Judith H Burrell

The effects of a converting enzyme inhibitor (Captopril) and angiotensin II on fetal renal function

1 Renal function was studied in chronically catheterized fetal sheep (119–128 days gestation), before and during treatment of the ewe with the angiotensin converting enzyme (ACE) inhibitor, Captopril, which crosses the placenta and blocks the fetal renin angiotensin system. 2 An i.v. dose of 15 mg (about 319 μg kg−1) of Captopril to salt‐replete ewes followed by an infusion to the ewe of 6 mg h−1 (about 128 μg kg−1 h−1) caused a fall in fetal arterial pressure (P < 0.01), and a rise in fetal renal blood flow (RBF) from 67.9 ± 5.6 to 84.9 ± 8.3 ml min−1 (mean ± s.e.mean) (P < 0.05). Renal vascular resistance and glomerular filtration rate (GFR) fell (P < 0.01); fetal urine flow (P < 0.01) and sodium excretion declined (P < 0.05). 3 Ewes were treated for the next 2 days with 15 mg Captopril twice daily. On the 4th day, 15 mg was given to the ewe and fetal renal function studied for 2 h during the infusion of Captopril (6 mg h−1) to the ewe. Of the 9 surviving fetuses, 3 were anuric and 3 had low urine flow rates. When 6 μg kg−1 h−1 of angiotensin II was infused directly into the fetus RBF fell from 69 ± 10.1 ml min−1 to 31 ± 13.9 ml min−1, GFR rose (P < 0.05) and urine flow (P < 0.01) and sodium excretion increased in all fetuses. 4 It is concluded that the small fall in fetal arterial pressure partly contributed to the fall in fetal GFR but in addition, efferent arteriolar tone fell so that the filtration pressure fell further. Thus maintenance of fetal renal function depends on the integrity of the fetal renin angiotensin system. These findings explain why use of ACE inhibitors in human pregnancy is associated with neonatal anuria.

Angiotensin receptor subtypes in the uterine artery during ovine pregnancy

This study was undertaken to determine if changes in receptor density or affinity could account for the reduced vascular sensitivity to angiotensin II seen during pregnancy. Angiotensin receptor subtypes in the uterine arteries of non-pregnant, pregnant and postpartum ewes were investigated using saturation and competition receptor binding techniques with the specific receptor antagonists, losartan (DuP-753) and PD-123319 (S)1-[[4-(dimethylamino)-3-methylphenyl]-methyl]-5-(diphenylacetyl )-4,5,6,7-tetrahydro-1H-imidazo(4,5-c)pyridine-6-carboxylic acid, ditrifluoroacetate, monohydrate). Receptor density and affinity of total angiotensin receptors, as well as the angiotensin AT1 and AT2 receptor subtypes in uterine arteries were compared with those in the mesenteric artery and aorta. The uterine artery contains both AT1 and AT2 receptor subtypes, whereas the mesenteric artery and aorta contain primarily the AT1 receptor subtype. In uterine arteries from pregnant sheep, angiotensin receptor density was increased because AT2 receptors were increased. AT1 receptor density was not altered. This change was not seen in the aorta. In the uterine artery, receptor affinity for [Sar1,Ile8]angiotensin II decreased in mid-gestation (IC50 7.7 +/- 1.2 x 10(-9) M) compared with non-pregnant ewes (IC50 3.0 +/- 0.6 x 10(-9) M, P = 0.006), and there was decreased affinity of angiotensin AT1 receptors for losartan during pregnancy (IC50 2.8 +/- 1.0 x 10(-4) M) compared with non-pregnant ewes (IC50 2.2 +/- 1.3 x 10(-6) M, P = 0.025). Our results show changes in the density and affinity of the angiotensin receptor subtypes in the uterine artery which could explain its reduced responsiveness to circulating angiotensin II during pregnancy.

Effects of gestation on ovine fetal and maternal angiotensin receptor subtypes in the heart and major blood vessels.

Previous studies in fetal sheep have concluded that (a) the vascular AT1 angiotensin II (Ang II) receptor subtype is present in the external umbilical artery, but not in other systemic blood vessels, and (b) carotid arterial rings contract in vitro in response to Ang II. These contractions are blocked by the AT1 specific receptor antagonist losartan. The aim of the present study was to resolve the apparent contradiction of these earlier conclusions, by examining the distribution of Ang II receptor subtypes in different regions of the ovine fetal cardiovascular system, and to find out at what stage in development AT1 receptors first appear. We measured AT1 and AT2 receptors in hearts, carotid arteries, aortae and umbilical vessels from fetal sheep aged 65‐144 days (term ∼150 days), and in hearts and aortae from lambs, and adult pregnant and non‐pregnant ewes. Both AT1 and AT2 receptors were present in aortae of fetuses > 118 days gestation, and carotid arteries of fetuses > 121 days gestation, while in younger fetuses only AT2 receptors were found. The proportion of carotid artery and aortic AT1 receptors increased with age, while the proportion of AT2 receptors decreased. The internal umbilical artery contained both subtypes, but there was no relationship between receptor density and gestational age. The external umbilical artery had only AT1 receptors. The highest density of Ang II receptors was found in the fetal heart where the AT2 subtype predominated. The density of fetal cardiac Ang II receptors declined with age (r= ‐0.44, P < 0.02) due to the decrease in the AT2 subtype. The density in late gestation fetal hearts was greater than in lamb or adult hearts (P < 0.001). Our study shows that fetal systemic blood vessels contain AT1 receptors, and we have documented for the first time that the appearance of AT1 receptors is both different in different regions of the fetal cardiovascular system and is developmentally regulated. Together with the in vitro contractile studies, this suggests that Ang II can play an important role in fetal blood pressure regulation via AT1 receptors in the fetal systemic vasculature, as well in the umbilicoplacental vessels.

Effects of intrafetal IGF-I on growth of cardiac myocytes in late-gestation fetal sheep

Intrafetal insulin-like growth factor (IGF)-I promotes cardiac hypertrophy in the late-gestation fetal sheep; whether these effects are sustained is unknown. IGF-I was infused for 4 days at 80 microg/h from 121 to 125 days of gestation, and its effects at 128 days, 3 days after the infusion stopped, were determined by comparison with untreated fetal sheep. After IGF-I treatment, fetal weights were similar to those in control fetuses but kidney weights were bigger (P < 0.05), as were spleen weights of male fetuses (P < 0.05). Cardiac myocytes were larger in female than male fetal sheep (P < 0.001). IGF-I increased male (P < 0.001) but not female myocyte volumes. IGF-I did not alter the proportions of uni- or binucleated right or left ventricular myocytes. Female fetal sheep had a greater proportion of binucleated cardiac myocytes than males (P < 0.05). IGF-I-treated fetuses had a slightly greater proportion of right ventricular nuclei in cell cycle phase G(2)/M and a reduced proportion of G(0)/G(1) phase nuclei (P < 0.1). Therefore, evidence for IGF-I-stimulated cardiac cell hyperplasia in fetal sheep in late gestation was limited. In conclusion, the greater sizes and larger proportion of binucleated cardiac myocytes in female fetal sheep suggest that myocyte maturation may occur earlier in females than in males. This may explain in part the male sex-specific responsiveness of cardiac hypertrophy to IGF-I in late gestation. If IGF-I-stimulated cardiomyocyte growth is accompanied by maturation of contractile function, IGF-I may be a potential therapeutic agent for maintaining cardiac output in preterm males.

Analgesic nephropathy in fischer 344 rats: comparative effects of chronic treatment with either aspirin or paracetamol

&NA; This study has compared the relative nephrotoxicity of chronic treatment with aspirin or paracetamol in an animal model. Changes in renal structure and urinary concentrating ability were examined in female Fischer 344 rats after continuous treatment with either aspirin (120–230 mg/kg body wt/day), or paracetamol (140–210 mg/kg body wt/day), and were compared with age‐matched untreated control rats. Renal morphological changes were examined after 40–83 weeks of analgesic treatment, using light and electron microscopy. Aspirin caused renal papillary necrosis and a decrease in urinary concentrating ability, whereas paracetamol alone did not cause significant renal damage. Aspirin produced damage to the interstitial cells and matrix, particularly in the mid‐papillary region, followed by changes to the thin limbs of the loop of Henle and medullary capillary endothelium. These structural changes were similar to those described previously,19 when continuous treatment with combined aspirin and paracetamol was studied in the same animal model.

Experimental Analgesic Nephropathy: Changes in Renal Structure and Urinary Concentrating Ability in Fischer 344 Rats Given Continuous Low Doses of Aspirin and Paracetamol

&NA; Long‐term treatment with aspirin and paracetamol produced renal papillary necrosis in female Fischer 344 rats. Aspirin (230 mg/kg body weight/day) and paracetamol (380 mg/kg body weight/day) were dissolved in drinking water and given continuously for up to 65 weeks. Renal morphological changes were examined between 21 weeks and 65 weeks of commencement of analgesic treatment using light and electron microscopy, and were compared with age‐matched controls. Structural damage initially occurred in the mid‐papillary region, and specifically involved the interstitial cells and interstitial matrix. Necrosis of the epithelium of the thin limbs of the loop of Henle was present only after interstitial changes were well established. Cortical interstitial fibrosis and tubular atrophy occurred after renal papillary changes were observed. There was no evidence of significant vascular damage. Urinary concentrating ability was measured sequentially during the period of analgesic treatment. A decrease in urine concentrating ability was present when early changes to the interstitial cells and matrix were observed, and concentrating ability continued to decrease in parallel with increasing morphological damage. This study describes an animal model of analgesic‐induced nephropathy, enabling early morphological changes to be studied and correlated with renal functional changes.

Effect of cortisol on fetal ovine vascular angiotensin II receptors and contractility

The renin angiotensin system is important in the regulation of fetal blood pressure. This study investigated the expression of angiotensin AT(1) and AT(2) receptors in the ovine fetal heart, aorta and umbilical artery, and how these receptors are affected by cortisol. Cortisol infusion into the fetus has previously been shown to cause an increase in fetal blood pressure. We hypothesised that this effect of cortisol is mediated by upregulation of the angiotensin AT(1) receptor. Binding studies performed on tissues with intact endothelium demonstrated both receptor subtypes in the fetal aorta and right ventricle, although the latter contained mainly angiotensin AT(2) receptors. In contrast, only angiotensin AT(1) receptors were found in the umbilical artery. Cortisol infusion into fetuses (3 mg/day for 3-5 days) caused a physiological increase in plasma cortisol levels to 29+/-4 nM. This was associated with an increase in systolic pressure (57.8+/-1.7 vs. 52.2+/-1.5 mm Hg, P<0.05), but cortisol had no effect on the density or affinity of angiotensin receptors, nor on the in vitro contractile responses of carotid and umbilical arterial rings to 5-microM angiotensin II. In conclusion, this study has demonstrated differential expression of angiotensin AT(1) and AT(2) receptors in the different regions of the ovine fetal cardiovascular system and that the angiotensin AT(1) receptor is functional. The lack of any effect of low doses of cortisol on these receptors and on the contractility of isolated fetal vessels to angiotensin II suggests cortisol acts by other mechanisms to raise fetal arterial pressure.

Effects of one-clip, one-kidney hypertension in chronically catheterized pregnant ewes.

1. Hypertension secondary to renal disease was studied in non‐pregnant and pregnant ewes to determine whether there were any changes in arterial pressure and the distribution of cardiac output and, in particular, whether uteroplacental blood flow was affected.

The uridine nucleotides constitute a natriuretic pressor system.

1. Uridine monophosphate was tested in the conscious rat for natriuretic properties and an immunoperoxidase technique was used to localize uridine‐containing compounds in the rat kidney.

Effects of infusions of ACTH in the chronically catheterized pregnant ewe and her fetus.

To study the effects of elevated maternal levels of adrenocorticotropic hormone (ACTH) on the fetus, nine chronically catheterized pregnant ewes (132 +/- 0.9 days of gestation) were infused intravenously for 3 days with Synacthen (5 micrograms.kg-1.day-1). Four ewes were given 0.15 M saline intravenously over the same period. ACTH induced hypertension in the ewe. Mean arterial pressure (MAP) increased from 101 +/- 4.4 to 114 +/- 3.9 mmHg at 48 h (P < 0.05); cardiac output increased from 8.6 +/- 0.5 to 10.4 +/- 1.0 l/min after 24 h (P < 0.05). Within 2-4 h, maternal cortisol levels increased from 24.6 +/- 6.3 to 287 +/- 30 nM (P < 0.05) and remained high. Fetal plasma cortisol levels increased from 20 +/- 4.5 to 60 +/- 4.5 nM (P < 0.05) within 2-4 h and then increased further. Fetal MAP was increased at 24 h. There was no effect on fetal blood gases or pH. Ewes became hyperglycemic and lactacidemic by 24 h (P < 0.05), and the fetuses were also hyperglycemic and lactacidemic (P < 0.05) at this time. There were no changes in fetuses carried by saline-infused ewes. Both ewes and fetuses had raised plasma osmolalities and, since hematocrit fell, retained fluid. Ewes became hypokalemic; the fetuses did not, but there was an increase in fetal K excretion. Thus ACTH-induced hypertension in the ewe had minimal effects on fetal MAP, fetal blood gas status, and pH. The fetus, however, did show many of the other effects of maternal glucocorticoid and mineralocorticoid excess, partly because its cortisol levels were increased but also as a consequence of metabolic and endocrine changes in the ewe.To study the effects of elevated maternal levels of adrenocorticotropic hormone (ACTH) on the fetus, nine chronically catheterized pregnant ewes (132 ± 0.9 days of gestation) were infused intravenously for 3 days with Synacthen (5 μg ⋅ kg-1 ⋅ day-1). Four ewes were given 0.15 M saline intravenously over the same period. ACTH induced hypertension in the ewe. Mean arterial pressure (MAP) increased from 101 ± 4.4 to 114 ± 3.9 mmHg at 48 h ( P < 0.05); cardiac output increased from 8.6 ± 0.5 to 10.4 ± 1.0 l/min after 24 h ( P < 0.05). Within 2-4 h, maternal cortisol levels increased from 24.6 ± 6.3 to 287 ± 30 nM ( P < 0.05) and remained high. Fetal plasma cortisol levels increased from 20 ± 4.5 to 60 ± 4.5 nM ( P < 0.05) within 2-4 h and then increased further. Fetal MAP was increased at 24 h. There was no effect on fetal blood gases or pH. Ewes became hyperglycemic and lactacidemic by 24 h ( P < 0.05), and the fetuses were also hyperglycemic and lactacidemic ( P < 0.05) at this time. There were no changes in fetuses carried by saline-infused ewes. Both ewes and fetuses had raised plasma osmolalities and, since hematocrit fell, retained fluid. Ewes became hypokalemic; the fetuses did not, but there was an increase in fetal K excretion. Thus ACTH-induced hypertension in the ewe had minimal effects on fetal MAP, fetal blood gas status, and pH. The fetus, however, did show many of the other effects of maternal glucocorticoid and mineralocorticoid excess, partly because its cortisol levels were increased but also as a consequence of metabolic and endocrine changes in the ewe.