Effects of fetal ovine adrenalectomy on sympathetic and baroreflex responses at birth

Studies were performed to test the hypothesis that the absence of adrenal glucocorticoids late in gestation alters sympathetic and baroreflex responses before and immediately after birth. Fetal sheep at 130-131 days gestation (term 145 days) were subjected to bilateral adrenalectomy before the normal prepartum increase in plasma cortisol levels. One group of fetuses (n = 5) received physiological cortisol replacement with a continuous infusion of hydrocortisone (2 mg x day(-1) x kg(-1) for 10 days), whereas the other group received 0.9% NaCl vehicle (n = 5). All animals underwent a second surgery 48 h before the study for placement of a renal nerve recording electrode. Heart rate (HR), mean arterial blood pressure (MABP), renal sympathetic nerve activity (RSNA), and baroreflex control of HR and RSNA were studied before and after cesarean section delivery. At the time of study (140-141 days gestation), fetal plasma cortisol concentration was undetectable in adrenalectomized (ADX) fetuses and 58 +/- 9 ng/ml in animals receiving cortisol replacement (ADX + F). Fetal and newborn MABP was significantly greater in ADX + F relative to ADX animals. One hour after delivery, MABP increased 13 +/- 3 mmHg and RSNA increased 91 +/- 12% above fetal values in ADX + F (both P < 0.05) but remained unchanged in ADX lambs. The midpoint pressures of the fetal HR and RSNA baroreflex function curves were significantly greater in ADX + F (54 +/- 3 and 56 +/- 3 mmHg for HR and RSNA curves, respectively) than ADX fetuses (45 +/- 2 and 46 +/- 3 mmHg). After delivery, the baroreflex curves reset toward higher pressure in ADX + F but not ADX lambs. These results suggest that adrenal glucocorticoids contribute to cardiovascular regulation in the late-gestation fetus and newborn by modulating arterial baroreflex function and sympathetic activity.     

Effects of fetal behavioral states on renal sympathetic nerve activity and arterial pressure of unanesthetized fetal sheep

Fetal behavior, renal sympathetic nerve activity (RSNA), mean arterial pressure (MAP), and heart rate (HR) were studied 1-3 days after surgery in seven fetal sheep (aged 127-136 days). Five behavioral states were defined from chart recordings of electrocortical (electrocorticographic; ECoG) activity and eye, limb, and breathing movements. Most records were of high-voltage ECoG (HV) or low-voltage (LV) ECoG with breathing (LVB); 6.7 +/- 1.7% were LV ECoG with no breathing (LV0). RSNA was lower in LV0 (P < 0.001) and greater in LVB than in HV (P < 0.05). MAP was lower in both LV states than in HV and when the fetuses went from LV to HV (P < 0.001 to P < 0.03). HR was highest in HV (P < 0.001). In HV and LVB and when the fetus went from LV to HV, MAP and HR were inversely related (P = 0.012-0.003). In LVB and from LV to HV there were direct relationships between MAP and RSNA (P = 0.0014, P = 0.08), and when the fetus went from LV to HV there was also an inverse relationship between HR and RSNA (P = 0.02). Thus fetal RSNA, MAP, and HR are affected by behavioral state as is fetal cardiovascular control. The increase in RSNA during fetal breathing showed that there was an altered level of fetal RSNA associated with fetal breathing activity.     

Purinergic contribution to circulatory, metabolic, and adrenergic responses to acute hypoxemia in fetal sheep

This study investigated the effects on femoral vascular resistance, blood glucose and lactate levels, and plasma catecholamine concentrations of fetal treatment with an adenosine receptor antagonist during acute hypoxemia in fetal sheep during late gestation. Under anesthesia, seven fetal sheep were instrumented between 117 and 118 days gestation (term is approximately 145 days) with vascular and amniotic catheters and an ultrasonic probe around a femoral artery. Six days after surgery, all fetuses were randomly subjected to a 3-h experiment consisting of 1 h of normoxia, 1 h of hypoxemia, and 1 h of recovery. This was done during either intravenous infusion of vehicle or the adenosine receptor antagonist [8-(p-sulfophenyl)-theophylline; 8-SPT] dissolved in vehicle. During vehicle infusion, all fetuses responded to hypoxemia with bradycardia, an increase in arterial blood pressure, and femoral vasoconstriction. Increases in blood glucose and lactate concentrations and in plasma epinephrine and norepinephrine concentrations also occurred in all fetuses during hypoxemia. Fetal treatment with 8-SPT markedly attenuated the bradycardic, hypertensive, vasoconstrictor, glycemic, and adrenergic responses to hypoxemia, but it did not affect the increase in blood lactate concentrations during hypoxemia. These data show that adenosine is involved in the mechanisms mediating fetal cardiovascular, metabolic, and adrenergic responses to hypoxemia in fetal sheep. Fetal treatment with 8-SPT mimics the effects of carotid sinus nerve section on fetal cardiovascular function during hypoxemia, suggesting a role for adenosine in mediating fetal cardiovascular chemoreflexes.     

Fetal blood volume responses to fetal haemorrhage: autonomic nervous contribution

The purpose of this study was to determine whether the autonomic nervous system contributes to short-term fetal blood volume restitution following fetal haemorrhage. Unanaesthetized, chronically catheterized fetal sheep averaging 130 days gestation (term = 145-150 days) were studied 5 days after catheter implantation. Blockade of the autonomic nervous system was produced with a bolus intravenous injection of hexamethonium. In 7 blocked fetuses, an average of 17.6% of the initial blood volume was removed over 5 min and this was not different from the 16.5% blood volume removed in 7 control fetuses. However, blood volume restoration averaged 44.6% after 30 min in the blocked fetuses and this was significantly less than the 60.2% volume restoration in the control fetuses (P less than 0.05). Thus blood volume restoration averaged 74% of control in the blocked fetuses. Following blood reinfusion over 5 min, blood volume was transiently elevated in the control group, but averaged 3.4, 4.7, and 5.3% below normal (P less than 0.05) at 10, 20 and 30 min post reinfusion in the blocked group. In addition, arterial pressure was lower after haemorrhage in the blocked group and increased twice that in the control group during reinfusion. Thus, the present data suggest that the fetal autonomic nervous system plays a significant role in mediating the short-term changes in fetal blood volume and vascular pressures following fetal haemorrhage and reinfusion.     

妊娠晚期胎羊宫内外科手术的实验研究

目的　以妊娠晚期孕羊为研究对象 ,探讨宫内外科手术的可行性 ,为开展胎儿外科研究建立动物实验模型。方法　通过对妊娠 12 0d左右的 4只双胎孕羊进行宫腔手术 ,切除双胎中 1只胎羊的脚趾或唇部皮肤 ,观察妊娠结果。并取分娩后羔羊脑组织及手术切除部位皮肤作组织形态学观察和评价。结果　妊娠羊经阴道足月分娩后羔羊均成活 ,脚趾及唇部切除处未见疤痕形成 ,皮肤组织形态学观察对照组与实验组未见明显差异 ;脑组织形态学检查仅有 1例实验组羔羊有轻度病理学变化 ,与对照羔羊有差异 ,其余 3例实验羔羊与对照羔羊脑组织结构均正常 ,未见有明显的病理学改变。结论　妊娠羊经宫内胎羊外科手术后能够持续妊娠状态并经阴道自然分娩 ,产出羔羊的精神状态及病理组织学观察结果表明 ,利用妊娠晚期孕羊进行胎儿外科动物实验研究是可行的 ,为进一步开展人类先天性畸形胎儿的早期治疗奠定了动物实验基础     

Complexity of heart rate fluctuations in near-term sheep and human fetuses during sleep.

We investigated how the complexity of fetal heart rate fluctuations (fHRF) is related to the sleep states in sheep and human fetuses. The complexity as a function of time scale for fetal heart rate data for 7 sheep and 27 human fetuses was estimated in rapid eye movement (REM) and non-REM sleep by means of permutation entropy and the associated Kullback-Leibler entropy. We found that in humans, fHRF complexity is higher in non-REM than REM sleep, whereas in sheep this relationship is reversed. To show this relation, choice of the appropriate time scale is crucial. In sheep fetuses, we found differences in the complexity of fHRF between REM and non-REM sleep only for larger time scales (above 2.5 s), whereas in human fetuses the complexity was clearly different between REM and non-REM sleep over the whole range of time scales. This may be due to inherent time scales of complexity, which reflect species-specific functions of the autonomic nervous system. Such differences have to be considered when animal data are translated to the human situation.     

Increased myometrial contracture frequency at 96-140 days accelerates fetal cardiovascular maturation

Fetal cardiovascular responses to an altered intrauterine environment of increased myometrial contractures induced by oxytocin (OT) pulses to the ewe over the final 50 days of gestation were studied in chronically instrumented sheep. Ewes received saline (Cntl) or long-term OT treatment (LTOT, 600 microU x kg(-1) x min(-1) in 5-min pulses every 20 min) from 96 days gestational age. Fetal baroreflex responses to sodium nitroprusside (SNP) and phenylephrine (PE) were studied at 133 days gestation. OT increased contractures in LTOT ewes. Fetal blood pressure (FBP) was higher, and fetal heart rate (FHR) and slope of daily change in FBP and FHR were lower in LTOT fetuses. Fetal SNP-induced hypotension resulted in a narrow R-R interval variation range in LTOT fetuses; Cntl fetuses showed early breakdown in compensation. Baroreflex response slope during PE-induced fetal hypertension was lower in LTOT than in Cntl fetuses. Although the cortisol-to-ACTH ratio was lower in LTOT fetuses, fetal plasma ACTH and cortisol changes were similar in control and LTOT fetuses. We hypothesize that contracture-induced alterations in the intrauterine environment accelerate fetal cardiovascular development through mild hypoxemia, repetitive fetal pituitary-adrenal stimulation, and/or physical stimulation.     

Relationships among Doppler-derived umbilical artery absolute velocities, cardiac function, and placental volume blood flow and resistance in fetal sheep

We hypothesized that umbilical artery (UA) absolute blood flow velocities measured by Doppler ultrasonography reflect placental volume blood flow (Q(UA)) and placental vascular resistance (R(UA)) in a late gestation fetal sheep model. In addition, we examined the relationships between umbilical artery absolute blood flow velocities and parameters of fetal cardiac function. Twenty-six sheep fetuses were instrumented at 112-132 days of gestation. After a 5-day recovery period, experiments were performed under general anesthesia in 16 normal fetuses, in 5 fetuses after maternal administration of phenylephrine, and in 5 fetuses after placental embolization. The Q(UA) and arterial blood pressures were measured using a transit-time ultrasonic flow probe and a catheter placed into the descending aorta, respectively. UA peak systolic velocity (PSV), end-diastolic velocity (EDV), time-averaged maximum velocity (TAMXV), pulsatility index (PI), mean velocity (V(mean)), fetal cardiac output, ventricular ejection forces, and the proportion of isovolumetric relaxation time (IRT%) in the cardiac cycle were measured with the use of Doppler ultrasonography. Significant positive linear correlations were found between UA EDV, TAMXV, and V(mean) versus Q(UA), whereas UA PI had a significant negative correlation with Q(UA). Significant negative correlations were shown between UA EDV, TAMXV, and V(mean) versus R(UA). A significant positive correlation was present between UA PI and R(UA). Doppler-derived UA parameters did not correlate with fetal arterial blood pressures, cardiac output, ventricular ejection forces or IRT%. In fetal sheep, Doppler-derived UA PI and absolute velocities, except PSV, are closely related to directly measured Q(UA) and R(UA), validating the use of noninvasive Doppler velocimetry in the assessment of placental circulation.     

Ontogeny of the circadian variation of plasma prolactin in sheep

The ontogeny of circadian rhythms is unknown. The newborn sheep has a circadian rhythm of temperature; to study the ontogeny of other rhythms, we examined the 24-h variation of plasma prolactin concentration in fetal and newborn sheep. To this effect, we measured plasma prolactin concentration in chronically catheterized fetuses (n = 7) and in newborn lambs raised under short day nycthemeral (12 light:12 dark, n = 13) or constant light conditions (n = 5). Indwelling catheters were implanted into the jugular vein and carotid artery of late gestation fetuses (0.9 gestation) and newborns (5-29 days old). Experiments were performed 4 or more days after surgery. Lambs were kept in a canvas sling and were fed cow's milk either by mouth or through a nasogastric catheter at established time intervals. Haematocrit, pH, and blood gases were measured before and after the experiments in all cases and remained within normal values. Lights were on and room temperature was maintained constant during the whole experiment. Samples were obtained every 1-2 h for 24 h in fetuses and newborn lambs under nycthemeral conditions and every hour for 48 h in newborn lambs kept under constant light. Plasma prolactin was measured by radioimmunoassay. The presence of a 24 h rhythm was determined by Cosinor analysis. Fetuses, aged 129 +/- 6 days (SD) n = 7, showed a variation in plasma prolactin concentration with a period of 24 h that fits the equation: plasma prolactin (ng ml-1) = 97.0 + 15.4 cos 15 (t-23.0), P = 0.035.(ABSTRACT TRUNCATED AT 250 WORDS)     

The role of hypothalamic input on corticotroph maturation in fetal sheep

Corticotropin-releasing hormone receptor type 1 (CRH-R1) expression and vasopressin type 1b (V1b) receptor protein decrease in late-gestation fetal sheep. Because hypothalamo-pituitary disconnection (HPD) has been demonstrated to prevent the morphological maturation of corticotrophs, we hypothesized that hypothalamic input is necessary for the maturational changes in CRH-R1 and V1b receptor levels. We measured CRH-R1 and V1b receptor expression in the anterior pituitaries of fetuses at 140 days gestational age (dGA) that underwent HPD or sham surgery at 120 dGA. CRH-R1 mRNA decreased similarly in HPD and sham-operated fetuses compared with 120 dGA naive fetuses. However, CRH-R1 protein levels were elevated in HPD fetuses compared with sham and were not different from 120 dGA values. V1b protein levels decreased similarly in HPD and sham-operated fetuses compared with 120 dGA naive fetuses. We conclude that hypothalamic input to the pituitary is necessary for the decrease in CRH-R1 receptor protein levels in late-gestation fetal sheep. However, hypothalamic input is not necessary for the decrease in V1b receptor expression seen in late gestation.     

Pulmonary feedback and gestational age-dependent regulation of fetal breathing movements

Prenatal lung development requires fetal breathing movements (FBM). To investigate the dependence of FBM on feedback originating from the lung, we hypothesized that pneumonectomy stimulates FBM. Time-dated fetal sheep underwent bilateral pneumonectomy, unilateral pneumonectomy, or sham surgery at 125-130 days gestation. The incidence of FBM decreased in sham-operated fetuses at 142 days versus 130 days (p = 0.013), but was unchanged across all gestational ages in bilaterally pneumonectomized fetuses (p > or = 0.52). In unilaterally pneumonectomized fetuses, the incidence of FBM remained unchanged until 139 days and was higher than that of the bilaterally pneumonectomized fetuses at 130-136 days gestation (p < or = 0.03). The amplitude of integrated diaphragmatic electromyographic activity (integralEMG(di)) and total respiratory output (frequency of breathing x integralEMG(di)) were lower in pneumonectomized fetuses versus sham-operated fetuses at later gestational ages (p < 0.05). These decreases in integralEMG(di) and total respiratory output were most pronounced at 142 days in bilaterally pneumonectomized fetuses versus sham-operated fetuses (p = 0.006 and 0.016, respectively). Low-voltage electrocortical activity (ECoG) increased, and high-voltage ECoG decreased, in unilaterally pneumonectomized fetuses compared with sham-operated fetuses (p = 0.04). In conclusion, we provide new evidence that feedback from the fetal lung modulates the incidence and various components of phrenic nerve output, suggesting a positive feedback mechanism between FBM and lung development.     

Continuous measurement of biparietal distance in the intact and hypophysectomized fetal sheep using ultrasound

Piezoelectric transducers were implanted into the parietal bones of intact (n = 4) and hypophysectomized (n = 8) fetal sheep of approximately 110-120 days gestational age (term 145-150 days). Intertransducer distance was determined by measuring the time taken for an ultrasonic pulse, generated by one transducer, to elicit a piezoelectric response in an opposing transducer. The limit of sensitivity of the timer was +/- 0.033 microsec. The ultrasonic velocity through fetal sheep brain tissue was 1549.6 +/- 2.2 m.s-1 (SEM; n = 33). This velocity remained constant throughout the entire period studied in both intact and hypophysectomized fetuses. At this velocity, the sensitivity of the measuring device was +/- 0.05mm. The ultrasonic transit time was measured daily between 0900 and 1100h until term in all fetuses. Three hypophysectomized fetuses were allowed to remain in utero until day 163 of gestation. The mean biparietal distance growth rate prior to day 135 for the intact and hypophysectomized fetuses was 0.25 +/- 0.03 and 0.27 +/- 0.025 mm/day respectively. These values were not significantly different (P greater than 0.05). A significant decrease (P less than 0.05) in growth rate was detected in both experimental groups between days 135 and 147 and was more pronounced in the sham (0.05 +/- 0.04 mm/day) than in the hypophysectomized (0.14 +/- 0.03 mm/day) group. However, the growth rate of the sham animals after day 135 was not significantly different from that of the hypophysectomized animals. In the three hypophysectomized fetuses killed at day 163 the biparietal distance growth was maintained at 0.12 +/- 0.005 mm/day. We conclude that fetal biparietal distance growth is pituitary independent from day 110 of gestation and that this technique for measuring distance is a valid and extremely accurate method for the continuous measurement of this parameter of fetal growth and may have further applications in other areas of growth research.     

Canine fetal heart rate: Do accelerations or decelerations predict the parturition day in bitches?

Ultrasonography is a safe and efficient technique for monitoring fetal development and viability. One of the most important and widely used parameters to verify fetal viability is the fetal heart rate (HR). In human medicine, the fetal HR normally oscillates during labor in transient accelerations and decelerations associated with uterine contractions. The present study investigated whether these variations also occur in canine fetuses and its relationship to parturition. A cohort study was conducted in 15 pregnant bitches undergoing two-dimensional high-resolution ultrasonographic examination during the 8th and 9th week of gestation. Fetal HR was assessed in M-mode for 5 minutes in each fetus in all bitches. In addition, the bitches were monitored for clinical signs of imminent parturition. Associations between the HR, antepartum time, and delivery characteristics were evaluated with a Poisson regression model. Fetal HR acceleration and deceleration occurred in canine fetuses and predicted the optimal time of parturition. These findings can help veterinarians and sonographers better understand this phenomenon in canine fetuses.     

Circulating insulin-like growth factor-I and -II concentrations are not dependent on pituitary influences in the midgestation fetal sheep

Studies were performed to investigate the possible role of pituitary factors on the regulation of circulating concentrations of insulin-like growth factor-I and -II in the midgestation sheep fetus. Four fetuses were decapitated at 59-64 days of gestation and fetal serum obtained at sacrifice at 90-102 days of gestation. Insulin-like growth factor-I and -II concentrations were similar in these samples to those from 6 control fetuses (83-102 days). A further 4 fetuses were studied following electrolytic destruction of the median eminence of the hypothalamus at 108-110 days of gestation. Four sham operated controls were also studied. Circulating growth hormone concentrations were markedly reduced (P less than 0.01) by destruction of the median eminence. However neither insulin-like growth factor-I nor -II levels differed from those of sham operated fetuses. We conclude that, in the midgestation fetal sheep, growth hormone is not essential for the maintenance of circulating concentrations of insulin-like growth factor-I or -II.     

Periconceptional nutrition programs development of the cardiovascular system in the fetal sheep

It has been proposed that fetal adaptations to intrauterine nutrient deprivation permanently reprogram the cardiovascular system. We investigated the impact of restricted periconceptional nutrition and/or restricted gestational nutrition on fetal arterial blood pressure (BP), heart rate, rate pressure product, and the fetal BP responses to ANG II and the angiotensin-converting enzyme inhibitor captopril during late gestation. Restricted periconceptional nutrition resulted in an increase in fetal mean arterial BP between 115 and 125 days gestation (restricted 41.5 +/- 2.8 mmHg, n = 12; control 38.5 +/- 1.5 mmHg, n = 13) and between 135 and 147 days gestation (restricted 50.5 +/- 2.2 mmHg, n = 8; control 42.5 +/- 1.9 mmHg, n = 10) as well as an increase in the rate pressure product in twin, but not singleton, fetuses between 115 and 147 days gestation. Mean BP and fetal plasma ACTH were also positively correlated in twin, but not singleton, fetuses. This is the first demonstration that maternal undernutrition during the periconceptional period results in an increase in fetal arterial BP. This increase occurs concomitantly with an increase in fetal ACTH but is not dependent on activation of the fetal renin-angiotensin system.     

The retino-hypothalamic tract is involved in prolactin regulation in fetal sheep.

To investigate the role of the retino-hypothalamic tract on fetal prolactin regulation, we examined the effect of ocular enucleation on fetal plasma prolactin. Eleven fetuses of Suffolk ewes were chronically catheterized during fall, and six of them were subjected to bilateral ocular enucleation. All ewes were kept at 12h:12h light:dark cycle (lights on at 0800 and off at 2000). The experiments were performed 5-9 days after surgery (GA control fetuses 125 +/- 1.5, optical enucleation 121.3 +/- 1.5 days). Blood samples were taken from fetuses hourly around the clock, and plasma prolactin and cortisol were measured by radioimmunoassay (RIA). Luteinizing hormone (LH) and Growth hormone (GH) were measured in pooled plasma samples from control and enucleated fetuses by RIA. Average plasma prolactin was 5-fold lower in enucleated than in control fetuses (9.6 +/- 0.5 and 54.2 +/- 3.3 ng/ml, SEM; P < 0.005). Both control and enucleated fetuses presented circadian rhythm of prolactin with acrophase between 1400 and 1830 h. An enucleated fetus was tested for response of prolactin to TRH. Prolactin increased as described in the literature. There was no change in plasma concentration of cortisol, LH or GH after ocular enucleation. Our data indicate that the optical pathway participates in prolactin regulation in the fetal sheep.     

Long-term hypoxia increases leptin receptors and plasma leptin concentrations in the late-gestation ovine fetus

This study was designed to test the hypothesis that long-term hypoxia (LTH) increases fetal plasma leptin and fetal adipose or placental leptin expression and alters hypothalamic and adrenocortical leptin receptor (OB-R) expression. Pregnant ewes were maintained at high altitude (3,820 m) from day 30 to approximately 130 days of gestation. Reduced Po2 was maintained in the laboratory by nitrogen infusion through a maternal tracheal catheter. On day 132, normoxic control and LTH fetuses underwent surgical implantation of vascular catheters (n=6 for each group). Five days after surgery, maternal and fetal arterial blood samples were collected for leptin, insulin, and glucose analysis. Placental tissue, periadrenal fat, and fetal hypothalami and adrenal glands were collected from additional control (n=7) and LTH (n=8) fetuses for analysis of leptin mRNA by quantitative, real-time, RT-PCR (qRT-PCR). There was a significant (P<0.03) elevation in fetal plasma leptin in the LTH fetuses (3.5+/-0.7 ng/ml) vs. control (1.1+/-0.1 ng/ml). There were no differences in either glucose or insulin concentrations between the two groups. Periadrenal adipose leptin mRNA was significantly higher in the LTH group compared with control, as was placental leptin expression. The levels of leptin mRNA in adipose were approximately 70 times higher vs. placenta. LTH significantly reduced expression of OB-Ra (short-isoform) in the hypothalamus (P=0.0156), while resulting in a significant increase in adrenal OB-Rb (long-form) expression (P<0.03). Our data suggest that leptin is a hypoxia-inducible gene in the ovine fetus and OB-R expression is altered by LTH. These changes may be responsible in part, for our previously observed alterations in fetal hypothalamic-pituitary-adrenal function following LTH.     

Long-term hypoxia enhances proopiomelanocortin processing in the near-term ovine fetus

Secondary stressors in long-term hypoxic (LTH) fetal sheep lead to altered function of the hypothalamic-pituitary-adrenal axis. Although ACTH is considered the primary mediator of glucocorticoid production in fetal sheep, proopiomelanocortin (POMC) and 22-kDa pro-ACTH (22-kDa ACTH) have been implicated in the regulation of cortisol production in the ovine fetus. This study was designed to determine whether POMC expression and processing are altered after LTH. Pregnant ewes were maintained at high altitude (3,820 m) from day 30 of gestation to near term, when the animals were transported to the laboratory. Reduced Po2 was maintained by nitrogen infusion through a maternal tracheal catheter. On days 139-141, fetal anterior pituitaries were collected from normoxic control and LTH fetuses. We measured POMC and corticotrophin-releasing factor type 1 receptor (CRF1-R) mRNA using quantitative real-time PCR, and we used Western blot analysis for quantitation of ACTH, ACTH precursor, and CRF1-R proteins. We measured plasma ACTH1-39 using a two-site immunoradiometric assay specific for ACTH1-39. Plasma ACTH precursors were measured by ELISA. Anterior pituitary POMC mRNA levels were not different between groups, whereas CRF1-R levels were significantly higher in the LTH anterior pituitaries compared with control (P<0.05). In contrast, protein levels of POMC, CRF1-R, 22-kDa ACTH, and ACTH1-39 were significantly lower in the LTH group. Plasma concentrations of both ACTH precursors and ACTH1-39 were significantly elevated in LTH fetuses, whereas the ratio of plasma precursors to ACTH was significantly lower. We conclude that LTH results in enhanced POMC processing and/or release to ACTH and increased hypothalamic drive.     

Effect of hyperoxia (PaO2 50-90 mmHg) on fetal breathing movements in the unanaesthetized fetal sheep

Hypoxia inhibits fetal breathing movements but after birth it stimulates breathing. These differences have long been thought to involve central nervous inhibitory mechanisms. Such mechanisms might exert a tonic inhibition of fetal breathing movements at normal fetal PaO2 and the rise in PaO2 at birth might lift this inhibitory effect. To test this hypothesis 7 fetal sheep were chronically instrumented at 125-130 days for recording electrocortical activity (ECoG), and the electromyograph (EMG) activity of the diaphragm and neck muscles. Catheters were placed in a fetal carotid and a brachial artery and in the fetal trachea. For an extracorporeal membrane oxygenation system a 12 F gauge silastic catheter was placed in the right atrium for draining fetal blood and a 9.6 F gauge catheter was placed in a carotid artery to return oxygenated blood. Three days after operation the fetuses were connected to the extracorporeal membrane oxygenation system and fetal PaO2 was raised to 65.2 +/- 4.4 mmHg (SEM) for 6 to 19 h without changing pH or PaCO2. Neither the incidence of high voltage ECoG (48.5 +/- SEM 2.0% vs 52.8 +/- 3.3%) nor of fetal breathing movements (37.3 +/- 2.6% vs 23.8 +/- 5.9%) changed during the periods of hyperoxia. Since fetal breathing movements did not become continuous, we conclude that the lower PaO2 in the fetus compared to the neonate does not exert a tonic inhibitory influence on fetal breathing movements.