Biomedical subjects

K P Conrad

Publications and source records attributed to K P Conrad.

At least 19 recordsLinked to original sources

PubMed2000

Expression of endothelial nitric oxide synthase by extravillous trophoblast cells in the human placenta.

Previous reports have documented the expression of endothelial nitric oxide synthase (eNOS) expression by the syncytiotrophoblast layer of the villus in the human placenta. In contrast, the underlying villous cytotrophoblast cells do not express the enzyme. Because extravillous cytotrophoblasts have not been as extensively investigated, our objective was to test whether these cells express eNOS. Using both a mouse monoclonal and a rabbit polyclonal antibody, we demonstrated immunoreactive eNOS in trophoblast cell columns emanating from anchoring villi in second trimester placentae. Cytokeratin positive trophoblast cells lying beneath remnant anchoring villi, lining decidual blood vessels and scattered throughout the basal plate of normal term and pre-eclamptic placentae also expressed immunoreactive eNOS. By Western analysis, the monoclonal and polyclonal antibodies were shown to be absolutely and relatively specific for eNOS, respectively. The finding of immunoreactive eNOS expression by extravillous trophoblast cells was substantiated by in situ hybridization. Using riboprobes generated from a bovine eNOS cDNA, we demonstrated specific hybridization in the endothelium of blood vessels in the umbilical cord, thus validating the in situ hybridization methodology, as well as specific hybridization in the extravillous trophoblast cells of the basal plate in normal term placenta. In conclusion, several different populations of extravillous trophoblast cells in the basal plate of the human placenta express eNOS.

Animals↗

PubMed2000

Expression, ontogeny, and regulation of hypoxia-inducible transcription factors in the human placenta.

Placental hypoxia likely plays an important role in both normal placental development and pathology. Yet, the molecular mechanisms of hypoxia signaling in this organ are virtually unexplored. Therefore, we investigated the expression of the hypoxia inducible transcription factors (HIF) in normal human placentas spanning the first trimester to term. Several key observations emerged: 1) HIF-1 alpha and -2 alpha mRNA were present in placentas of all gestational ages but with greater variability during early pregnancy; 2) overall, HIF-1 alpha mRNA was expressed at a constant level in all placentas, whereas HIF-2 alpha mRNA increased significantly with gestational age; 3) both HIF-1 alpha and -2 alpha protein decreased significantly with gestational age; and 4) HIF-1 alpha and -2 alpha immunoreactivity were overlapping in cellular distribution being expressed by the syncytiotrophoblast, villous cytotrophoblast, and fetoplacental vasculature with both nuclear and cytoplasmic localization. Next, we studied the regulation of these transcription factors by oxygen using placental villous explants in culture from first-trimester and term placentas. The major findings were 1) HIF-1 alpha and -2 alpha protein, but not mRNA, was induced by hypoxia in the placental villous explants; 2) HIF-1 alpha DNA-binding activity was also stimulated by hypoxia; and 3) glucose transporter-1 mRNA (a known target of HIF) was also increased by hypoxia in placental villous explants. We suggest that physiological hypoxia contributes to the increased expression of HIF-1 alpha and -2 alpha protein in early placentas and that regulation of these transcription factors by hypoxia in the human placenta occurs at the level of protein and not mRNA.

Basic Helix-Loop-Helix Proteins↗

PubMed1999

Expression of the erythropoietin receptor by trophoblast cellsin the human placenta.

Nonclassical sites of erythropoietin (EPO) and erythropoietin receptor (EPO-R) expression have been described that suggest new physiological roles for this hormone unrelated to erythropoiesis. The recent finding of EPO expression by trophoblast cells in the human placenta prompted us to consider whether these cells also express EPO-R. With use of immunocytochemistry, EPO-R was identified in villous and extravillous cytotrophoblast cells, as well as in the syncytiotrophoblast at all gestational ages. EPO-R was also expressed by cells within the villous core, including endothelial cells of fetoplacental blood vessels. Placental tissues and isolated and immunopurified trophoblast cells, as well as trophoblast-derived choriocarcinoma Jar cells, expressed immunoreactive EPO-R on Western blot. EPO-R mRNA was also detected in the same placental tissues and trophoblast cells by nested-primer reverse transcription-polymerase chain reaction. Finally, EPO-R was functional insofar as the receptor was phosphorylated on tyrosine residues in response to exogenous EPO treatment of cultured trophoblast or Jar cells. Thus, the present findings support the hypothesis that trophoblast cells of the human placenta express EPO-R. In view of these results, taken together with previous work demonstrating EPO expression by the same cells, an autocrine role for this hormone in the survival, proliferation, or differentiation of placental trophoblast cells is proposed.

Blotting, Western↗

PubMed1999

Endothelin mediates renal vasodilation and hyperfiltration during pregnancy in chronically instrumented conscious rats.

Profound vasodilation of the kidneys and other nonreproductive organs transpires during early pregnancy. Because nitric oxide (NO) was found to mediate renal vasodilation and hyperfiltration in conscious pregnant rats, and endogenous endothelin (ET) was suggested to be vasodilatory in the renal circulation of nonpregnant rats, we tested whether endothelin mediates the NO-dependent changes in the renal circulation during pregnancy. Glomerular filtration rate (GFR) and effective renal plasma flow (ERPF) were measured in conscious pregnant and virgin rats before and during infusion of 30 micrograms/min RES-701-1 (a selective ETB receptor subtype antagonist). Baseline GFR and ERPF were significantly increased by 35% in gravid rats relative to virgin controls. During infusion of RES-701-1, the pregnant rats responded more robustly, showing a greater decline in both GFR and ERPF such that renal function converged in the two groups of rats. ERPF also converged in pregnant and virgin rats during infusion of SB-209760, a nonselective ETA/B receptor subtype antagonist. Combined infusion of Nomega-nitro-L-arginine methyl ester [L-NAME, an NO synthase (NOS) inhibitor] and RES-701-1 reduced GFR and ERPF to levels comparable to those reached with either agent given alone, suggesting inhibition of a common vasodilatory pathway. RES-701-1 and SB-209670 significantly lowered the cGMP content of small renal arteries from gravid and virgin rats in vitro, strengthening the link between the renal endothelial ETB receptor subtype and NO. Importantly, we showed that RES-701-1 is not a direct inhibitor of NOS. We conclude that endothelin mediates the NO-dependent changes in the renal circulation of conscious rats during pregnancy.

Animals↗

PubMed1999

Plasma and 24-h NO(x) and cGMP during normal pregnancy and preeclampsia in women on a reduced NO(x) diet.

We tested the hypothesis that nitric oxide (NO) biosynthesis increases during normal human pregnancy and decreases in preeclampsia. The major metabolites of NO, nitrate and nitrite (NO(x)), were measured in both the plasma and 24-h urine of women subjected to a reduced NO(x) diet. In this way, the plasma and urinary levels mainly reflected endogenous production rather than dietary intake. Moreover, we assessed cGMP, a second messenger of NO, in the same samples. Both NO(x) and cGMP assays were validated in our laboratory. We first conducted a cross-sectional study of nonpregnant women (n = 15), normal pregnant women in the first (n = 9), second (n = 17) and third (n = 22) trimesters, as well as women with preeclampsia (n = 15) and transient hypertension of pregnancy (n = 7). We also performed a serial study in the same women (n = 9) before, during, and after pregnancy. Taken together, the results of the two investigations suggested marked increases in cGMP production especially during the first trimester when the maternal circulation is rapidly vasodilating. In contrast, whole body NO production as estimated by the plasma level and urinary excretion of NO(x) was not elevated during the first trimester. Finally, unequivocal demonstration of reduced NO biosynthesis in preeclampsia was not forthcoming.

Adult↗

PubMed1999

Relaxin is a potent renal vasodilator in conscious rats.

The kidneys and other nonreproductive organs vasodilate during early gestation; however, the "pregnancy hormones" responsible for the profound vasodilation of the renal circulation during pregnancy are unknown. We hypothesized that the ovarian hormone relaxin (RLX) contributes. Therefore, we tested whether the administration of RLX elicits renal vasodilation and hyperfiltration in conscious adult, intact female rats. After several days of treatment with either purified porcine RLX or recombinant human RLX 2 (rhRLX), effective renal plasma flow (ERPF) and glomerular filtration rate (GFR) increased by 20%-40%. Comparable renal vasodilation and hyperfiltration was also observed in ovariectomized rats, suggesting that estrogen and progesterone are unnecessary for the renal response to rhRLX. The nitric oxide synthase inhibitor Nomega-nitro-L-arginine methyl ester completely abrogated the increase in ERPF and GFR elicited by chronic administration of purified porcine RLX. In contrast, the renal vasoconstrictory response to angiotensin II was attenuated by the RLX treatment. Short-term infusion of purified porcine RLX to conscious rats over several hours failed to increase ERPF and GFR. Plasma osmolality was consistently reduced by the chronic administration of both RLX preparations. In conclusion, the renal and osmoregulatory effects of chronic RLX administration to conscious rats resemble the physiological changes of pregnancy in several respects: (a) marked increases in ERPF and GFR with a mediatory role for nitric oxide; (b) attenuation of the renal circulatory response to angiotensin II; and (c) reduction in plasma osmolality.

Angiotensin II↗

PubMed1998

Circulating levels of immunoreactive cytokines in women with preeclampsia.

PROBLEM: Circulating inflammatory cytokines have been implicated in the pathogenesis of preeclampsia. To test this hypothesis, we measured plasma levels of immunoreactive tumor necrosis factor (TNF)-alpha and -beta, interleukin (IL)-1 alpha and -beta, and IL-6 and -10 in women with preeclampsia, in women with transient gestational hypertension, and throughout normal pregnancy. METHOD OF STUDY: Enzyme-linked immunosorbent assays were used and subjected to extensive validation studies. RESULTS: The median concentration of plasma TNF-alpha was increased by twofold in women with preeclampsia compared with that in normal third-trimester pregnancy (P < 0.001) and in women with gestational hypertension (P < 0.04). The median concentration of plasma IL-6 was increased by threefold in women with preeclampsia compared with that in normal third-trimester pregnancy (P < 0.001) and increased twofold compared with that in women with gestational hypertension (P < 0.1). There were no significant differences observed in the levels of plasma IL-1 beta and IL-10 between the preeclamptic and other subject groups. The level of IL-1 beta, but not the levels of IL-10, TNF-alpha, or IL-6, was significantly changed during normal pregnancy compared with the nonpregnant condition manifesting an overall decline (P < 0.04). TNF-beta and IL-1 alpha were not detected in any samples, possibly because of the low sensitivity of these particular immunoassays. CONCLUSION: Elevated levels of TNF-alpha and IL-6 may contribute to the putative endothelial dysfunction of preeclampsia.

Adult↗

PubMed1997

Placental cytokines and the pathogenesis of preeclampsia.

The authors explore the hypothesis that tumor necrosis factor-alpha (TNF-alpha) and possibly other inflammatory cytokines are overproduced by the placenta in response to local ischemia/hypoxia contributing to increased plasma levels, and subsequent endothelial activation and dysfunction in the pregnancy disorder, preeclampsia. It is widely held that inadequate trophoblast invasion and physiologic remodeling of spiral arteries initiate placental ischemia/hypoxia in preeclampsia. Furthermore, focal areas of placental hypoxia have been implicated in the production of "toxic" factor(s) by the placenta, which circulate and cause maternal disease. Placental trophoblast cells and fetoplacental macrophages normally produce TNF-alpha and interleukin-1 (IL-1), which are capable of producing endothelial cell activation and dysfunction. Hypoxia has recently been reported to increase TNF-alpha and IL-1 production by term villous explants from the human placenta. Placental cells also express erythropoietin (EPO), which is the prototype molecule for transcriptional regulation by hypoxia in mammals. Interestingly, TNF-alpha and IL-1 have DNA sequence homologous or nearly homologous to the hypoxia-responsive enhancer element of the EPO gene, thus providing a potential, but as of yet, untested molecular link between placental hypoxia and stimulation of cytokine production. Inflammatory cytokines overproduced by the placenta in response to hypoxia may then lead to increased plasma levels and endothelial activation and dysfunction in preeclampsia. The purpose of this short review is to critically evaluate the hypothesis that placental cytokines contribute to the pathogenesis of preeclampsia. Of note, the etiology of the disease presumably related to deficient trophoblast invasion is beyond the scope of this work.

Cytokines↗

PubMed1997

Nitric oxide and pregnancy.

This review will consider whether nitric oxide (NO) contributes to maternal systemic vasodilation during pregnancy, regulates uterine and fetoplacental blood flow, and is involved in uterine quiescence prior to parturition. Also, whether a deficiency of NO contributes to the hypertensive disorder of pregnancy, preeclampsia, will be considered. The biosynthesis of NO increases in gravid rats and sheep, but the status in normal human pregnancy and preeclampsia is controversial. NO contributes to maternal systemic vasodilation and reduced vascular reactivity during normal pregnancy; however, the relative contribution of NO is variable depending on the animal species, vascular bed, and vessel size. Impaired relaxation responses to acetylcholine, but not bradykinin or NO donors, are observed in small arteries from women with preeclampsia, suggesting a receptor or signal transduction defect, although NO may play little, if any, role here. Uterine arteries have increased endothelial nitric oxide synthase (NOS) activity, protein expression, and guanosine 3',5'-cyclic monophosphate production during pregnancy; however, whether these mediate uterine vasodilation during pregnancy remains to be established. NOS is expressed in the human placental syncytiotrophoblast and in the fetoplacental and umbilical vascular endothelium where basal production of NO contributes to low fetoplacental vascular resistance. Controversy exists over the status of placental NOS in preeclampsia, although an abnormality of umbilical NOS activity is likely. Finally, the uterus has NOS activity, which decreases at the end of gestation, and exogenous NO relaxes the myometrium, but whether endogenous NO contributes to uterine quiescence during pregnancy has yet to be confirmed.

Animals↗

PubMed1997

Intrinsic tone and passive mechanics of isolated renal arteries from virgin and late-pregnant rats.

The purpose of this study was to investigate whether there are alterations in the intrinsic properties of renal interlobar arteries during pregnancy. Renal interlobar arteries (internal diameter approximately 250 microns) from virgin and late-pregnant rats were mounted in a pressurized arteriograph system. Intrinsic tone was quantified as the percent difference in luminal diameter of each artery in the presence of physiological saline solution and while pharmacologically relaxed with papaverine. At pressures between 75 and 125 mmHg, tone was 35-50% less in arteries from pregnant rats (P < 0.05). Endothelial removal reduced tone in arteries from virgin rats but had no effect on arteries from pregnant rats. Analysis of stress-strain curves (rate constants: pregnant, 6.31 +/- 0.38; virgin, 7.81 +/- 0.78; P < 0.05) indicate that there is a decrease in arterial stiffness in gestation. Thus pregnancy is associated with a reduced intrinsic tone, possibly because of a reduction in an endothelial constrictor influence on the vascular smooth muscle in isolated rat renal interlobar arteries. This effect, coupled with the decreased arterial stiffness, demonstrates the significant arterial adaptation occurring during pregnancy.

Animals↗

PubMed1997

Hypoxia stimulates cytokine production by villous explants from the human placenta.

It has been hypothesized that inadequate placentation in the hypertensive disorder of pregnancy known as preeclampsia creates foci of placental ischemia/hypoxia leading to the elaboration of factors that compromise systemic endothelial function to produce disease sequelae. As tumor necrosis factor-alpha (TNF alpha) and interleukin-1 (IL-1) are inflammatory cytokines capable of eliciting endothelial cell dysfunction, we investigated whether the production of these inflammatory cytokines by cultured villous explants from the human placenta was affected by incubation in reduced oxygen (2% O2). The term placenta produced TNF alpha, IL-6, and low levels of IL-1alpha and IL-1beta under standard tissue culture conditions. Hypoxia significantly increased TNF alpha, IL-1alpha, and IL-1beta production by 2-, 6-, and 23-fold, respectively, but did not affect IL-6 production. Further, cytokines were immunolocalized to the syncytiotrophoblast layer as well as to some villous core cells. Hypoxic regulation of placental TNF alpha and IL-1beta production also appeared to differ based on gestational age. Finally, treatment with either cobalt chloride or an iron chelator mimicked the hypoxic response, suggesting that stimulation of placental cytokine production may involve a heme-containing, O2-sensing protein. These results suggest that placental hypoxia can lead to the elaboration of inflammatory cytokines, which may contribute to the pathophysiology of preeclampsia.

1,2-Dihydroxybenzene-3,5-Disulfonic Acid Disodium ↗

PubMed1996

Expression of erythropoietin by the human placenta.

Circulating levels of maternal erythropoietin (EPO) rise during gestation due to increased biosynthesis of the hormone. Our objective was to investigate the human placenta as a potential extrarenal site of EPO production. Using two monoclonal antibodies recognizing different antigenic determinants, we identified immunoreactive EPO associated with villous cytotrophoblast, endovascular and intravascular cytotrophoblast, cytotrophoblast cell columns, and syncytiotrophoblast of first and second-trimester placenta as well as syncytiotrophoblast and extravillous cytotrophoblast of normal third-trimester and preeclamptic placenta. In addition, cultured JAR (trophoblast-derived) choriocarcinoma cells, cytotrophoblasts isolated from term placenta, villous core cells, and possibly other nontrophoblast cells within the decidual basal plate expressed immunoreactive EPO. Using reverse transcription-polymerase chain reaction and EPO-specific primers, a 378 bp DNA product was amplified from placental tissues of various gestational ages, cytotrophoblasts isolated from term placenta, and JAR choriocarcinoma cells. The amplified product yielded restriction enzyme fragments of predicted sizes. On Southern analysis, hybridization was observed for two of these fragments in which the radiolabeled EPO cDNA probe did not overlap with the primer sequences. Finally, the JAR choriocarcinoma cells elaborated EPO into the culture medium as determined by enzyme-linked immunosorbant assay and expressed EPO mRNA as determined by Northern analysis, both of which were stimulated by hypoxia (15-20 torr). Taken together, these results suggest a new site of EPO expression: the trophoblast cell of the human placenta.

Enzyme-Linked Immunosorbent Assay↗

PubMed1996

Prostaglandins maintain renal vasodilation and hyperfiltration during chronic nitric oxide synthase blockade in conscious pregnant rats.

Rats demonstrate renal vasodilation and hyperfiltration in pregnancy. Because both NO and cGMP biosynthesis are increased in gravid rats and because acute administration of NO synthase inhibitors abrogates renal vasodilation and hyperfiltration, NO most likely mediates the renal circulatory changes of gestation. In the present study, we tested the effect of chronic inhibition of NO synthase on effective renal plasma flow (ERPF) and glomerular filtration rate (GFR) in chronically instrumented, conscious, gravid rats. Because gestation is a relatively long-term condition, we postulated that chronic withdrawal of NO would result in sustained inhibition of renal vasodilation and hyperfiltration. Contrary to our hypothesis, the renal circulatory changes of pregnancy were maintained during chronic blockade of NO synthase. That is, subcutaneous administration of 10 micrograms/min N omega-nitro-L-arginine methyl ester (NAME) for 48 hours did not significantly reduce GFR in either virgin or pregnant rats; thus, hyperfiltration persisted in the latter despite chronic NO synthase blockade. In contrast, ERPF was reduced and effective renal vascular resistance (ERVR) increased in both groups of rats during NAME administration but in a parallel fashion, such that renal vasodilation persisted in the gravid animals despite chronic inhibition of NO synthase. However, with superimposition of acute prostaglandin synthesis inhibition (meclofenamate, 10 mg/kg IV), renal vasodilation and hyperfiltration were abolished; ie, the combined treatments of chronic NO synthase blockade and acute prostaglandin synthesis inhibition led to the equalization of GFR, ERPF, and ERVR in conscious virgin and pregnant rats. Inhibition of prostaglandin synthesis alone had little affect on the renal circulation, as previously reported. In summary, prostaglandins are recruited to maintain renal vasodilation and hyperfiltration during chronic NO synthase blockade in conscious pregnant rats.

Animals↗

PubMed1995

Nitric oxide synthase activity in placentae from women with pre-eclampsia.

The syncytiotrophoblast (ST) cell layer of the human villous placenta expresses nitric oxide (NO) synthase. Because NO is a potent relaxant of vascular smooth muscle and inhibitor of platelet activity, we postulated that exaggerated intervillous aggregation of platelets and reduced fetoplacental blood flow in pre-eclampsia result from reduced expression of NO synthase (and production of NO) by the ST. Conversion of [3H]arginine to [3H]citrulline and Lineweaver-Burk transformation were used to derive the Vmax and K(M) of NO synthase. Contrary to our expectations, the Vmax was not significantly different between villous placenta obtained from nulliparous normal and pre-eclamptic women (n = 11 each). The Vmax and K(M) were 22.3 +/- 2.3 pmol/mg per min and 1.3 +/- 0.1 microns, and 22.0 +/- 2.7 pmol/mg per min and 1.4 +/- 0.1 microns, for villous placenta from the nulliparous normal and pre-eclamptic women, respectively. The Vmax and K(M) of placental NO synthase were also comparable among multiparous normal and pre-eclamptic women, as well as women with gestational hypertension. When compared with the enzyme activity of the villous, that of the basal plate was reduced by approximately one-half in all placentae. The calcium-independent activity was consistently 40-fold less than the calcium-dependent activity, and it was similar between villous and basal plate, and between placentae from normal and hypertensive women. We suggest that expression of NO synthase is not different in placentae obtained from normal and pre-eclamptic women.

Adult↗

PubMed1995

Nitric oxide biosynthesis during pregnancy: implications for circulatory changes.

1. The biosynthesis of NO and its second messenger, cGMP, increases from pre-pregnant levels during rat gestation. An increase in plasma level and urinary excretion of cGMP is also evident during human pregnancy. However, the relative contribution of the maternal vasculature and other tissues to increased NO and cGMP biosynthesis during gestation is uncertain. Consensus is lacking about the contribution of NO to reduced maternal vascular tone and reactivity during gestation in various organ beds; clearly, further investigation is still needed. That NO may also regulate vascular smooth muscle behaviour during pregnancy by altering membrane potential is another intriguing possibility. 2. The syncytiotrophoblast of the human placenta expresses significant NO synthase activity, and along with the fetoplacental endothelium undoubtedly contributes to NO production during pregnancy. 3. Finally, it should be emphasized that vascular studies in gravid animal models need to be extended to pregnant women.

Animals↗

PubMed1995

Metabolism of erythropoietin in conscious pregnant rats.

Circulating erythropoietin (EPO) concentration increases during human and rat gestation, thereby contributing to the expansion of red cell mass. However, the mechanism(s) underlying gestational increases of the hormone is unknown. Our objective was to define whether the elevated EPO levels are secondary to decreased metabolic clearance or to enhanced production. The half-life of the hormone was also measured. A bolus and a constant infusion of 125I-labeled recombinant human EPO (125I-rhEPO) were administered to chronically instrumented conscious pregnant and virgin rats. The metabolic clearance rate of the 125I-rhEPO was slightly but significantly higher in gravid rats than in the virgin control animals (0.13 +/- 0.01 vs. 0.10 +/- 0.01 ml/min). The plasma half-life of 125I-rhEPO was 2.9 +/- 0.1 h for the pregnant rats and 2.9 +/- 0.2 h for the virgin controls. To confirm these results obtained by using 125I-rhEPO, EPO-rich plasma was generated in anemic rats and administered to another group of conscious virgin and pregnant rats. The half-life of homologous EPO was 2.9 +/- 0.5 and 3.3 +/- 0.1 h for gravid and virgin rats, respectively (P = NS). We conclude that elevated circulating EPO in rat gestation is secondary to increased biosynthesis and not to decreased metabolic clearance.

Animals↗

PubMed1995

Acute blockade of nitric oxide synthase inhibits renal vasodilation and hyperfiltration during pregnancy in chronically instrumented conscious rats.

Because the kidneys are vasodilated and the endogenous production of nitric oxide is increased in gravid rats, we tested whether nitric oxide mediates the renal vasodilatory response to pregnancy. Chronically instrumented, conscious rats of gestational days 12-14 were studied concurrently with age-matched virgin control animals. GFR and effective renal plasma flow (ERPF) were determined by the renal clearances of inulin and para-aminohippurate before and during acute infusion of N omega-nitro-L-arginine methyl ester (NAME; 2, 20, and 50 micrograms/min) or NG-monomethyl-L-arginine (100 micrograms/min). Baseline GFR and ERPF were significantly increased, and effective renal vascular resistance was decreased by 30-40% in gravid rats compared with virgin controls. During infusion of all three dosages of NAME and NG-monomethyl-L-arginine, effective renal vascular resistance, GFR, and ERPF were equalized in the pregnant and virgin rats (the only exception being GFR during the 20 micrograms/min NAME infusion). When compared with virgin rats, the gravid animals were more responsive to nitric oxide synthase inhibition, showing a significantly greater decline in GFR and ERPF and rise in effective renal vascular resistance at each timepoint during the infusion of inhibitor. To exclude the possibility that nonspecific renal vasoconstriction per se led to equalization of renal function in the two groups of rats, we investigated angiotensin II. In contrast to the results observed with nitric oxide synthase inhibitors, pregnant rats were less responsive to the renal vasoconstrictory effects of angiotensin II, such that the baseline differences in renal parameters measured before infusion of the hormone were increased during the infusion. To determine whether nitric oxide synthase was inhibited to a similar extent in gravid and virgin rats, aortic and renal cortical cGMP content was assayed ex vivo at the end of inhibitor infusion. The lower 2-micrograms/min dose of NAME consistently reduced cGMP content of these tissues to comparable levels in the two groups of rats. In conclusion, we suggest that nitric oxide mediates reduced renal vascular resistance and hyperfiltration during pregnancy in conscious rats.

Amino Acid Oxidoreductases↗

PubMed1994

Effects of 17 beta-estradiol and progesterone on pressor responses in conscious ovariectomized rats.

Attenuation of pressor responsiveness to several administered vasoconstrictors is a constant feature of normal gestation in humans and other species, such as the rat. However, the mechanism of this physiological adaptation remains uncertain. Because plasma levels of 17 beta-estradiol (E2) and progesterone (P) increase markedly during pregnancy, we tested the hypothesis that these hormones may mediate the reduced pressor responses. Seven days after bilateral ovariectomy and chronic instrumentation of rats, the pressor responses of arginine vasopressin, angiotensin II, and norepinephrine were tested on two occasions > or = 48 h apart. Then E2, P, or a combination of E2 and P was administered by subcutaneous implantation of 21-day-release steroid pellets. Pressor responses were again tested at various times throughout the period of steroid treatment. The plasma concentrations of the steroids were assessed by radio-immunoassay, and doses of the hormones were given that both approximated and exceeded circulating levels found in our laboratory for gravid rats. Despite chronic elevation of plasma E2 and/or P, we did not observe consistent attenuation of pressor responsiveness in any of the steroid-treatment regimens, nor was a decline in mean arterial pressure observed, which is typically found in rats during late gestation. In conclusion, we are unable to support the hypothesis that E2 and/or P contributes to the diminished pressor responsiveness of rat pregnancy.

Angiotensin II↗