volume was significantly larger in the fetuses from uninephrectomized mothers than in those from sham-operated ones. On fetal day 20, the CF particles were clustered in the laminae rarae interna and externa of the glomerular basement membrane in the fetuses from uninephrectomized mothers, while the clusters were arrayed in three to four layers in the glomerular basement membrane in the fetuses from sham-operated ones. On fetal day 22, the CF particles noted in the lamina rara externa in the fetuses from uninephrectomized mothers were slightly larger in number than such particles in the age-matched control fetuses. The BUN concentration of the uninephrectomized pregnant rats was significantly higher than that of the sham-operated pregnant ones on each postoperative day. These results suggest that the development of the fetal renal glomerulus is accelerated by the elevated BUN level following maternal uninephrectomy when the fetal kidney is functional in effective filtration in the rat.

Renal tubular function in the gravid rat

Pregnancy in the rat is accompanied by enhanced reabsorption of salt and water throughout most, if not all, of the gestational period. Many mechanisms have been suggested but definitive answers are still awaited. The major area of controversy centres around the detection of changes at term. There is general agreement that, at least in mid-gestation, the increase in reabsorption can be attributed to increases in the proximal tubules, the loop of Henle and the collecting duct. The contribution of the proximal tubule to the increased reabsorption at term is still uncertain. Enhanced salt and water reabsorption is demonstrated in distal nephron segments irrespective of the stage of gestation. Micropuncture and microperfusion experiments have identified increased reabsorption of water, sodium and chloride in the loop of Henle, but it appears that there is net addition of glucose, urea and potassium to the tubular fluid in this segment which, at least for potassium and glucose, offsets to some extent increased reabsorption by the proximal tubule. Altered renal handling of other solutes (uric acid, calcium and magnesium) also occurs throughout pregnancy but the mechanisms responsible and nephron sites involved remain to be investigated. Attempts to attribute altered reabsorption to direct renal effects of changes in maternal hormones are inconclusive. Prolactin mimics some of the pregnancy-associated increases in reabsorption following chronic administration to male and non-pregnant female rats. These effects might be due to a direct renal action of the hormone or even to the volume expansion following its dipsogenic action.

The renin-angiotensin system during pregnancy in chronically instrumented, conscious rats

Whether the renin-angiotensin system is activated during rat gestation is controversial. Therefore we serially assessed plasma renin activity in unrestrained, chronically instrumented conscious rats during pregnancy and the postpartum period. Plasma renin activity was 3.26 +/- 0.30, 2.80 +/- 0.31, and 2.70 +/- 0.26 ng.ml-1.hr-1 on gestational days 6, 12, and 20, respectively. When the same rats were studied after delivery, plasma renin activity was 1.87 +/- 0.29, 1.81 +/- 0.09, and 2.31 +/- 0.35 ng.ml-1.hr-1 on postpartum days 3, 6, and 11, respectively. Levels measured during pregnancy were significantly greater than in the postpartum period (p less than 0.05 or less than 0.01). We then evaluated potential functional consequences of the renin-angiotensin system in gravid rats. Near term, renal hemodynamics fall from the peak levels of midgestation; we tested whether angiotensin II mediates this apparent vasoconstriction. Captopril (1.5 mg/kg, 1.5 mg.kg-1.hr-1) was acutely administered to lower circulating angiotensin II. The drug produced an 80% inhibition of angiotensin I pressor response, a tenfold elevation in plasma renin activity, but caused the same degree of mild renal vasodilation in rats whether they were virgin or pregnant. We also tested whether prior occupancy of receptors by endogenous hormone or receptor downregulation mediates the attenuated pressor response to angiotensin II observed during late pregnancy. Acute administration of captopril failed to augment refractory pressor responsiveness. Chronic treatment with enalaprilat (2.0 mg.kg-1.day-1 by osmotic minipump) also did not restore pressor responsiveness. But, in our hands, chronic administration of enalaprilat most likely failed to lower plasma angiotensin II. In summary, we suggest that during rat gestation (1) the renin-angiotensin system is activated, (2) angiotensin II does not mediate the apparent renal vasoconstriction observed near term, (3) prior receptor occupancy by endogenous hormone is not responsible for the attenuated pressor response to angiotensin II, and (4) long-term treatment with enalaprilat can produce hypotension without reducing plasma concentrations of angiotensin II.

Sodium and water reabsorption in the proximal and distal nephron in conscious pregnant rats and third trimester women

1. Sodium and water reabsorption were determined in virgin controls and rats at 9 and 20 days of pregnancy during a water diuresis induced and maintained by infusion of 2.5% dextrose and in normally hydrated women during the third trimester and again 2-3 months after delivery. 2. Reabsorption from the proximal tubules and distal nephron segments was assessed using the clearance of lithium as a marker of the sodium and water delivery from the proximal tubules to the loops of Henle. 3. Glomerular filtration rate and whole-kidney reabsorption of sodium and water were significantly increased at both stages of pregnancy in rats and late in pregnancy in women when compared to the respective non-pregnant control. Fractional reabsorption was unaltered in pregnant rats but increased in pregnant women. 4. Proximal tubule sodium and water reabsorption were increased in pregnant rats and women; fractional reabsorption was unaltered. 5. Absolute reabsorption of sodium and water in distal nephron segments were increased during pregnancy in both rats and women. Although increases in fractional reabsorption were detected in pregnant women, this was not the case at either stage of gestation in rats.

Renal tubular function in the gravid rat

Pregnancy in the rat is accompanied by enhanced reabsorption of salt and water throughout most, if not all, of the gestational period. Many mechanisms have been suggested but definitive answers are still awaited. The major area of controversy centres around the detection of changes at term. There is general agreement that, at least in mid-gestation, the increase in reabsorption can be attributed to increases in the proximal tubules, the loop of Henle and collecting duct. The contribution of the proximal tubule to the increased reabsorption at term is still uncertain. Enhanced salt and water reabsorption is demonstrated in distal nephron segments irrespective of the stage of gestation. Micropuncture and microperfusion experiments have identified increased reabsorption of water, sodium and chloride in the loop of Henle, but it appears that there is net addition of glucose, urea and potassium to the tubular fluid in this segment which, at least for potassium and glucose, offsets to some extent increased reabsorption by the proximal tubule. Altered renal handling of other solutes (uric acid, calcium and magnesium) also occurs throughout pregnancy but the mechanisms responsible and nephron sites involved remain to be investigated. Attempts to attribute altered reabsorption to direct renal effects of changes in maternal hormones are inconclusive. Prolactin mimics some of the pregnancy-associated increases in reabsorption following chronic administration to male and non-pregnant female rats. These effects might be due to a direct renal action of the hormone or even to the volume expansion following its dipsogenic action.

Effects of prolactin on Na-K-ATPase activity along the rat nephron

To test prolactin (PRL) action on osmoregulation in mammals, we evaluated in the rat the effect of this hormone on a major enzyme in renal regulation of water and electrolyte: renal Na-K-ATPase. Enzyme activity was determined by cytochemistry in medullary ascending limb (MAL) and distal convoluted tubule (DCT) from rats treated either by bromocriptine, or by PRL. Three hours after a bromocriptine injection (0.1 mg/100 g IP) a significant decrease of Na-K-ATPase activity is observed in both MAL (80% of control values, p less than 0.001) and DCT (78% p less than 0.01). Reciprocally, a significant (p less than 0.001) increase in enzyme activity is induced 3 h after a single PRL injection (140 micrograms/100 g IM), in both segments (MAL: 165%, DCT: 172% of control activities) and persists 6 h after the injection (MAL: 130%, DCT: 118%). Na-K-ATPase activity was correlated to plasma PRL levels (r = 0.78 in DCT, r = 0.89 in MAL). A direct effect of PRL on the tubule is suggested by results from experiments in which PRL, at various concentrations, is added in vitro on renal slices before Na-K-ATPase activity measurements. The increase in Na-K-ATPase activity exhibits a log-dose dependency with PRL concentration (p less than 0.01) and is still observed when AVP antagonist is added before PRL incubation, ruling out the possible role of AVP contamination of PRL. These results suggest a direct effect of PRL on renal Na-K-ATPase in MAL and DCT.

Effect of pregnancy on plasma renin activity and glomerular synthesis of prostaglandins and thromboxane in rats

Normal pregnancy in women and rats is characterized by an increased glomerular filtration rate. Because prostaglandin glomerular synthesis has been reported to be increased in 2 other circumstances with glomerular hyperfiltration (streptozotocin-induced diabetes and high protein dietary intake in Heymann nephrilis), we studied prostaglandin biosynthesis in glomeruli obtained from pregnant Wistar rats in comparison with non pregnant rats during the estrus cycle. Comparatively to the 3 first phases of the cycle, and wether or not arachidonic acid was present, PGE2 and PGF2 alpha production rates were found significantly higher in diestrus 2 (2-3 fold increase for both) at 15 days of pregnancy (2 fold increase for PGE2, 5-6 fold increase for PGF2 alpha) and at 21 days of pregnancy (5-6 fold increase for both). On the other hand synthesis of TXB2 was not increased during pregnancy nor during diestrus 2. Plasma renin activity (PRA) was increased during pregnancy. It is concluded that in presence of increased PRA, the increased synthesis of PGE2 may be the hormonal factor explaining, at least in part, the hemodynamic mechanism of glomerular hyperfiltration which has been previously described by micropuncture techniques and characterized by increased renal plasma flow, increased glomerular filtration pressure and decreased ultrafiltration coefficient.

Renal hemodynamics during pregnancy in chronically catheterized, conscious rats

Two types of experiments were performed, cross-sectional and longitudinal. In the cross-sectional studies, rats were mated, later prepared surgically, and then 5 or more days after surgery, each examined twice during days 11 to 15 or days 18 to 20 of gestation. Nonpregnant rats matched for age and prepregnant weight served as controls. In the longitudinal studies, rats were catheterized and, starting 6 days later, examined twice; then the same rats were mated and each was studied on days 5, 8, 12, 16, and 20 of gestation, as well as on day 5 postpartum. In the cross-sectional studies, glomerular filtration rate (GFR) and effective renal plasma flow (ERPF) were elevated by approximately 26% and 20%, respectively, above nonpregnant controls at 11 to 15 days of gestation (GFR, 2739 +/- 94 vs. 2181 +/- 134 microliters . min-1, P less than 0.005; ERPF, 9367 +/- 295 vs. 7785 +/- 422 microliters . min-1, P less than 0.01). By 18 to 20 days of gestation, GFR and ERPF had returned to levels that were not significantly different from nonpregnant values. The longitudinal studies confirmed these findings in every respect and further revealed that GFR and ERPF were elevated above nonpregnant values as early as day 5 of gestation (P less than 0.005). Thereafter, they rose to peak values, at 12 and 16 days of gestation, of 3122 +/- 144 and 10,584 +/- 541 microliters . min-1, and then returned to nonpregnant levels by day 20 of gestation.(ABSTRACT TRUNCATED AT 250 WORDS)