Colin T. Jones

Acth peptides and the development of the fetal adrenal

Abstract The response to ACTH 1–39 of adrenal cells from fetal sheep, monkeys and guinea pigs were compared with that of cells from adult glands. Although the cortisol production rate was lower from the fetal cells they responded to physiological rises in ACTH concentration by large increases in output. The proportional increase in cortisol production was of the same order in the fetal and adult tissue. The in vitro responses of fetal tissue contrasts with the data available on in vivo function. Analysis of ACTH in fetal plasma and pituitary show it to be present in high-molecular-weight peptides and ACTH 1–39 At the time of adrenal hypertrophy and increase cortisol secretion prior to birth the concentration ratio in plasma of ACTH 1–39 /“big” peptides increases markedly. The big peptides inhibit the response to ACTH 1–39 of the fetal adrenal cells. These peptides may be responsible for suppressing the fetal adrenal in vivo and the fall in their relative contribution, late in gestation, to the total ACTH pool may be one of the factors initiating parturition.

The appearance, properties, and functions of gluconeogenic enzymes in the liver and kidney of the guinea pig during fetal and early neonatal development

Abstract The changes in the activity and properties of the four gluconeogenic enzymes have been followed during development of the guinea pig. Pyruvate carboxylase was almost exclusively mitochondrial and kinetically identical to the adult liver enzyme and did not appear in significant activity until after day 50 when it rose to values several times higher than those in the adult liver, then fell after birth. Little activity was detected in the fetal kidney. Phosphoenolpyruvate carboxylase appeared in the fetal liver from day 30 on, both in the mitochondrial and cytoplasmic fractions. The cytoplasmic enzyme was kinetically and chromatographically identical to the mitochondrial enzyme of the fetal and maternal liver. After birth the activity of the cytoplasmic enzyme increased and that of the particulate enzyme fell. Fetal kidney activity appeared several days before birth. Fructose 1,6-diphosphatase and glucose 6-phosphatase appeared in the fetal liver and kidney after day 40; the former showed no postnatal change while the latter rose 10-fold after birth. Fetal liver fructose 1,6-diphosphatase was more sensitive to AMP and fructose 1,6-diphosphate inhibition but was chromatographically indistinguishable from the maternal liver enzyme. Despite the presence of the gluconeogenic enzymes, gluconeogenesis and glyconeogenesis were not detected in the fetal liver until 7–9 days before birth. While the synthesis of glyceride-glycerol from 3-carbon compounds was detected from 35–40 days onwards and some of the gluconeogenic enzymes participate in that pathway, gluconeogenesis was not detected in the fetal kidney.

Is there a gloxylate cycle in the liver of the fetal guinea pig

Abstract Studies on the effect of the inhibitor of fatty acid oxidation (+)-octanoylcarnitine on the perfused liver of the 48–51 days fetal guinea pig indicate that the oxidation of endogenous fatty acids is a major source of carbon for the citric acid cycle and for synthesis of hexose. Consistent with this the liver can convert isocitrate to glyoxylate and glyoxylate to malate and may therefore operate a glyoxylate cycle allowing the net production of sugars from acetyl-CoA.

DEVELOPMENT OF FETAL PITUITARY-ADRENAL-FUNCTION

Abstract The factors responsible for the growth of the fetal adrenal, and its increase in cortisol secretion during late pregnancy have been examined in experimental studies in sheep. The fetal pituitary is necessary for these changes to occur. However, the increase in cortisol secretion at term is not preceded by an increase in ACTH in fetal plasma. Prolactin increases in parallel with fetal cortisol. ACTH is elevated in fetal plasma during late pregnancy, despite the demonstration of a negative feedback relationship with cortisol, suggesting an overriding stimulus to ACTH. Comparison of fetal ACTH levels during Synacthen (β1–24 ACTH) infusion into intact and hypophysectomized fetal lambs demonstrates endogenous ACTH release at this time. The possible mechanisms by which negative feedback may be overridden are discussed. Studies in vivo and in vitro show that there is a maturation of fetal adrenal sensitivity during late pregnancy. In vivo at days 120–130, ACTH provokes little or no increase in the fetal adrenal secretion of cortisol, whereas intra-fetal infusion of prostaglandin E2, but not PGF2α, provokes a rapid 300% increase in the cortisol concentration in fetal plasma. These findings indicate that the fetal adrenal has the pathway necessary for cortisol production by day 130 p.c., and suggest that adrenal maturation is a key factor in the release of cortisol which precedes parturition.

Pyruvate kinase isoenzymes in tissues of the developing guinea pig

Abstract Pyruvate kinase activities and isoenzymes have been followed during the development of the fetal and neonatal guinea pig. The kinetic properties of the adult isoenzymes were not substantially different from those previously reported for the rat except pyruvate kinase 1 isolated from liver was far less sensitive to l -alanine inhibition. The kinetic properties of the isoenzymes isolated from the fetal tissues were the same as those of the adult. Fetal liver contained pyruvate kinase 1 and 4 in comparable activity throughout the period of gestation studied. Up to five bands of activity in the region of pyruvate kinase 3 and 4 were detected in the brain and muscles of developing guinea pig after electrophoresis. Early in gestation the bands with mobility close to pyruvate kinase 4 were predominant; during development these disappeared and bands of activity with mobility close to pyruvate kinase 3 were seen. The properties and distribution of the pyruvate kinase isoenzymes are discussed in relation to the control of glycolysis in developing tissue. The possible molecular significance of multiple forms between pyruvate kinase 3 and 4 is discussed.

Functional activation by glucagon of glucose 6-phosphatase and gluconeogenesis in the perfused liver of the fetal guinea pig

The fetal livers of several species have the enzymic capacity for gluconeogenesis [l-4]. Despite this, de novo glucose synthesis has not been demonstrated, and glucose output from the perfused fetal liver [5-71 is probably the result of glycogenolysis rather than gluconeogenesis. Studies with the perfused fetal guinea pig liver have indicated that at 50 days gestational age no gluconeogenesis occurs in the presence of a range of gluconeogenic substrates, even though the liver has moderate activities of the gluconeogenic enzymes. The reasons for this are unclear. Glucagon is a potent activator of gluconeogenesis in adult livers [8] and stimulated the incorporation of [14C]alanine into glucose by the perfused liver of the near-term (63 day) guinea pig fetus [9]. Thus the effects of glucagon on the perfused fetal guinea pig liver at 50 days have been investigated. The results indicate that glucagon stimulates gluconeogenesis and causes functional activation of glucose 6-phosphatase.

Hexokinase isoenzymes in tissues of the adult and developing guinea pig

Abstract Changes in the activities and isoenzyme distribution of hexokinase were determined in a number of tissues during the development of the guinea pig. The total activity in the fetal liver showed a large fall during the second half of gestation to reach adult values by term. With normal diet the fetal, neonatal, and adult livers had isoenzymes I and III but little or no detectable IV (glucokinase). The fetal liver had predominantly type I, but the proportion of type III increased during development. The kinetics of the guinea pig isoenzymes were similar to those reported for the rat. Two additional isoenzymes with mobility between I and II were detected in the fetal liver and blood. They appear to have kinetic properties similar to type I. Detectable liver glucokinase activity was induced by glucose administration to adult guinea pigs. The total activity in kidney, brain and skeletal muscle showed a postnatal rise while in the fetal heart it was high and declined after birth. These tissues contained predominantly type I with varying proportions of type III hexokinase. The ratio of particulate-bound to soluble hexokinase varied from tissue to tissue. All except the liver showed a significant increase in binding after birth. The changes are discussed in relation to the control of glucose utilization in the fetal and neonatal periods.

Pyruvate kinase isoenzymes in tissues of the human fetus

Four major isoenzymes of pyruvate kinase (EC 2.7.1.40) have been identified in various animal tissues which, based on increasing mobility, have been labelled PK4* (alternatively M,), PK3 (alternatively M,), PK2 and PKl (alternatively liver L) [l-7]. The presence of pyruvate kinase isoenzymes in tissues of the fetal rat [8-l 31 and fetal guinea pig [ 141 has been described. The fetal rat liver possesses little PKl activity particularly early in gestation while we found that PKl represented a major portion of the total pyruvate kinase activity of the fetal guinea pig liver. Moreover Balinsky et al. [ 151 recently demonstrated that PKl may represent a major portion of total activity in the 20 week human fetal liver. In addition to the 4 major isoenzymes fetal guinea pig tissues were found to possess additional pyruvate kinase isoenzymes of mobilities intermediate between PK3 and PK4 [14]. The present work describes further studies on the pyruvate kinase isoenzymes in fetal tissues. Pyruvate kinase activity has been measured in tissues from human fetuses and the distribution of isoenzymes determined by electrophoresis and chromatography. PKI and a previously unidentified isoenzyme have been found in substantial amounts in the fetal liver. The pattern in the other tissues is discussed in relation to the changes observed during development.

Placental Metabolism and Endocrine Effects in Relation to the Control of Fetal and Placental Growth

In the search for factors that will explain the close interrelationship between placental and fetal growth there have been relatively few candidates providing clear pathways of regulation (Gluckman 1986; Jones 1989). The strongest candidate has been placental lactogen, but convincing evidence that it is a clear placental signal regulating fetal growth is awaited. The other strong candidates, the insulin-like growth factors (IGFs), also await clear evaluation as far as a role in coordinating fetal-placental growth is concerned (Gluckman 1986; Jones 1989). They are produced by both fetal and placental tissue (D’Ercole et al. 1980; Fant et al. 1986; Gluckman 1986; Jones 1988) and their concentration changes in reponse to manipulations of prenatal growth rate (Gluckman 1986; Jones 1989; Jones et al. 1987; Vileisis et al. 1986), but more detailed information on their biochemical actions on placental and fetal tissues is required before their precise significance can be ascertained.

Lipid synthesis in vivo by tissues of the maternal and foetal guinea pig.

The rate of lipid biosynthesis in vivo was determined in pregnant guinea pigs after maternal and foetal injections of 3H2O. Synthesis in the maternal tissues was low and in the foetal liver and adipose tissues relatively high. In the foetal liver it reached a peak at about two-thirds of gestation, whereas that in the foetal adipose tissue occurred later. These results were used to support the view that lipid synthesis in the foetal guinea-pig liver at two-thirds of gestation is largely from short-chain fatty acids, whereas in foetal adipose tissue glucose is probably the major substrate.