Christine Baylis

Adverse Interactions Between Pregnancy and a New Model of Systemic Hypertension Produced by Chronic Blockade of Endothelial Derived Relaxing Factor (EDRF) in the Rat

Chronic blockade of the endogenous EDRF system has previously been reported to produce a model of hypertension and renal damage in rats. The present studies were conducted in conscious chronically catheterized rats to compare the renal hemodynamic and blood pressure responses to pregnancy in normal rats and during chronic EDRF blockade. In both moderate and severe hypertension produced by EDRF blockade, the normal late gestational fall in blood pressure is absent and blood pressure remains elevated at term. There was no midterm renal vasodilation or increase in GFR in the chronically hypertensive rats, also proteinuria developed close to term in hypertensive animals. The normal pregnant rat shows a marked plasma volume expansion as evidenced by significant falls in hematocrit close to term. This was suppressed in the hypertensive animals, particularly in the more severely hypertensive group. The chronically EDRF blocked hypertensive animals had a relatively poor maternal and fetal outcome compared to norm...

Male gender predisposes to development of endotoxic shock in the rat.

OBJECTIVE After intravenous (i.v.) injection of lipopolysaccharide (LPS) macrophages release nitric oxide (NO) due to the expression of the inducible NO synthase (iNOS). After LPS NO is abundantly produced also in the cardiovascular system and may contribute to the development of hypotension and shock. Since the immune response, the synthesis of NO and the regulation of blood pressure (BP) differ between males and females, in the present study the effect of LPS on BP, renal function, the plasma and urinary concentration of the metabolites of NO as well as the splenic and aortic expression of the iNOS gene were compared between male and female rats. METHODS BP and renal function were measured in anesthetized rats following the i.v. injection of LPS (E. coli, 4 mg/kg). The NO2- and NO3- (metabolites of NO=NOx) concentration was measured by the Griess reaction. The iNOS gene expression was studied by RT-PCR. RESULTS Four hours after LPS, BP of males (n=9) was reduced by 63+/-12 mmHg versus 10+/-4 in females (n=7, P<0.005). Aminoguanidine, a selective inhibitor of iNOS, prevented the reduction of BP in males. The plasma concentration of NOx (P(NOx)), microM) was lower in hypotensive males (128+/-20) than in normotensive females (235+/-29, P<0.005). Males also exhibited lower urinary NOx excretion (U(NOx)V) after LPS (P<0.001 vs. females). Prior castration of males provided protection against hypotension (fall of BP: -4+/-4 mmHg, n=6, P<0.02 versus males) and resulted in higher P(NOx) as well as U(NOx)V (both P<0.001 versus males and not different from females). Prior ovariectomy (n=5) had no influence on the hemodynamic and NOx response to LPS. Male rats displayed enhanced aortic iNOS/beta-actin ratio relative to females after LPS (n=3 in each group, P<0.05). CONCLUSIONS (1) Male gender may sensitize to LPS-induced shock and (2) sensitivity of males to endotoxin is associated with an attenuated, not exaggerated total rate of NO synthesis.

The mechanisms of proteinuria in aging rats

Aging is associated with the appearance of a selective proteinuria which cannot be attributed to any specific underlying renal disease. The present studies were conducted in conscious, chronically catheterized young (3-4 months), non-proteinuric male rats and old (22-25 months), proteinuric males to determine the mechanism(s) of the proteinuria. Compared with young males, old proteinuric rats had increased blood pressure, reduced glomerular filtration rate (GFR) and renal plasma flow and heavy proteinuria. Fractional clearance of neutral dextran (D) and anionic dextran sulfate (DS) were both significantly increased at the 36 A molecular radius in old rats; the increase in DS fractional clearance being greater than the increase in D fractional clearance. The proteinuria of aging is therefore due to moderate increases in glomerular permeability and, more importantly, to loss of fixed glomerular polyanion. Striking glomerular morphologic changes were also evident in the old rats including thickening of the glomerular basement membrane and extensive glomerular sclerosis.

Effects of administered thromboxanes on the intact, normal rat kidney.

The effects of administered thromboxanes on the intact, normal rat kidney were studied. Euvolemic male rats received intraarterial infusions of thromboxane B2 (TXB2) and the stable thromboxane A2 analog (U46619) and the effects on renal function were investigated, using glomerular micropuncture and whole-kidney clearance techniques. Both TXB2 and U46619 were renal vasoconstrictors and lowered GFR by reducing renal plasma flow rate; U46619 was much more potent that TXB2. Neither agent caused any marked change in the glomerular capillary ultrafiltration coefficient (Kf). Thus in the rat, the thromboxanes reduce filtration rate by increasing renal vascular resistance and without exerting a marked influence on Kf.

Proximal tubular metalloprotease activity is decreased in the senescent rat kidney

Proximal tubule brush border membrane-enriched fractions (BBM) from young (4 months) and old (20-22 months) male Sprague-Dawley rats were prepared by differential centrifugation and metalloprotease activity was measured using radioiodinated insulin B chain as substrate. Proteolytic activity is expressed as Units (U), where 1 U = 1 microgram insulin B chain degraded per min and the specific activity is the U per mg BBM protein used in the assay. Total proteolytic activity (measured at pH 7) was decreased 2-fold in BBM from old rats (2.47 +/- 0.11 vs 4.71 +/- 0.35 U/mg BBM protein, p less than 0.01). The chelator, 1,10-phenanthroline, completely inhibited the proteolytic activity in both groups, suggesting that the BBM insulin B chain-degrading activity in both old and young rats was entirely due to metalloproteases. In the presence of thiorphan, a specific inhibitor of the metalloprotease endopeptidase 24.11, approximately 60% inhibition of proteolytic activity occurred in both groups. Thus, total metalloprotease and endopeptidase 24.11 activities are markedly diminished in the proximal tubule of the senescent rat kidney.

Glomerular Effects of Pregnancy in a Model of Glomerulonephritis in the Rat

These studies were designed to investigate the functional and morphologic glomerular effects of pregnancy in a model of experimentally induced glomerulonephritis (GN) in the Munich-Wistar rat. Virgin and midterm (day 12) pregnant rats were studied during the autologous phase of anti-glomerular basement membrane antibody GN and were compared with normal control virgin and pregnant rats. In this model of moderately intense GN, glomerular filtration rate (GFR) and single-nephron GFR were not different from normal controls; however, glomerular micropuncture revealed that glomerular capillary blood pressure was elevated and the glomerular capillary ultrafiltration coefficient, Kfi, was reduced in rats with GN compared with normal rats. In addition a substantial proteinuria was present, and there was morphologic evidence of a moderate and fairly uniform GN. The superimposition of pregnancy to midterm (11 to 13 days) had no effects on the proteinuria or glomerular morphology in rats with GN. Also, pregnancy did not further alter the increased glomerular blood pressure or reduced Kf that occur in this model of GN. Despite the presence of an underlying glomerular disease, the maternal kidney was able to increase renal plasma flow. When underlying mild GN is present, 11 to 13 days of pregnancy does not appear to worsen the disease in the short term. Other studies are needed to determine the long-term renal effects of repeated pregnancy when underlying GN is present.

Basal and stimulated plasma atrial natriuretic peptide (ANP) concentrations and cardiac and contents in old and young rats

To investigate the ability of the ageing heart to release atrial natriuretic peptide (ANP) we compared the response of awake, trained, chronically catheterized old (20-21 months) and young (4 months) rats to an acute, hypertonic saline challenge. There were no differences between young and old rats in basal plasma concentration of sodium (PNa; old: 141 +/- 3 meq/l; young: 143 +/- 3 meq/l) or ANP (old: 61 +/- 5 pg/ml; young: 67 +/- 12 pg/ml). Five minutes after acute saline challenge, PNa rose in both groups (old: 146 +/- 2 meq/l; young 149 +/- 1 meq/l) and approximately 3-fold increases in plasma ANP levels (182 +/- 24 pg/ml; young: 179 +/- 42 pg/ml). Hearts of old and young rats were assayed for atrial and ventricular ANP content. Atrial ANP levels were similar in old and young rats (13.5 +/- 3.6 vs. 24.9 +/- 8.7 micrograms/g atrial tissue), whereas ventricular ANP content was approximately 4-fold higher in old vs. young rats (153.7 +/- 39.3 vs. 47.5 +/- 6.4 ng/g ventricular tissue). Thus, the ageing rat heart responds equally as well as the young rat to an acute NaCl challenge.

Renal autoregulation in midterm and late-pregnant rats

OBJECTIVES The normal kidney autoregulates (that is, maintains) renal blood flow over a wide range of arterial blood pressures. During normal pregnancy blood pressure falls and the kidney vasodilates. The purpose of this study was to investigate whether renal autoregulatory ability was influenced during pregnancy in the normal rat. STUDY DESIGN Experiments were conducted on three groups of anesthetized rats, studied in the virgin state, at midterm, and late in pregnancy (gestation lasts 22 days in the rat). Renal blood flow was measured by electromagnetic flow probe at normal renal perfusion pressures, during elevation in renal perfusion pressure with bilateral carotid occlusion, and at subnormal renal perfusion pressures after application of an aortic snare. RESULTS Blood pressure was similar in midterm and virgin rats but lower in late pregnancy in the control state. Control renal blood flow was similar between virgin and late-pregnant rats but significantly elevated at midterm. During elevation in blood pressure with bilateral carotid occlusion, autoregulation was maintained in all three groups. During graded aortic occlusion, renal autoregulatory ability was lost. The autoregulatory threshold was between 90 and 100 mm Hg for midterm pregnant and virgin rats and was slightly lower at about 88 mm Hg for late-pregnant rats. CONCLUSIONS Midterm pregnant rats are able to autoregulate renal blood flow as well as virgins are in spite of underlying gestational renal vasodilation. The slight shift in renal autoregulatory threshold seen in late-pregnant rats may prevent the kidney from hypoperfusion during late-gestation hypotension.

Renal effects of administered atrial natriuretic peptide in the conscious, aging rat.

Studies were performed in conscious, chronically catheterized male Sprague-Dawley rats to investigate the effect of administered atrial natriuretic peptide (ANP) on blood pressure, renal hemodynamics and urinary electrolyte excretion. Studies were performed on young adult (3-4 month old) rats and on aging rats (18-24 months of age). Low dose ANP (80 ng/kg/min for 60 min) had no effects on renal hemodynamics in either young or old rats and produced only a slight blood pressure reduction in young animals. No effect on urinary electrolyte excretion was evident in young rats whereas in the old animals, low dose ANP produced large rises in the rate of sodium excretion, fractional excretion of sodium and urine flow rate. A four fold higher dose of ANP evoked a moderate natriuretic and a marked antihypertensive response in young rats. Time control studies indicated that time alone had no influence on urinary sodium excretion rate, the fractional excretion of sodium or urine flow rate. These studies indicate a much enhanced sensitivity to the natriuretic effects of administered ANP by the kidneys of old rats.

Hemodynamic and renal effects of U-46619, a TXA2/PGH2analog, in late-pregnant rats

The vasoconstrictor effects of pressor agents are attenuated during pregnancy. Thromboxane A2 (TXA2) is produced in great quantities during hypertension in pregnancy, and therefore it is important to know whether pregnancy modifies the pressor effects of TXA2. The TXA2 analog U-46619 was infused in anesthetized, acutely prepared and conscious, chronically prepared late-pregnant and nonpregnant female rats to examine its systemic hemodynamic and renal effects. Mean arterial pressure (MAP) and total peripheral resistance (TPR) were lower in anesthetized pregnant than nonpregnant rats (P < 0.01). The infusion of U-46619 into the aortic arch resulted in elevation of MAP only in pregnant rats, due to a greater elevation of TPR (60 +/- 17%) compared with nonpregnant rats (36 +/- 6%, P < 0.05). The pressor effect of intravenously infused U-46619 was also enhanced in conscious pregnant versus nonpregnant rats, and the increase in renal vascular resistance was undiminished. U-46619 increased hematocrit and plasma protein concentration more during pregnancy, which suggested greater reduction of plasma volume. The urinary excretion of sodium (-1.49 +/- 0.25 vs. -0.54 +/- 0.24 micromol/min) and water was reduced more in pregnant than nonpregnant rats during U-46619 (P < 0.01). Thus the MAP and renal effects of the TXA2 analog are exaggerated during pregnancy in the rat.The vasoconstrictor effects of pressor agents are attenuated during pregnancy. Thromboxane A2(TXA2) is produced in great quantities during hypertension in pregnancy, and therefore it is important to know whether pregnancy modifies the pressor effects of TXA2. The TXA2 analog U-46619 was infused in anesthetized, acutely prepared and conscious, chronically prepared late-pregnant and nonpregnant female rats to examine its systemic hemodynamic and renal effects. Mean arterial pressure (MAP) and total peripheral resistance (TPR) were lower in anesthetized pregnant than nonpregnant rats ( P < 0.01). The infusion of U-46619 into the aortic arch resulted in elevation of MAP only in pregnant rats, due to a greater elevation of TPR (60 ± 17%) compared with nonpregnant rats (36 ± 6%, P < 0.05). The pressor effect of intravenously infused U-46619 was also enhanced in conscious pregnant versus nonpregnant rats, and the increase in renal vascular resistance was undiminished. U-46619 increased hematocrit and plasma protein concentration more during pregnancy, which suggested greater reduction of plasma volume. The urinary excretion of sodium (-1.49 ± 0.25 vs. -0.54 ± 0.24 μmol/min) and water was reduced more in pregnant than nonpregnant rats during U-46619 ( P < 0.01). Thus the MAP and renal effects of the TXA2 analog are exaggerated during pregnancy in the rat.