The incidence of congenital heart defects in Asian children is significantly higher than in non-Asian, however little data are available for other anomalies. The Fetal Management Unit at St Mary's Hospital is a tertiary referral centre for prenatal diagnosis in the north-west region. Using data collected after routine prenatal ultrasonography between 1996-2001, we show that in a defined population there was a significant reduction in the incidence of central nervous system (CNS) anomalies over this period but not in other anomalies. Furthermore, fetal congenital anomalies were diagnosed in a higher proportion of Asian than non-Asian women, with CNS, cardiac, bowel, thoracic and facial anomalies and hydrops being statistically significant. Monitoring local trends in a multiethnic community is essential for appropriate counselling, providing parents with an informed choice and in assessing the effectiveness of interventions.

The placenta is the organ in which the two microcirculations, maternal and fetal, are opposed and across which most, but not all transfer takes place. For any permeant to cross the placenta, it has to traverse, or bypass, several lipid bilayer barriers and several fluid compartments. For some, transfer may also involve the release from or uptake by blood cells, plasma complexes, or carrier proteins; or metabolic interconversion within the placenta. The flux (rate of transfer) through each of the barriers is a function of both the ease of movement through it (its permeability) and on the difference in relevant electrical, chemical, hydrostatic, or thermal driving force across it. Movement through the fluid compartments is likely to be a function of diffusion and convection and is usually assumed not to be rate limiting. Overall, the rate will depend on the perfused surface area of the placenta. The capacity of the placenta to transfer solutes and water has to be sufficient to allow fetal growth. Moreover, it is likely that the transfer capacity of the placenta also changes during the course of pregnancy.

This study investigates P-gp activity in placental villous fragments and the possibility of upregulating its expression and function by retroviral transduction. In fresh fragments, cyclosporin A caused a significant increase in 3H-vinblastine accumulation (187 ± 48% at 180 min n = 4), consistent with multi-drug resistance activity. After 7 days in culture, villous fragments showed a similar increase in 3H-vinblastine accumulation (143 ± 10% at 180 min n = 4), which was not significantly different from that in fresh tissue. Following transduction, immunohistochemistry revealed increased P-gp expression. However, the distribution of the protein differed from that in controls, with P-gp being located throughout the tissue as opposed to the normal specific location on the maternal facing plasma membrane. Transduced explants showed a significantly larger increase in 3H-vinblastine accumulation in the presence of cyclosporin A than control explants (245 ± 15.5% at 180 min, n = 4), suggesting reduced capacity to efflux vinblastine. This study demonstrates P-gp activity in intact placental tissue which is maintained in explant culture. Retroviral transduction of P-gp to such tissue leads to increased but undirected expression of the protein. The consequent increased activity at sites such as the basal, fetal facing, plasma membrane probably explains the increased substrate accumulation within the tissue.

Homeostasis and functional regulation are among the most characteristic and essential features of organisms. Intracellular pH is especially critical, and organisms of all types have evolved elaborate and metabolically costly mechanisms for its stabilization. Of necessity, those mechanisms vary widely in order to be effective in types as diverse as bacteria and mammals, living in habitats that range from aqueous to xeric and from well-aerated to anoxic.

To address the hypothesis that impaired ATP synthesis rates caused by changes in the creatine kinase system is an important mechanism underlying cardiac failure, we measured total creatine kinase activity, isoenzyme composition and creatine content in two animal models of hypertrophy with cardiac dysfunction, the spontaneously hypertensive rat in the transition to failure and the creatine-depleted hyperthyroid rat heart challenged by hypoxia. During the transition from stable compensated hypertrophy to failure characterized by decreased functional capacity, we found that total creatine kinase activity and particularly mitochondrial creatine kinase activity decreased. The decrease in functional capacity, the further increase in heart size and the derangements in the creatine kinase system did not occur if these animals were treated for 6 months with the antihypertensive agents, guanethidine or hydralazine. These results suggest that changes in the creatine kinase system occur coordinately with the transition to failure. To assess whether the changes in the creatine system may be causally linked to decreased functional capacity, we used 31P NMR spectroscopy of isolated perfused hearts to define the high energy phosphate content and cardiac performance of creatine-depleted (approximately 50%) hypertrophied hearts challenged by hypoxia. These hearts displayed greater susceptibility to hypoxic injury with regard to both systolic and diastolic function during and following hypoxia. We also measured total creatine kinase activity in right ventricular biopsy specimens from patients with various forms of cardiomyopathy and low ejection fractions, and found a positive correlation between total creatine kinase activity and ejection fraction. Taken together, these results support the hypothesis that decreasing the energy reserve for ATP synthesis renders the heart more susceptible to systolic and diastolic failure.

Liver glutaminase is stimulated by an increase in NH4+ concentration and NH4+ is an absolute requirement for activity at approximate physiological concentrations of phosphate and glutamine. Increases in the concentration of NH4+ cannot, however, overcome the inhibitory effect of a decrease in pH. In addition, the concentration of NH4+ required for half-maximal rate decreases as pH increases. This decrease is the result of two factors: a direct effect of pH on the apparent affinity of the enzyme for NH4+, and an indirect effect of pH brought about by an increase in the apparent affinity of the enzyme for phosphate which results in a further decrease in the M0.5 for NH4+. In addition, liver glutaminase responds strongly to the concentration of citrate over a physiologically relevant range at approximate physiological concentrations of NH4+, phosphate, and glutamine. An increase in citrate concentration stimulates glutaminase by increasing the affinity of the enzyme for glutamine. The apparent affinity of the enzyme for citrate increases as pH increases. The strong response of liver glutaminase to pH, NH4+, and citrate and the fact that the hydrolysis of glutamine can supply metabolites and effectors for urea synthesis suggest a possible regulatory role of glutaminase in ureagenesis.

The activity of rat liver glutaminase from sedimented fractions of freeze-thawed mitochondria is strongly affected by variation in the Mg2+ concentration within the approximate physiological range of activators. A rise in the Mg2+ concentration stimulates glutaminase by increasing the apparent affinity of the enzyme for its positive modifier phosphate. With the addition of 4 mM Mg2+ the M0.5 for phosphate activation decreased from 18 to 9.5 mM at pH 7.1, 10 to 5.8 mM at pH 7.4 and 6.4 to 4.0 mM at pH 7.7. The result is an increase in the apparent affinity of the enzyme for glutamine. With the addition of 4 mM Mg2+ the S0.5 of glutaminase for glutamine decreased from 24 to 13 mM at pH 7.1, 14 to 9.6 mM at pH 7.4, and remained unchanged at 8.2 mM at pH 7.7. Since Mg2+ stimulates glutaminase, as does a rise in pH (Szweda, L.I. and Atkinson, D.E. (1989) J. Biol. Chem. 264, 15357–15360), by increasing the apparent affinity of the enzyme for phosphate, it reduces the inhibitory effect of a decrease in pH and/or phosphate concentration over a physiologically relevant range.

The activity of rat liver glutaminase from sedimented fractions of freeze-thawed mitochondria is strongly affected by variation in pH over a physiologically relevant range at approximate physiological concentrations of activators. As pH increases from 7.1 to 7.7 at 0.7 mM ammonium and 10 mM phosphate, the S0.5 for glutamine decreases 3.5-fold, from 38 to 11 mM. This results in an 8-fold increase in reaction velocity at 10 mM glutamine. In addition, the M0.5 for phosphate activation decreases from 21 to 8.9 mM as pH increases from 7.1 to 7.7. This apparent effect of pH on the affinity of glutaminase for phosphate is similar to previous reports of the pH effect on activation by ammonium (Verhoeven, A. J., Van Iwaarden, J. F., Joseph, S. K., and Meijer, A. J. (1983) Eur. J. Biochem. 133, 241-244; McGivan, J. D., and Bradford, N. M. (1983) Biochim. Biophys. Acta 159, 296-302). Glutaminase does not respond to variation in pH between 7.1 and 7.7 when phosphate and ammonium are saturating. The effects of the two modifiers are additive. Each is still effective, as is pH, when the other is saturating. Therefore, it appears that the effects of pH on the apparent affinity of the enzyme for ammonium and phosphate account for the enzyme's response to pH. These results may help explain previous reports of minimal effects of pH on glutaminase at saturating concentrations of related substances (McGivan, J. D., Lacey, J. H., and Joseph, K. (1980) Biochim. J. 192, 537-542; Horowitz, M. L., and Knox, W. E. (1968) Enzymol. Biol. Clin. 9, 241-255; McGivan, J. D., and Bradford, N. M. (1983) Biochim. Biophys. Acta 759, 296-302). Glutaminase binds glutamine cooperatively with Hill coefficients ranging from 1.7 to 2.2, which suggests at least two and probably three or more interacting binding sites for glutamine. The strong response of liver glutaminase to pH and the fact that the reaction can supply metabolites for urea synthesis suggest a possible regulatory role of glutaminase in ureagenesis.

Catabolism of protein produces CO2, NH4+, and HCO3-. Mammals readily lose CO2 through the lungs, but the bicarbonate produced in metabolism of a typical diet (in humans, approximately 1 mol/day from approximately 100 g of protein) would cause alkalosis if not disposed of. Air-breathing animals solve this problem by incorporating NH4+ into organic compounds in which N is not protonated; thus each NH4+ ion loses a proton in the course of the synthesis. These protons serve to titrate HCO3-. In mammals, ureagenesis is the pathway by which protons are liberated from NH4+. The rate of ureagenesis therefore determines the rate of disposal of bicarbonate, and must be an important factor in the maintenance of pH homeostasis. Ammonium ion that is not needed for urea synthesis is packaged into glutamine by the liver. Hepatic glutamine synthetase is localized in the last rank of cells around the pericentral venule; thus glutamine synthetase cannot compete for NH4+ or interfere with the control of pH by urea synthesis. Ammonium excretion in the urine does not represent excretion of acid, and is not stoichiometrically related to renal generation of bicarbonate. The quantitatively major processes by which the HCO3-/CO2 ratio, and hence the pH, is regulated in blood and interstitial fluid are excretion of CO2 through the lungs and disposal of HCO3- as a consequence of ureagenesis in the liver.

ARTICLESUreagenesis and pH homeostasisD. E. AtkinsonD. E. AtkinsonPublished Online:01 Jun 1986https://doi.org/10.1152/ajprenal.1986.250.6.F1128MoreFiguresReferencesRelatedInformationPDF (858 KB)Download PDF ToolsExport citationAdd to favoritesGet permissionsTrack citations ShareShare onFacebookTwitterLinkedInWeChat Previous Back to Top Next Download PDF FiguresReferencesRelatedInformationSee PDF More from this issue > Volume 250Issue 6June 1986Pages F1128-F1130 Copyright & PermissionsCopyright © 1986 the American Physiological Societyhttps://doi.org/10.1152/ajprenal.1986.250.6.F1128PubMed3717351History Published online 1 June 1986 Published in print 1 June 1986 Metrics Downloaded 16 times 1 CITATION 1 Total citation 0 Recent citations n/a Field Citation Ratio 0.05 Relative Citation Ratio publications0supporting0mentioning0contrasting0Smart Citations0000Citing PublicationsSupportingMentioningContrastingView CitationsSee how this article has been cited at scite.aiscite shows how a scientific paper has been cited by providing the context of the citation, a classification describing whether it supports, mentions, or contrasts the cited claim, and a label indicating in which section the citation was made.