Long-term hypoxia alters calcium regulation in near-term ovine myometrium.

Previous studies showed that long-term hypoxia (LTH) during pregnancy alters myometrial contractility. The present study was designed to test the hypothesis that LTH during pregnancy suppresses myometrial contractility in sheep by affecting the calcium signaling cascade. Pregnant sheep were maintained at high altitude (3820 m) from Day 30 to Day 139 of gestation, when the animals were killed for collection of myometrial tissue. Tissue was also collected from age-matched, normoxic controls. Circular and longitudinal layers were separated, and strips from each layer were mounted in a muscle bath. After pretreatment with 10(-8) M oxytocin, the strips were exposed to increasing half- or quarter-log doses of nifedipine (L-type calcium-channel blocker), ruthenium red, ryanodine (blockers of inositol 1,4,5-trisphosphate-insensitive calcium stores), or 2-nitro-4-carboxyphenyl-N,N-diphenylcarbamate (NCDC; phospholipase C inhibitor). Area under the contraction curve was analyzed, and pD(2) (log of concentration yielding 50% of maximum response) values and maximum relaxation responses were calculated. The maximum relaxation response to nifedipine was increased in both longitudinal (P < 0.01) and circular (P < 0.05) myometrial layers from LTH compared to control tissue, whereas no difference was observed in response to ruthenium red or ryanodine. The maximum relaxation response to NCDC was lower in the LTH circular layer (P < 0.05). Together, these data are indicative of an increase in the dependence of ovine uterine smooth muscle on extracellular calcium influx through the L-type, voltage-gated calcium channels following LTH. This appears to occur not through an increase in L-type calcium channels but, rather, through a possible decline in importance of the oxytocin-induced, phospholipase C-mediated pathway, resulting in a greater proportion of extracellular calcium contributing to contraction. Layer-dependent differences also exist between the circular and longitudinal myometrium in response to phospholipase C inhibition.

Long-term hypoxia changes myometrial responsiveness and oxytocin receptors in the pregnant ewe: differential effects on longitudinal versus circular smooth muscle.

Previous studies from our laboratory demonstrated that long-term hypoxia (LTH) altered in vitro contractile responses to oxytocin in full-thickness myometrial strips from pregnant sheep. The present study was designed to determine, first, if the reduced contractile response to oxytocin following LTH is the result of combined effects on longitudinal and circular smooth muscle or if the effect is specific to a single muscle layer and, second, if the reduced contractile response to oxytocin following LTH is caused by changes in oxytocin-receptor protein. Pregnant ewes were maintained at high altitude (3820 m) from Day 30 to Days 137-142 of gestation, when the ewes were killed for collection of myometrial tissue. Tissue was also collected from age-matched, normoxic controls. Longitudinal and circular layers were separated, length-tension curves generated to determine optimal resting tension, and all strips exposed to increasing half-log doses of oxytocin ranging from 10-12 to 10-6.5 M. The expression of oxytocin-receptor protein was measured using Western blot analysis. We found that LTH did not affect KCl-induced contraction of either smooth muscle layer, whereas the sensitivity of both myometrial layers to oxytocin was altered. A decreased maximum contractile response of the circular layer to oxytocin was also observed. Additionally, LTH decreased expression of oxytocin-receptor protein in the circular layer and increased levels in the longitudinal layer. Results from the present study indicate that LTH alters contractile responses and oxytocin-receptor protein expression in a layer-specific manner in the pregnant sheep myometrium.

Fetectomy alters maternal pituitary-adrenal function in pregnant rhesus macaques.

The interplay between the fetus and mother may play a key role in the regulation of primate pregnancy and parturition. This study was designed to test the hypothesis that fetectomy alters maternal pituitary-adrenal function. Between 117 and 122 days of gestation (term = 167 days), six rhesus macaques underwent surgery for catheter implantation. At surgery the fetuses were removed while the membranes and placenta were left in situ. Six additional intact catheterized pregnant animals served as controls. Animals were maintained under a 12L:12D cycle with lights-on from 0700 to 1900 h. Beginning at least 1 wk after surgery, maternal arterial blood samples were collected at 3-h intervals for 24 h for hormone and catecholamine analysis. This sampling protocol was repeated at weekly intervals until cesarean delivery at 151-157 days of gestation. Following fetectomy, plasma ACTH, dehydroepiandrosterone sulfate (DHEAS), and cortisol levels were significantly lower (36%, 35%, and 44%, respectively) compared with control animals (P;lt 0.05). Despite a significant reduction in overall levels, the rhythm in maternal plasma cortisol was maintained following fetectomy. Plasma dopamine and norepinephrine were also depressed (P;lt 0.05), whereas epinephrine remained unaffected. Our data clearly demonstrate the role of the fetus in the regulation of the maternal pituitary-adrenal axis during gestation. This interaction plays a significant role in the regulation of maternal endocrine function that may influence the initiation of labor.

Chronic hypoxia and developmental regulation of cytochrome c expression in rats.

OBJECTIVE: To test the hypothesis that chronic hypoxia upregulates cytochrome c expression in heart, brain, and liver of fetal and maternal rats. METHODS: Time-dated pregnant Sprague-Dawley rats were divided into normoxic and hypoxic (48 hours of 10.5% oxygen from days 19 to 21) groups, and were killed on day 21. Tissue levels of cytochrome c in heart, brain, and liver were determined by using monoclonal antiserum for cytochrome c. RESULTS: Chronic hypoxia caused a decrease in fetal body weight (5.3 +/- 0.1 to 4.7 +/- 0.1 g) and an increase in heart/body weight ratio (0.0048 +/- 0.0001 to 0.0061 +/- 0.0002). Cytochrome c levels were 4-, 2.6-, and 13-fold higher in heart, liver, and brain, respectively, of the mother than of the fetus. Chronic hypoxia did not change cytochrome c levels in maternal tissues but caused a 70% increase and 54% decrease in cytochrome c levels in the fetal heart and liver, respectively. No difference was observed in the fetal brain. CONCLUSIONS: The results suggest that expression of cytochrome c is tissue specific and developmentally regulated. Chronic hypoxia showed differential regulation of cytochrome c levels both developmentally and tissue specifically. The increased sensitivity of cytochrome c in fetal tissue to chronic hypoxia is likely to represent a fetal adaptive mechanism to the stress of chronic hypoxia.

Maturation of spontaneous and agonist-induced uterine contractions in the peripartum mouse uterus.

This study tested the hypothesis that the uterus achieves maximum contractile capabilities before the onset of labor. Basal and agonist-stimulated contractions were assessed in uterine strips on Day 15 or 18 of pregnancy, the day of parturition, or 1 day postpartum (n = 4-13 per group). Spontaneous contractions were evident in all groups (n = 4-13 per gestational group); contraction frequency was greater in peripartum groups than in virgin controls ( approximately 4.6 versus 2.8/200 sec). Peak amplitude was nearly 9-fold higher on Days 15 and 18 and over 30-fold higher in the postpartum and 1 day postpartum groups than in nonpregnant mice. Maximum frequency and peak amplitude were achieved in response to 10(-6) to 10(-8) M oxytocin or arginine vasopressin (OT(max) or AVP(max)). Frequency of contractions in response to OT(max) peaked on Day 18 and then declined. Contraction amplitude increased 5-fold on Day 15, declined on the day of birth (equivalent to nonpregnant level), then rebounded to peak on postpartum Day 1. AVP(max) similarly increased frequency and amplitude of contractions, except that maximum contraction amplitude occurred postpartum. Thus, an endogenous oscillator, residing in the uterus, sustains high basal and agonist-induced contraction frequency during pregnancy. Although acceleration of this pacemaker occurred before term, the data suggest that peripartum increases in contraction amplitude characterize the transition to the powerful synchronous contractions of parturition.

Endoscopic intrauterine surgery in primates: overcoming technical obstacles.

Current protocols for fetal surgery require cesarean section and partial fetal extraction, both of which impart significant risks to the mother and fetus. Endoscopic fetal surgery is less invasive and will likely reduce some of these risks, but the technical difficulties and feasibility in a primate model have yet to be explored fully. Four pregnant baboons (95 days gestation) were anesthetized, their uteruses exposed via an abdominal incision, and blunt-tipped flanged endoscopic ports inserted. Amniotic fluid was removed, and warmed saline was infused to dilate the uterus. To evaluate instrumentation and wound closure, the tip of the snout was externalized and bilateral cleft lip-like defects made. The lips were then endoscopically repaired by suture (Endostitch, U.S. Surgical) or unique nonpenetrating clips (VCS, U.S. Surgical). The saline was then removed, amniotic fluid returned, and the ports carefully removed. After 4 weeks, the fetuses were delivered and evaluated. Eight cleft lip-like defects were successfully repaired in all four cases. Operative time averaged 83 min. No infections, amniotic leaks, or adhesions developed. Survival was 50% with two fetuses delivering within 48 hours postoperatively: one from preterm labor, the other with fetal demise from retroperitoneal hemorrhage after operative blunt abdominal trauma. We demonstrate the feasibility of endoscopic fetal surgery in primates. The use of blunt-tipped flanged ports provides a fluid tight seal and allows appropriate closure of the fetal membranes, but requires laparotomy and uterine exposure. Distension of the uterus with warmed saline affords a larger operating field, enhancing visualization and instrumentation of the fetus. Grasping the fetus through the exposed uterus gives excellent control for repair. However, such control is also needed in a percutaneous approach. Further instrumentation development is needed to accomplish similar control for the percutaneous approach.

Functional determination of oxytocin affinity in near-term pregnant rat myometrium: effect of chronic hypoxia.

We designed the present study to determine: (1) if phenoxybenzamine can be used as an irreversible blocker for oxytocin receptors, and as such to determine oxytocin affinity, (2) if prolonged hypoxic exposure alters oxytocin receptor coupling efficacy of oxytocin receptors to post-receptor mediated mechanisms in the rat myometrium. Rats were exposed to room air (control), or to continuous hypoxia (10.5% O2) from day 19 through day 21 (2-day exposure). On day 21, one uterine horn was removed and used for in vitro study of myometrial contractile responses to oxytocin, while the other was used for oxytocin receptor analysis. In normoxic tissues, phenoxybenzamine (20 microM) decreased the maximum contractile response (EMAX) to oxytocin (155+/-17 vs. 66+/-19 g s/cm2) and oxytocin binding sites (BMAX: 253+/-35 vs. 114.9+/-21.3 fmol/mg protein). A similar degree of reduction in EMAX and BMAX were observed in hypoxic tissues. The oxytocin dissociation constant (KA) in the normoxic rat was 2.8+/-0.7 nM, which was not different from the chronic hypoxic rat (3.3+/-0.9 nM). Analysis of receptor occupancy-response curves indicated no oxytocin receptor reserve in both normoxic and hypoxic myometrium. However, for a given fraction of the total oxytocin receptors occupied, hypoxic tissue elicited a lower contractile response to oxytocin. We conclude that: (1) phenoxybenzamine is a useful tool to functionally study oxytocin receptor kinetics, (2) prolonged hypoxic exposure does not affect the oxytocin affinity, (3) no spare receptors for oxytocin are present in the rat myometrium, and (4) prolonged exposure to hypoxia decreases oxytocin receptor-effector coupling efficiency in rat myometrium.

Fetal and maternal adaptations to chronic hypoxia: prevention of premature labor in response to chronic stress.

Both mother and fetus have the remarkable ability to adapt to conditions of chronic hypoxia during the course of gestation. One of these adaptations appears to be mechanisms that prevent premature delivery despite the chronic stress of hypoxia. Our studies in the chronically hypoxic sheep revealed that the fetal adrenal is less responsive to ACTH stimulation. This in turn may prevent a premature rise in cortisol that would normally trigger labor and delivery. In the rat, the myometrium is affected with a decrease in contractile sensitivity to oxytocin following chronic hypoxia. This response is mediated by a significant reduction in myometrial oxytocin receptors. Our preliminary studies have also suggested that this blunting of myometrial responsiveness also occurs in the chronically hypoxic sheep. Taken together, these data indicate an adaptive response by both mother and fetus to prevent preterm delivery in the face of a chronic stress.

Suppression of myometrial contractile responses to oxytocin after different durations of chronic hypoxia in the near-term pregnant rat.

OBJECTIVE: In light of our previous finding that chronic hypoxia decreases the myometrial contractile response to oxytocin in the near-term pregnant rat, we designed the current study (1) to investigate the effect of duration of hypoxic exposure on the contractile response to oxytocin and oxytocin binding sites and (2) to examine the effect of prolonged hypoxia on the contractile response to aluminum fluoride. STUDY DESIGN: Rats were exposed to room air (control) or to continuous hypoxia (10.5% oxygen) from day 19 through day 21 (48-hour exposure), from day 20 through day 21 (24-hour exposure), or midday 20 through day 21 of gestation (12-hour exposure). On day 21 the uterine horns were used for oxytocin receptor analysis and for in vitro study of myometrial contractile responses to cumulative doses of oxytocin (10(-10) to 10(-6) mol/L) or aluminum fluoride (0.5 to 4.0 mmol/L sodium fluoride in 10 mumol/L aluminum chloride). RESULTS: The maximal contractile tensions for the control and 12-hour exposure showed no difference. In contrast, 24-hour hypoxic exposure resulted in a reduction of the maximal contractile tension from 143 +/- 11 (control) to 116 +/- 7 gm x sec/cm2. By 48 hours the maximal contractile tension was reduced even further, to 44 +/- 13 gm x sec/cm2. Oxytocin binding sites followed a similar trend with values changing from 256.9 +/- 34.9 for control to 122.9 +/- 26.1 and 84.9 +/- 21.3 fmol/mg protein for the 24- and 48-hour exposure groups, respectively (p < 0.01, analysis of variance), with no change in the 12-hour group. The contractile responses to aluminum fluoride were not altered. CONCLUSIONS: The suppression in the myometrial contractile response to oxytocin and oxytocin binding sites depends on the duration of hypoxic exposure. Chronic hypoxic exposure did not affect the myometrial response to aluminum fluoride.

Pharmacokinetic and fetal cardiovascular effects of enalaprilat administration to maternal rhesus macaques.

OBJECTIVE: The purpose of this study was to determine the extent of placental transfer of the angiotensin-converting enzyme inhibitor enalaprilat and the effects on maternal and fetal cardiovascular parameters. STUDY DESIGN: Between gestational days 122 and 126 (term 167 days) five rhesus macaques underwent surgery for implantation of maternal and fetal vascular catheters. At least 4 days after surgery maternal and fetal blood pressures and heart rates were recorded for 1 hour. This was followed by a 5-minute maternal venous infusion of saline solution vehicle and recording for an additional hour. Enalaprilat was then infused over 5 minutes through the maternal femoral artery at doses of 0.05, 0.1, or 0.2 mg/kg. Maternal and fetal arterial blood samples were collected for determination of blood gas status and plasma enalaprilat concentrations. RESULTS: Enalaprilat rapidly crossed the placenta, and fetal values for areas under the concentration time curve were 50% to 65% of maternal values across dose groups. Drug was retained in the fetal plasma approximately threefold to fourfold longer than in maternal plasma. Maternal heart rate, blood pressure, arterial Po2 and pH were unchanged after enalaprilat infusion, as were fetal heart rate and blood gases. In contrast, fetal arterial pressure decreased significantly (19% to 23%, p < 0.01) after maternal treatment with 0.1 and 0.2 mg/kg and remained depressed throughout the 6-hour study interval. At 0.05 mg/kg fetal arterial pressure was decreased by 13% from baseline; differences were not significantly different (p > 0.05). CONCLUSIONS: Results from this study indicate that enalaprilat rapidly crosses the primate placenta with a single intravenous administration to the mother, resulting in significant and prolonged reduction of fetal arterial pressure. Because maternal cardiovascular parameters were unaffected, enalaprilat appears to have a direct effect on fetal arterial pressure.

Effect of constant light on fetal and maternal prolactin rhythms in sheep.

A 24-h rhythm of plasma PRL is present in fetal sheep. This rhythm is synchronized to an environmental clue (zeitgeber). We determined whether the light-dark cycle (L:D) is a zeitgeber for the fetal PRL rhythm and, if so, whether the mother might convey this zeitgeber to the fetus. We kept nine ewes (twin pregnancies) in constant light (L:L) and five ewes (singleton) in 14:10 L:D from 110 days gestation. Fetuses and mothers were catheterized at 119 days gestation. Blood samples were taken hourly for 24 h after 16 days under L:L or L:D. A mean 24-h rhythm of PRL was found (by RIA) in fetuses under L:D, but not in those under L:L. However, fetuses under L:L showed individual 24-h PRL rhythms (cosinor analysis) whose acrophases were distributed around the clock. Nonsynchronized rhythms persisted after 23 and 30 days of L:L. Acrophases of PRL rhythms within a set of twins were closer than those between sets, suggesting that twins were responding to a common signal. These findings indicate that the L:D cycle is a zeitgeber for the PRL rhythm in fetal sheep and suggest that the mother might convey the zeitgeber.

Effect of chronic hypoxia on myometrial responsiveness in the pregnant rat.

Mechanisms involving the timing of normal parturition are not well understood in most animal species. To gain a greater understanding of the mechanisms, we employed hypoxia to perturb the normal system of parturition. The present study was designed to investigate the effects of chronic hypoxia on myometrial contractility in the near-term pregnant rat. Rats were exposed to room air (control) or to continuous hypoxia (10.5% O2) either from experimental days 19 through 21 (2-day exposure) or from experimental days 15 through 21 (6-day exposure). On day 21, blood was collected for hormone assays, and the uterine horns were collected from each dam. One horn was snap-frozen in liquid nitrogen for oxytocin (OT) receptor analysis, and the other was used for in vitro assessment of myometrial contractile responses to cumulative doses of OT or arginine vasopressin (AVP). Hypoxic exposure resulted in approximately 60% reduction of the maximal myometrial contractile response to OT and a significant reduction in OT binding sites from 256.9 +/- 34.9 to 84.9 +/- 21.3 fmol/mg protein (P<0.01). In contrast, the contractile response to AVP was unaffected after exposure to chronic hypoxia (P> 0.05). Additionally, we observed no difference in the plasma concentrations of estrogen, progesterone, and corticosterone. We conclude that chronic hypoxia decreased the effectiveness of OT-specific contractile mechanisms, at least partially through a decrease in OT binding sites.

Endothelin-1 potentiates the in vitro contractile response of pregnant human myometrium to oxytocin.

OBJECTIVE: This study was designed to test the hypothesis that endothelin-1 pretreatment of human myometrium at subcontractile doses in vitro will enhance the contractile response to oxytocin. STUDY DESIGN: In vitro contractile oxytocin dose-response curves were generated by use of myometrial strips collected from nonpregnant women (n = 7), pregnant patients at elective cesarean section (n = 7), and patients in active labor (n = 7) in the presence or absence of 10(-9) mol/L endothelin-1. Contractile responses were analyzed by on-line computer, and data were normalized to the maximum response to potassium. RESULTS: Pretreatment with endothelin-1 significantly increased the maximal contractile response of pregnant myometrium (p < 0.01 compared with control). In marked contrast myometrium from nonpregnant patients was unaffected by endothelin-1 pretreatment. Values for the two-point discrimination and Hill coefficient were not different among the treatment groups. CONCLUSION: Endothelin-1 potentiates the oxytocin response of myometrium from pregnant but not nonpregnant women. We speculate that a high circulating level of a uterotonin-like oxytocin may not be necessary to initiate labor. The synergistic interaction between different uterotonins may be sufficient.

Regulation of cell-cell communication mediated by connexin 43 in rabbit myometrial cells.

The aim of this study was to evaluate the rapid regulation of cell-cell communication by using the microinjection of purified cAMP-dependent protein kinase (protein kinase A), the Ca(2+)-phospholipid-dependent protein kinase (protein kinase C), or the inhibitor proteins (PKI and CKI) that are, respectively, specific for each of these enzymes. Gap junction phenotypes of myometrial tissue and cells were studied by means of immunocytochemistry with antibody to connexin 43 (alpha 1; Cx43). Cells were enzymatically disaggregated from myometrium of nonpregnant, mid-pregnant (Day 14), and late-pregnant (Day 29) rabbit uteri (n = 8 per group) and seeded at high density such that after 4 days, cultures had the appearance of a cross-sectioned myometrium. Purified proteins and their subunits were microinjected, and intercellular communication was evaluated by monitoring Lucifer Yellow dye transfer. Cultures were treated with 0.5 mM 8Br-cAMP (8-bromo adenosine 3',5' cyclic monophosphate) or 10 microM OAG (1-oleoyl-2-acetyl-sn-glycerol), which, respectively, activate protein kinase A and protein kinase C. Immunoreactive Cx43 and cell-cell communication were examined 5 min to 2 h later. Cx43 was detected in myometrial cryosections and cultured cells by indirect immunofluorescence, and its expression increased with gestation. Exposure to 8Br-cAMP increased the amount of immunoreactive Cx43. Basal dye transfer was minimal in nonpregnant cells, increased in cells of mid-pregnant uteri, and was maximal in late-pregnant cells. Treatment with 8Br-cAMP enhanced transfer in mid- and late-pregnant cells but had no obvious effect on cells from nonpregnant animals. OAG treatment inhibited dye transfer in greater than 95% of the cells tested irrespective of pregnancy status. PKI inhibited cell-cell communication within 2 min and up to 40 min. Injection of free catalytic subunit of protein kinase A following PKI inhibition restored communication within 2-3 min, with maximal transfer in 4-5 min. Protein kinase C inhibited communication, which resumed in < 3 min after injection of CKI. We conclude that rabbit myometrial cells engage in Cx43-mediated cell-cell communication and that this process increases during pregnancy. Further, activators of protein kinase A or injected free catalytic subunit rapidly enhances cell-cell communication, whereas activators of protein kinase C or the enzyme itself diminishes this process.

Fetectomy alters maternal endocrine and uterine activity rhythms in rhesus macaques during late gestation.

OBJECTIVE: The current study was designed to test the hypothesis that fetectomy will eliminate or substantially alter rhythms in maternal estradiol concentrations and subsequently reduce or eliminate uterine activity rhythms. STUDY DESIGN: Six rhesus macaques underwent surgery for catheter implantation between days 117 and 122 of gestation (term = 167 days). At surgery the fetuses were removed and the membranes and placenta remained intact. Thirteen additional catheterized pregnant animals served as controls. Maternal arterial blood samples were collected for hormone analysis at 3-hour intervals for 24 hours, starting at 9 AM. This sampling protocol was performed four times at weekly intervals until 151 to 157 days' gestation. RESULTS: A significant rhythm (p < 0.01) in estradiol was determined in the control animals with peak concentrations observed in the morning hours whereas the progesterone peak was observed at night. In the fetectomy group mean plasma estradiol concentrations decreased significantly from 312 +/- 34 to 110 +/- 8 pg/ml throughout the study (p < 0.01). Despite a trend toward elevated morning levels, the estradiol rhythm was ablated. The uterine contractile rhythm observed in the control animals with peak activity between 10 PM and midnight (p < 0.01) was also ablated after fetectomy. Basal concentrations of progesterone were significantly lower than control values. CONCLUSIONS: (1) Fetectomy resulted in the elimination of the maternal estradiol rhythm. (2) The uterine activity rhythm was lost after fetectomy. These data suggest that the fetus, by supplying precursors of estrogen, may play an indirect role in the regulation of maternal estradiol rhythms, which in turn appear to play a key role in regulating uterine activity rhythms.

Is uterine blood flow controlled locally or systemically in the pregnant rabbit?

OBJECTIVE: We tested the hypothesis that uterine blood flow is regulated by systemic circulating factors. The alternative hypothesis is that uterine blood flow is regulated by local factors. STUDY DESIGN: Adult female New Zealand White rabbits were subjected to a unilateral tubal ligation and thereafter allowed to become pregnant (n = 9). A group of nonpregnant one-tube-ligated animals served as controls (n = 8). On day 21 of gestation uterine blood flow in the pregnant and nonpregnant uterine horns were measured with 15 microns microspheres. The concentration of prostaglandin E2 metabolites were measured in blood from the uterine veins and from the arterial circulation. RESULTS: Absolute uterine blood flow in the pregnant uterine horn was 12.9 +/- 4.7 versus 5.2 +/- 1.4 ml in the nonpregnant horn (p < 0.05). However, when expressed by blood flow per gram of tissue they were not different (p > 0.1). The uterine blood flow for the nonpregnant uterine horn in the pregnant animals was the same as that of the horns from nonpregnant animals. The level of prostaglandin E metabolites was greater in the uterine vein draining the pregnant horn compared to the nonpregnant horn (p < 0.05). CONCLUSION: These data support the conclusion that the increase in uterine blood flow observed during pregnancy is controlled largely by local factors induced by pregnancy.

Changes in ovine fetal adrenocortical responsiveness after long-term hypoxemia.

This study tested the hypothesis that in the fetus long-term hypoxemia induces premature adrenocortical maturation and augments adrenal responsiveness to adrenocorticotropin hormone (ACTH). Pregnant ewes were exposed to high altitude (3,820 m) from 30 to 120 days gestation, when surgery was performed. Maternal arterial pressure of O2 (PaO2) was maintained at approximately 60 Torr by N2 infusion through a tracheal catheter. Fetal PaO2 was significantly lower in the hypoxemic (21 +/- 0.2 Torr) vs. normoxic (26 +/- 0.4 Torr) fetuses (P < 0.01). Between 125 and 140 days, basal ACTH and cortisol concentrations were similar in both groups. To assess changes in adrenal responsiveness, we challenged the fetuses with ACTH (100 ng/kg body wt, iv bolus) at 126 and 136 days. At 126 days, after ACTH challenge, fetal plasma ACTH peaked at similar values (275 +/- 43 and 250 +/- 26 pg/ml) in normoxic and hypoxemic fetuses, respectively. Plasma cortisol subsequently increased to 84 +/- 8 and 44 +/- 6 ng/ml in these groups. At 136 days, after ACTH challenge, plasma ACTH peaked at 379 +/- 57 and 336 +/- 21 pg/ml in normoxic and hypoxemic fetuses, respectively. Although plasma cortisol concentration in normoxic fetuses increased to 180 +/- 21 ng/ml, levels in hypoxemic fetuses only reached 62 +/- 12 ng/ml (P < 0.05 compared with normoxic). Catecholamine concentrations were not significantly different between the two groups. These data do not support the hypothesis that adrenocortical maturation occurs prematurely, augmenting adrenal responsiveness to ACTH after exposure to long-term hypoxemia. Rather, the ability of the fetus to respond to an ACTH challenge is blunted.

Relationship of circadian rhythms of uterine activity with term and preterm delivery.

OBJECTIVE: Our aim was to document the presence or significance of circadian uterine activity rhythms in pregnant women who delivered at term and preterm. STUDY DESIGN: We measured uterine activity in 19 women divided into a control group (low risk for preterm labor, term delivery, n = 7), a group at high risk for preterm labor, term delivery (n = 6), and a group at high risk for preterm labor, preterm delivery (n = 6). Patients were hospitalized for 24 hours every 2 weeks from 26 weeks' gestation until delivery. Uterine activity was measured continuously by external tocodynamometer. RESULTS: Patients delivering at term demonstrated a nocturnal surge (4 to 7 AM) in uterine activity the last 80 days before delivery (p < 0.05, analysis of variance). Patients delivered preterm showed an initial nocturnal surge of uterine activity similar to those delivered at term, but this disappeared 24 days before delivery (p > 0.05, analysis of variance). CONCLUSION: Uterine activity nocturnal surges normally precede term delivery. These surges are lost in women who deliver prematurely.

Cardiovascular responses to adenosine in fetal sheep: autonomic blockade.

The mechanism by which adenosine increases heart rate was investigated in 21 chronically catheterized fetal sheep (> 0.8 term). Intra-arterial infusion of adenosine (0.16 mg.min-1.kg fetal wt-1) for 1 h significantly increased fetal heart rate within 5 min with maximum values of approximately 68 beats/min above the control mean of 163 +/- 8 beats/min. The average diastolic blood pressure was reduced only during the first 10 min of infusion, and the average systolic and mean arterial pressures were not significantly affected. Mean venous pressure rose by approximately 48% after 20 min of adenosine infusion, but all other measurements did not differ significantly from the control value. The mean hemoglobin concentration during the last 30 min of infusion was increased by approximately 8%. Plasma concentrations of norepinephrine and epinephrine were elevated only during the first 30 min of adenosine administration, to values as high as 2.3 and 5 times the respective control mean. Adenosine significantly increased mean fetal heart rate by about 15-20 beats/min in fetuses with autonomic ganglion blockade or combined cholinergic, alpha-, and beta-adrenergic receptor blockade. Intra-arterial infusion of CGS 21680C, an A2-adenosine receptor agonist, also produced a fetal tachycardia of approximately 86 beats/min above the control mean and increased intrinsic fetal heart rate by approximately 38 beats/min. It is concluded that approximately 75% of the positive chronotropic effects of adenosine are produced by A2-receptor stimulation of the autonomic nervous system and that approximately 25% of the rise in heart rate induced by adenosine may be caused by activation of A2-receptors in myocardium.