Fetal endocrine responses to prolonged reduced uterine blood flow are altered following bilateral sectioning of the carotid sinus and vagus nerves

Long-term hypoxia alters endocrine and physiologic responses to umbilical cord occlusion in the ovine fetus

OBJECTIVE

This study was designed to determine the effect of umbilical cord occlusion (UCO) on fetal endocrine responses in the long-term hypoxemic (LTH) ovine fetus.

METHODS

Pregnant ewes were maintained at high altitude (3820 m) from day 30 of gestation. Normoxic control and LTH fetuses were catheterized, and an inflatable occluder was placed on the umbilical cord at day 132 of gestation. In the LTH group, maternal oxygen tension was maintained at approximately 60 mmHg by nitrogen infusion through a maternal tracheal catheter. On day 137, two 5-minute UCOs were performed. On day 139, the study was repeated with a 10-minute UCO.

RESULTS

Basal adrenocorticotropic hormone (ACTH) levels and peak responses to the first 5-minute UCO were not different between control and LTH fetuses (17.6 +/- 4.0 to 418.8 +/- 41.3 in controls, 25.7 +/- 4.0 to 530.0 +/- 93.0 pg/mL in LTH fetuses). A similar pattern was observed during the second UCO. Basal cortisol levels were similar in both groups. In response to UCO, a significant increase in cortisol was observed in both groups, but peak concentrations in the LTH group were significantly higher than those in the control group (23.9 +/- 4.8 versus 14.8 +/- 2.9 ng/mL, respectively, P <.05). The second occlusion also increased cortisol concentrations, but no differences were observed between groups. After the 10-minute UCO, the ACTH and cortisol responses were similar to the first 5-minute occlusion, with higher cortisol levels in the LTH fetuses.

CONCLUSION

Despite similar ACTH responses to UCO, the cortisol response was greater in the LTH fetuses than in normoxic controls. LTH appears to result in enhanced adrenal sensitivity to a secondary stressor or altered cortisol metabolism.

Long-term hypoxia alters ovine fetal endocrine and physiological responses to hypotension

Exposure to long-term hypoxia (LTH) results in altered cortisol responses in the ovine fetus. The present study was designed to test the hypothesis that LTH alters adrenal responsiveness to fetal hypotension. Pregnant ewes were maintained at high altitude (3,820 meters) from day 30 of gestation. Normoxic control and LTH fetuses were catheterized on day 132 of gestation. In the LTH group, maternal Po(2) was maintained comparable to that observed at altitude ( approximately 60 mmHg) by nitrogen infusion through a tracheal catheter. On day 137, fetuses received a 5-h saline infusion followed by infusion of sodium nitroprusside to reduce fetal arterial pressure by 30-35% for 10 min. The study was repeated on day 139 of gestation with a continuous cortisol infusion (10 microg/min). Hypothalamic and pituitary tissues were collected from additional fetuses for assessment of glucocorticoid receptors. During the saline infusion in response to hypotension, plasma ACTH increased over preinfusion mean values in both groups (P < 0.05). Plasma cortisol concentrations increased in both groups concomitant with increased ACTH secretion. However, peak values in the LTH fetuses were significantly higher compared with controls (P < 0.05). During the cortisol infusion, the ACTH response was eliminated in both groups, with ACTH levels significantly lower in the LTH group (P < 0.05). Glucocorticoid receptor binding was not different between groups. These results demonstrate an enhanced cortisol response to hypotension in LTH fetuses that does not appear to be the result of an increase in negative feedback sensitivity of the hypothalamic-pituitary-adrenal axis.

Mechano- and chemoreceptor modulation of renal sympathetic nerve activity at birth in fetal sheep

Physiological responses at birth include increases in heart rate (HR), blood pressure, sympathetic nerve activity, and circulating vasoactive peptides. The factors mediating these responses are not known. To test the hypothesis that afferent input from peripheral mechanoreceptors (arterial and cardiopulmonary baroreceptors) and chemoreceptors contribute to the sympathoexcitatory and hormonal responses at birth, we studied the effects of sinoaortic denervation (SAD) and SAD with vagotomy (Vx) on changes in HR, mean arterial blood pressure (MABP), renal sympathetic nerve activity (RSNA), and catecholamine, arginine vasopressin (AVP), and ANG II levels at birth in term sheep. One hour after delivery by cesarean section, RSNA increased by 168 +/- 49 and 192 +/- 32% (relative to fetal values) in SAD and SAD-Vx animals, respectively. Significant increases in HR (18 +/- 5 and 20 +/- 6%) and MABP (24 +/- 4 and 20 +/- 5%) were also observed 1 h after delivery in SAD and SAD-Vx lambs, respectively. These responses are similar to those seen in intact sheep delivered at the same gestational age. AVP levels markedly increased after birth (19.8 +/- 6.7 to 136.1 +/- 75.9 pg/ml) in SAD-Vx lambs, whereas SAD animals displayed no change in AVP concentrations. Plasma ANG II also did not change after birth in either group, although levels were consistently higher (P < 0.01) in SAD compared with SAD-Vx animals. In the presence of SAD, Vx resulted in significantly greater plasma levels of norepinephrine, although levels did not change after birth in either group. The epinephrine responses at birth were similar in both groups of animals. The present data suggest that afferent input from peripheral chemoreceptors and mechanoreceptors contributes little to the hemodynamic and sympathetic responses after delivery by cesarean section. On the other hand, these peripheral mechanisms appear to be involved in modulating endocrine responses at birth.

Altered fetal cardiovascular responses to prolonged hypoxia after sinoaortic denervation

This study examines the role of the peripheral chemoreceptors in mediating fetal cardiovascular responses to prolonged hypoxia secondary to reduced uterine blood flow (RUBF). Fetal sheep were chronically instrumented for continuous heart rate (FHR), blood pressure (FBP), and carotid blood flow (CBF) measurements after bilateral sectioning of the carotid sinus and vagus nerves (denervated, n = 7) or sham denervation (intact, n = 7). Four days postoperatively, uterine blood flow was mechanically restricted, reducing fetal arterial oxygen saturation by 47.3% (P < 0.01). An initial bradycardia was observed in intact (184.0 +/- 10.7 to 160.5 +/- 10.7 beats/min, not significant) but not denervated fetuses, followed by a tachycardia (180.0 +/- 2.2 to 193.7 +/- 2.7 beats/min, P < 0.05). FHR increased in denervated fetuses (175.5 +/- 8.7 to 203. 0 +/- 17.9 beats/min, P < 0.05). FBP increased transiently in intact fetuses from 45.1 +/- 1.0 to 55.4 +/- 3.0 mmHg at 2 h (P < 0.01), whereas denervated fetuses demonstrated a decrease in FBP from 47.1 +/- 4.2 to 37.2 +/- 3.7 mmHg (not significant). CBF increased (P < 0. 05) in both intact and denervated fetuses from 39.3 +/- 2.8 and 29.7 +/- 3.8 ml. min-1. kg-1 to 47.7 +/- 0.4 and 39.1 +/- 0.3 ml. min-1. kg-1, respectively, whereas carotid vascular resistance decreased only in denervated fetuses (1.7 +/- 0.1 to 1.1 +/- 0.02 mmHg. ml-1. min. kg-1, P < 0.05). We conclude that the peripheral chemoreceptors play an important role in mediating fetal cardiovascular responses to prolonged RUBF.

The effect of repeated acute hypoxaemia on fetal cardiovascular development in the sheep

1. Hypoxaemia during intrauterine life may be important in the development of cardiovascular diseases in later life. Thus it was the aim of this study to investigate the effect of repeated acute hypoxia on the cardiovascular development and growth of the fetus. 2. Fourteen fetal sheep (105-109 days gestational age) were instrumented with amniotic and vascular catheters, an electrocardiogram (ECG) electrode and a flow probe around the femoral artery. Seven animals were given repeated acute isocapnic hypoxaemia (Pa,O2 reduced to ca. 13 mmHg) for 1 h every day for 14 days and they were compared to seven animals which remained normoxic throughout with respect to fetal mean arterial blood pressure (MAP), fetal heart rate (FHR), and fetal baro- and chemoreflexes. 3. No differences were found between the two groups of fetuses in FHR, MAP, baro- or chemoreflexes, femoral blood flow, femoral vascular resistance or fetal growth. 4. Repeated acute hypoxaemia of a moderate degree over a period of 2 weeks in late gestation does not affect cardiovascular development or growth in the fetal sheep.