Reduced systolic pressure load decreases cell-cycle activity in the fetal sheep heart.

The fetal heart is highly sensitive to changes in mechanical load. We have previously demonstrated that increased cardiac load can stimulate cell cycle activity and maturation of immature cardiomyocytes, but the effects of reduced load are not known. Sixteen fetal sheep were given either continuous intravenous infusion of lactated Ringer solution (LR) or enalaprilat, an angiotensin-converting enzyme inhibitor beginning at 127 days gestational age. After 8 days, fetal arterial pressure in the enalaprilat-infused fetuses (23.8 +/- 2.8 mmHg) was lower than that of control fetuses (47.5 +/- 4.7 mmHg) (P < 0.0001). Although the body weights of the two groups of fetuses were similar, the heart weight-to-body weight ratios of the enalaprilat-infused fetuses were less than those of the LR-infused fetuses (5.6 +/- 0.5 g/kg vs. 7.0 +/- 0.6 g/kg, P < 0.0001). Dimensions of ventricular myocytes were not different between control and enalaprilat-infused fetuses. However, there was a significant decrease in cell cycle activity in both the right ventricle (P < 0.005) and the left ventricle (P < 0.002) of the enalaprilat-infused fetuses. Thus, we conclude a sustained reduction in systolic pressure load decreases hyperplastic growth in the fetal heart.

Persistent changes in arterial blood gases in fetal sheep.

Two anaesthetic protocols were compared using pregnant sheep. In both groups of animals, anaesthesia was induced using an intravenous (i.v.) injection of diazepam and ketamine. The ewes were then intubated for positive pressure ventilation using 0.8 L/min of nitrous oxide and 2 L/min oxygen with 1.1-1.8% halothane. If the ewe showed any signs of awakening, one of two protocols was followed. First, the halothane concentration was increased to 2-3% until the ewe was completely anaesthetized. Second, the halothane concentration was not altered, but the ewe was given doses of i.v. diazepam (0.1 mg/kg) and ketamine (1 mg/kg) until again completely anaesthetized. At the completion of surgery, maternal recovery was rapid and similar between the two groups. However, five days after surgery, the fetal arterial Po(2) and oxygen content of the fetuses receiving additional halothane (1.9 +/- 0.2 kPa and 4.4 +/- 1.0 mL/100 mL) were statistically significantly depressed when compared with the fetuses receiving additional diazepam and ketamine (2.9 +/- 0.1 kPa and 7.0 +/- 0.5 mL/100 mL). These results led us to conclude that certain anaesthetic protocols, in spite of good maternal recovery, can lead to deleterious effects upon the fetus that persist for at least five days after surgery.

The steady state concentration gradients of an electron-dense marker (ferritin in the three-layered hemochorial placenta of the rabbit.

Ferritin was injected into the fetal or the maternal circulation of 27-29-day-pregnant rabbits. After the occurrence of a quasi-steady state, the placentas were prepared for electron microscopy. Ferritin particles were counted in the electron micrographs in the fetal capillaries, in the maternal blood spaces, and in the two interstitial compartments of the three-layered placenta. Under the circumstances of the experiments (excessively elevated plasma ferritin concentrations), no evidence was found for nondiffusional transport of radiolabeled ferritin. Comparison of the standing concentration gradients in the placentas, recorded after maternal and after fetal injection, showed that the interstitial spaces "excluded" ferritin; the plasma-interstitial space ferritin partition coefficients were 10 in the basement membrane space and 3 in the space between the cyto- and syncytiotrophoblasts. 55% of the total concentration gradient across the rabbit placenta occurred across the fetal endothelium, about 45% across the cytotrophoblast, and less than 5% across the syncytiotrophoblast. These figures are believed to reflect the relative contributions of these three layers to the total diffusional resistance in the rabbit placenta. When compared to previous data on the relative contributions of these three layers for small ions and molecules, the present data lead to the conclusion that discrimination of molecular size is a function of the fetal capillary endothelium alone.

Fetal infusions of plasma cause an increase in umbilical vascular resistance in sheep.

Earlier studies suggested that the fetal placental circulation is relatively inert with fetal placental flow increasing or decreasing with perfusion pressure. Subsequent studies have demonstrated that the placenta may not be an unreactive vascular bed. The present study was undertaken to determine if plasma infusion-induced hypertension increased fetal placental flow in proportion to the driving pressure across the fetal placental circulation. Six fetal sheep were operated on at 118-122 days to place intravascular catheters and a flow sensor on the common umbilical artery. Starting 6 days later, the fetuses were infused with adult sheep plasma. During the 7-day-long infusion period, they received a total of 1515+/-217 (SD) ml of fluid and 93.2+/-12.0 g of protein. Fetal plasma protein concentrations increased from 34.2+/-2.3 to 77.0+/-9.7 g/l (P<0.0001). Fetal arterial blood pressures rose from 42+/-3 to 59+/-4 mmHg (P<0.01) and venous pressures rose from 2.2+/-0.5 to 4.8+/-0.8 mmHg (P<0.01). In spite of the large increase in driving pressure, fetal placental blood flow remained (statistically) constant (627+/-299 ml/min and 552+/-221 ml/min) while fetal umbilical resistance increased from 0.077+/-0.038 to 0.115+/-0.053 mmHg min/ml (P<0.01). On day 7, plasma renin activity had fallen from 6.7+/-4.2 ng/(ml/h) at preinfusion control to 0.6+/-0.6 ng/(ml/h) (P<0.05) and plasma angiotensin-II concentration had fallen from 33.2+/-26.6 to 6.2+/-3.9 pg/ml, although this fall was not statistically significant (P=0.07). Fetal placental flow did not increase with increased driving pressure across the fetal placental circulation. The increase in fetal placental resistance may be a response to the increase in arterial pressure since there was no increase in flow.

Filtration and diffusion across the immature placenta of the anaesthetized rat embryo.

Transplacental clearances of Cl-, Na+, SO4(2-), mannitol, raffinose and inulin were measured in anaesthetized rats of 14-19 days gestational age. Water flow across the placenta was calculated from embryonic (74 per cent/day) and extra-embryonic fluid (94 mg/day) growth. Before 16 days, more than half of the tracer contents of the conceptuses were located in the extra-embryonic fluids. Clearances were not demonstrably affected by the presence or polarity of an electrical charge on the tracer. Clearances fell off with increasing molecular weights but faster than the corresponding decreases in the coefficients of free diffusion. The dimensions of the paracellular transplacental passages were calculated with the Patlak equation and equivalent pore theory. Pore radii were not significantly different from 3.3 nm at any gestational age but in the last 2 days, the geometric constant (number of pores per gram x pore area/pore length) increased threefold, to 31 cm/gram placental weight. The calculated parameters were insensitive to deletion of Na+ and Cl- from the data set. In haemochorial placentae, steric restriction does not prevent the entry into the embryo of solutes circulating in maternal blood, except for macromolecules.

Flexner's Na+ clearances and the histological structure and age of the placenta in various species.

1939 and the early 1940s saw the publication of a remarkable series of papers on placental transfer. For the first time, a group of investigators made a quantitative and systematic study of the transfer of a simple, presumably inert physiological solute, the sodium ion, in a number of different species at various gestational ages. The driving force behind these studies was Louis Barkhouse Flexner.

Transplacental clearances of inert hydrophilic tracers in rabbits of 18 days gestation.

Transplacental clearances were measured for radiolabelled Cl-, SO4(2-), mannitol, sucrose, raffinose, PEG-900 and inulin. Maternal placental blood flows were measured with radiolabelled microspheres. At 18 days of gestation (term 31 days), the fetuses and extra-fetal fluids were growing at 55 and 32 per cent per day, accounting for a net transplacental filtration rate of 14.2 nl/sec per gram placental weight. Pore theory and a least squares fit of the Patlak equation yielded an equivalent pore radius of 1.75 nm. It was demonstrated that the clearance of the largest tracer, inulin, was 30 times higher than it would have been in the absence of net filtration. Comparison with literature data showed that there was a small increase in placental permeability per gram placenta between 14 and 18 days of gestation but that the increase between 18 and 28 days of gestation was about 14-fold for Cl- and 300-fold for inulin. There was no evidence for a decreasing equivalent pore radius in the course of gestation from 14-18 days.

Current topic: water volume of the ovine conceptus; point of view.

The sheep is the only species for which there is sufficient information to justify an overview of maternofetal water transfer. Current information points to the following conclusions: (1) the site of water exchange between mother and conceptus is mainly the placenta; (2) the pressure that initiates transfer to the fetus is the osmotic pressure of solutes that are present in higher concentrations in fetal plasma than in maternal plasma, such as urea, fructose, amino acids, bicarbonate and lactate; (3) maternal ultrafiltrate thus attracted into the conceptus arrives there mostly denuded of electrolytes and is strongly hypotonic; thus it is the transfer of electrolyte that restrains the transfer of water; (4) maternofetal transfer of water is strongly facilitated by angiotensin I in the fetal circulation by a sequence of events that has not yet been elucidated; (5) the net combined osmotic and hydrostatic pressure that drives water across the placenta is of the order of only 20 to 80 mm Hg; (6) it is necessary to make a sharp distinction between transfer from mother to conceptus and transfer from one compartment to another compartment within the conceptus. Finally, the review lists certain precautions to be taken in the further study of water exchange between mother and conceptus.

Angiotensin mediated interaction of fetal kidney and placenta in the control of fetal arterial pressure and its role in hydrops fetalis.

Fetal cardiovascular control is effected by an interaction of the fetal somatic and placental circulations. Three primary regulatory mechanisms are involved: transplacental transfer of extracellular fluid, driven by a difference in hydrostatic and oncotic pressures; modulation of fetal placental and somatic vascular resistances by means of blood pressure controlled production of angiotensin; and somatic autoregulation of flow. A systems analysis incorporates these and other fetal cardiovascular functions and this analysis was modelled for computer simulation. Given physiologically plausible values for known cardiovascular parameters in the fetal sheep, the model reproduced in detail a variety of experimental protocols with known outcomes; these included the normal fetus, the fetus after bilateral nephrectomy, the nephrectomized fetus infused with angiotensin, the intact fetus infused with NaCl solutions, the fetus with lymphatic obstruction and the severely anaemic fetus. The systems analysis demonstrated that fetal cardiac failure constituted the strongest stimulus for the formation of fetal oedema of any tested pathological intervention.

Electrophysiology of extrafetal membranes.

This paper reviews some of the equations that apply to the electrophysiology of the extrafetal membranes, and stresses the limitations that may apply to the use of these equations. A literature review of experimental data on potential differences between the fetus, the extrafetal fluid compartments and the mother leads to the conclusion that the transplacental potential difference is no more than a few millivolts and that, in some species, there is an electrical generator outside the placenta.

In vivo placental permeability to hydrophilic solutes as a function of fetal weight in the guinea pig.

The permeability of the placenta was measured with unlabelled inulin in unanaesthetized guinea pigs. Inulin was injected into the sows and the plasma concentrations were recorded at regular intervals. The inulin contents of the conceptuses were calculated from fetal weights, fetal plasma inulin concentrations and a measured inulin distribution volume of 180 ml/kg fetus. From fetal inulin contents and the time integral of the difference in maternal and fetal plasma concentrations, we calculated a mean inulin permeability of 29 +/- 9 (s.e.m.) nl/s per gram placental weight. The inulin permeability per gram placental weight rose statistically significantly with increasing fetal weight. Comparison with a similar rise observed for the smaller cyanocobalamin molecule studied previously led to the conclusions that in the last ten days of gestation the transplacental passages narrow while at the same time there are apparent changes in the numbers and/or lengths of these passages that suffice to increase the unrestricted permeability fourfold.

Interrelationships between the renin angiotensin system and uteroplacental blood flow--a recent perspective.

In pregnancy, the maternal circulating renin-angiotensin system (RAS) and uteroplacental tissue RAS has been thought to support maternal placental flow by raising maternal arterial pressure or changing placental vascular resistance. Also, the placenta or uterus may alter maternal circulating RAS. Recent studies in the authors' laboratory using chronically catheterized rabbits are compared with previous studies on interactions between the RAS and uteroplacental flow. When uterine driving pressure was reduced either mechanically or after converting enzyme inhibition, maternal placental flow decreased in proportion to change in driving pressure; myoendometrial flow did not change. Angiotensin II (AII) infusion to increase pressure by 21 +/- 2 mm Hg decreased placental but not myoendometrial flow. Thus, there is no evidence that maternal placental flow is autoregulated or supported by a specific renin-angiotensin mechanism. Normally, there is no net uterine release or uptake of active plasma renin activity, AI, or AII, but there is a small net release of trypsin-activated plasma renin activity (tPRA), presumably prorenin. Distal aortic occluder inflation produced upper-body hypertension, and uterine release of tPRA increased. There was a significant uterine arteriovenous concentration difference for AII during AII infusion. These methods are adaptable for studying interactions between uteroplacental flow and other vasoactive agents.

Convention for reporting amniotic fluid pressure.

A proposed convention sets zero pressure at atmospheric pressure at the level of the surface supporting the supine patient and takes the cm of water as the unit of measurement. This ensures that measurements made in different clinics will be comparable. Statements about 'higher' and 'lower' pressures, in different patients or in the same patient in different situations, should specify the level of the uterus to which the statement applies.

Excess extrafetal fluid without demonstrable changes in placental concentration gradients after week-long infusions of angiotensin into fetal lambs.

It is known that a week-long infusion of angiotensin into fetal sheep produces polyhydramnios. The purpose of the present experiments was to determine whether an increased osmotic force across the placental barrier could account for the excess transfer of water. Six fetuses with indwelling catheters were infused with angiotensin-I and one with angiotensin-II; all, except one fetus in the first group, developed gross polyhydramnios. None of the transplacental concentration differences of the small plasma solutes Na+, Cl-, HCO3-, K+, urea, or glucose showed a demonstrable change and the same was true of the transplacental difference in freezing point osmolality and for the transplacental difference in plasma protein concentration. It is concluded that the infusion of angiotensin at a low dose rate is a reliable protocol for producing polyhydramnios. However, the present findings lend no support to the hypothesis that a primary change in transplacental osmotic force is the cause of the increased transplacental water transfer in this form of polyhydramnios. Alternative hypotheses are discussed in the light of recent discoveries.

Insignificant response of the fetal placental circulation to arterial hypotension in sheep.

Infusion of the angiotensin-converting enzyme inhibitor enalaprilat into fetal sheep caused a profound arterial hypotension within days. Five fetal lambs were infused with enalaprilat for 8 days starting at day 128 of gestation. Total accumulated dose was 0.30 ± 0.11 mg/kg. Arterial pressure decreased from 43.6 to 25.6 mmHg; venous pressure did not change. Biventricular output was not statistically significantly changed; placental blood flow decreased almost in proportion to the decrease in pressure but the increase in somatic flow was not statistically significant. There were no significant changes in pressure 30 min after the initial 50-µg loading dose of enalaprilat. However, the arterial pressure responses to test doses of ANG I were largely abolished. After 1 day, however, there was a significant decrease in somatic vascular resistance, which became stronger with time, but almost no decrease in the placental resistance. We conclude that the fetal somatic circulation exhibits a slow but strong decrease in resistance but that the response to hypotension is weak or absent in the fetal placenta, possibly because it is already fully relaxed.

Diffusion permeability of the immature placenta of the rabbit embryo to inert hydrophilic molecules.

The clearances of a series of hydrophilic probes (radii of 2-10 A) were determined in rabbit placentas of 13-15 days of gestation (term, 31 days). Maternal placental blood flows were measured by means of radiolabeled microspheres. None of the transfers of the tracers were limited by maternal or embryonic arteriovenous differences in the placenta, and the clearances decreased with increasing molecular radius. The transplacental water filtration rate calculated from conceptual growth was comparable in magnitude to the clearances of the largest tracers. Application of Patlak's modification of the Hertzian equation and pore theory suggested an equivalent pore radius of 17 A. Placental permeability surface area products computed from this value decreased much more steeply with increasing molecular dimension than the measured clearances, suggesting a highly significant contribution by filtration despite the very large diffusional gradients that existed under the experimental conditions. The results indicate that the size selectivity of the hemochorial embryo placenta in vivo is not very significant, and that under normal conditions filtrate of maternal plasma constitutes a major contributor to embryonic supply.