Since the first identification of renin by Tigerstedt and Bergmann in 1898, the renin-angiotensin system (RAS) has been extensively studied. The current view of the system is characterized by an increased complexity, as evidenced by the discovery of new functional components and pathways of the RAS. In recent years, the pathophysiological implications of the system have been the main focus of attention, and inhibitors of the RAS such as angiotensin-converting enzyme (ACE) inhibitors and angiotensin (ANG) II receptor blockers have become important clinical tools in the treatment of cardiovascular and renal diseases such as hypertension, heart failure, and diabetic nephropathy. Nevertheless, the tissue RAS also plays an important role in mediating diverse physiological functions. These focus not only on the classical actions of ANG on the cardiovascular system, namely, the maintenance of cardiovascular homeostasis, but also on other functions. Recently, the research efforts studying these noncardiovascular effects of the RAS have intensified, and a large body of data are now available to support the existence of numerous organ-based RAS exerting diverse physiological effects. ANG II has direct effects at the cellular level and can influence, for example, cell growth and differentiation, but also may play a role as a mediator of apoptosis. These universal paracrine and autocrine actions may be important in many organ systems and can mediate important physiological stimuli. Transgenic overexpression and knock-out strategies of RAS genes in animals have also shown a central functional role of the RAS in prenatal development. Taken together, these findings may become increasingly important in the study of organ physiology but also for a fresh look at the implications of these findings for organ pathophysiology.

Physiology of local renin-angiotensin systems.

Choline is required to make essential membrane phospholipids. It is a precursor for the biosynthesis of the neurotransmitter acetylcholine and also is an important source of labile methyl groups. Mammals fed a choline-deficient diet develop liver dysfunction; however, choline is not considered an essential nutrient in humans. Healthy male volunteers were hospitalized and fed a semisynthetic diet devoid of choline supplemented with 500 mg/day choline for 1 wk. Subjects were randomly divided into two groups, one that continued to receive choline (control), and the other that received no choline (deficient) for three additional wk. During the 5th wk of the study all subjects received choline. The semisynthetic diet contained adequate, but no excess, methionine. In the choline-deficient group, plasma choline and phosphatidylcholine concentrations decreased an average of 30% during the 3-wk period when a choline-deficient diet was ingested; plasma and erthrocyte phosphatidylcholine decreased 15%; no such changes occurred in the control group. In the choline-deficient group, serum alanine aminotransferase activity increased steadily from a mean of 0.42 mukat/liter to a mean of 0.62 mukat/liter during the 3-wk period when a choline-deficient diet was ingested; no such change occurred in the control group. Other tests of liver and renal function were unchanged in both groups during the study. Serum cholesterol decreased an average of 15% in the deficient group and did not change in the control group. Healthy humans consuming a choline-deficient diet for 3 wk had depleted stores of choline in tissues and developed signs of incipient liver dysfunction. Our observations support the conclusion and choline is an essential nutrient for humans when excess methionine and folate are not available in the diet.

Choline, an essential nutrient for humans.

Choline: an essential nutrient for humans.

Choline is a dietary component essential for normal function of all cells. It, or its metabolites, assures the structural integrity and signaling functions of cell membranes; it is the major source of methyl-groups in the diet (one of choline's metabolites, betaine, participates in the methylation of homocysteine to form methionine); and it directly affects nerve signaling, cell signaling and lipid transport/metabolism. In 1998, the National Academy of Sciences, USA, issued a report identifying choline as a required nutrient for humans and recommended daily intake amounts. Eggs are an excellent dietary source of choline. Pregnancy and lactation are periods when maternal reserves of choline are depleted. At the same time, the availability of choline for normal development of the brain is critical. When rat pups received choline supplements (in utero or during the second week of life), their brain function changed, resulting in the lifelong memory enhancement. This change in memory function appears to be due to changes in the development of the memory center (hippocampus) in the brain. The mother's dietary choline during a critical period in brain development of her infant influences the rate of birth and death of nerve cells in this center. These changes are so important that we can pick out the groups of animals whose mothers had extra choline even when these animals are elderly. Thus, memory function in the aged rat is, in part, determined by what the mother ate. This is not the first example of a critical nutrient that must be present at a specific time in brain development. If folate isn't available in the first few weeks of pregnancy, the brain does not form normally. Thus, we suggest that pregnancy is a period when special attention has to be paid to dietary intake.

Choline: Needed for Normal Development of Memory

Choline, a micronutrient found in food, serves as the starting material for several important metabolites that play key roles in fetal development, particularly the brain. Although human beings' requirement for choline is unknown, an Adequate Intake level of 425 mg/day was established for women with upward adjustments to 450 and 550 mg/day during pregnancy and lactation, respectively. The importance of choline in human development is supported by observations that a human fetus receives a large supply of choline during gestation; pregnancy causes depletion of hepatic choline pools in rats consuming a normal diet; human neonates are born with blood levels that are three times higher than maternal blood concentrations; and large amounts of choline are present in human milk. The development of the central nervous system is particularly sensitive to choline availability with evidence of effects on neural tube closure and cognition. Existing data show that the majority of pregnant (and presumably lactating) women are not achieving the target intake levels and that certain common genetic variants may increase requirements for choline beyond current recommendations. Because choline is not found in most varieties of prenatal vitamins (or regular multivitamins), increased consumption of choline-rich foods may be needed to meet the high pre- and postnatal demands for choline.

Pre- and Postnatal Health: Evidence of Increased Choline Needs

SUMMARY

1. The concentration of free choline and cephalin-N-methyltransferase activity of the maternal and foetal liver and placenta of rats in late pregnancy were determined.



2. The choline concentration of the liver fell from a mean of 130 nmol/g in adult non-pregnant rats to 38 nmol/g in late pregnancy.



3. The choline concentrations of the foetal liver and placenta had mean values of 135 and 442 nmol/g, respectively.



4. There was a significant 24% increase in the cephalin-N-methyltransferase activity of the liver during pregnancy. The enzyme activity in the foetal liver was 7% that of the maternal liver, and activity was undetectable in the placenta.



5. The results suggest that, during pregnancy, the maternal liver supplies choline to the placenta and foetal organs, like the liver, and that the demand for maternal liver choline far exceeds the supply available.

Free choline concentration and cephalin-N-methyltransferase activity in the maternal and foetal liver and placenta of pregnant rats.

Angiotensin converting enzyme (ACE) activity in fetal lung and placenta was determined by the method of Cushman and Cheung (1971) during the second trimester of human pregnancy and throughout the rat gestation. The enzyme activity in the neonatal rat lung was also determined during the first 19 days of life. The enzyme activity in both tissues of both species increased with gestation. The activity in human fetal lung at the end of the second trimester was already 70% of that present in the adult human lung while rat fetal lung enzyme activity at term was only 15% of the adult value. The activity in the term placenta of the human and rat was respectively 13% and 5% that of the adult lung value. Developmental increases in enzyme activity continued in the neonatal rat lung till adult value at about 19 days postpartum. The pattern of fetal lung enzyme development in the rat resembled that of the rabbit fetal lung as determined by other investigators using different techniques but was different from that of the human. The findings support the suggestion that ACE in the lung and placenta play important roles in the maintenance of circulatory homeostasis during the latter part of gestation, at birth and early postpartum, albeit at a different extent in different species.

Development of angiotensin converting enzyme in fetal lung and placenta of the rat and human.

Summary

1. The concentration of free choline and cephalin-N-methyltransferase activity were determined for the rat maternal and foetal liver and placenta during gestation, and for the maternal and neonatal liver during the early postpartum period. Choline was determined by bioassay and the enzyme activity by using S-adenosyl-L-(14C-methyl)-methionine.



2. There was a significant and progressive fall in the choline concentration (nmol/g tissue) of the maternal liver from the control value of 130 (s.e.m. = 9) to 93 (s.e.m. = 9) after about 11 days gestation and to 38 (s.e.m. = 1) from about 17 days gestation to term; however, there was a gradual rise in the corresponding enzyme activity (counts/min) from a mean value of 561, s.e.m. = 7, (control to 618 (s.e.m. = 14) and then to 693 (s.e.m. = 19).



3. At 1 day postpartum both the choline concentration (166, s.e.m. = 9) and the enzyme activity (738, s.e.m. = 45) of the maternal liver increased appreciably and reached a peak before gradually declining to the control value at about 3 and 7 days postpartum, respectively.



4. The hepatic choline concentration of the foetus at 20 to 22 days gestation (135, s.e.m. = 6) was similar to adult (control) levels, although its cephalin-N-methyltransferase activity was very low (about 20% of control).



5. The choline concentration of the neonatal liver increased to 220 (s.e.m. = 21) and then to 272 (s.e.m. = 17) at 1 day and at about 3 days postpartum followed by a decline to the adult (control) value at about 7 days postpartum. The enzyme activity, however, increased continuously to attain adult levels at about 10 days postpartum.



6. The placental choline concentration was about three to four-fold greater than that of the control liver. The values obtained were 370, s.e.m. = 27 (14 to 16 days gestation), 426, s.e.m. = 27 (17 to 19 days gestation) and 442, s.e.m. = 18 at term. However, the cephaline-N-methyltransferase activity was undetectable in the placentae.



7. The choline concentration of the maternal plasma(5.3, s.e.m. = 0.7) remained essentially constant throughout gestation and in the early postpartum period.



8. The results strongly suggest that, during pregnancy, the rat maternal liver supplies free choline to the placenta and foetal liver and that the demand for maternal liver choline probably exceeds the supply available.

Changes in the concentration of free choline and cephalin-N-methyltransferase activity of the rat material and foetal liver and placeta during gestation and of the maternal and neonatal liver in the early postpartum period.

The anti-acetylcholinesterase (AChEase) activity of 15 opioids, namely morphine, nalorphine, oxymorphone, naloxone, naltrexone, N-methylnaloxone, levorphanol, levallophan, dextrorphan, dextrallophan, levo-methadone, dextro-methadone, pethidine, leu-enkephalin and U-50,488H was investigated. Among the opioids, the morphinans (in terms of their dissociation constants) are the strongest inhibitors of the enzyme. These are followed by the phenanthrenes and the morphones, though two members of the latter group are non-inhibitors. Levo-methadone, dextro-methadone, pethidine and leu-enkephalin are also non-inhibitors of the enzyme. The present results, together with our previous findings, support the suggestion that opioids which are poor or non-inhibitors of AChEase are strong inhibitors of the ACh-induced contraction of the toad rectus, whereas the reverse is true for inhibitors of AChEase.

Inhibition of acetylcholinesterase by various opioids.

SUMMARY

1. Intracerebroventricular captopril produces significantly different effects on the sleep electroencephalogram (EEG) of normotensive Wistar-Kyoto (WK) and spontaneously hypertensive rats (SHR).



2. The compound produces a definite pattern of decrease in the percentage and power of the lower frequency EEG of the SHR. Corresponding action on the WK was confined to a general decrease in the power of the EEG.



3. These findings suggest that certain components of the central renin-angiotensin system in the SHR are abnormal.

M. K. Sim

Papers

Free choline concentration and cephalin-N-methyltransferase activity in the maternal and foetal liver and placenta of pregnant rats.

Development of angiotensin converting enzyme in fetal lung and placenta of the rat and human.

Changes in the concentration of free choline and cephalin-N-methyltransferase activity of the rat material and foetal liver and placeta during gestation and of the maternal and neonatal liver in the early postpartum period.

Inhibition of acetylcholinesterase by various opioids.

Effects of captopril on the sleep eeg of spontaneously hypertensive rats