The effect of prolonged hypobaric hypoxia on growth of fetal sheep

The effect of prolonged hypobaric hypoxia on fetal sheep was studied. Pregnant ewes were subjected to an atmospheric pressure of 429 torr from 30 days to 135 days gestation (long-term study). Average fetal weight for the hypoxaemic group (3.35 +/- 0.53 kg; n = 4; mean +/- SD) was significantly lower than for the controls (4.23 +/- 0.29 kg; n = 7; P less than 0.05). A short-term study was undertaken with fetuses (n = 8) which were catheterized at 110 days gestation and whose dams were subjected to hypobaric hypoxia from 120 to 141 days gestation. The mean carotid PO2 of fetuses in the hypoxic group was 12.7 +/- 0.7 torr compared to 22.7 +/- 0.7 torr for the control group (n = 9; P less than 0.001) throughout the period of treatment. Fetal arterial oxygen content fell from 6.5 +/- 1.7 to 4.9 +/- 0.4 ml/dl (P less than 0.05), but rose to control values after 7 days due to an increase in fetal haemoglobin concentration (9.6 +/- 1.1 to 13.0 +/- 1.9 g/dl, P less than 0.001) and packed cell volume (33 +/- 3 to 45 +/- 4%, P less than 0.001). In the hypoxaemic fetuses, pH fell initially from 7.34 +/- 0.02 to 7.28 +/- 0.03 (P less than 0.05) and then recovered to 7.32 +/- 0.03 within 24 h. Mean fetal weight of the short-term hypoxic group was 3.46 +/- 0.72 kg compared to 4.15 +/- 0.51 for the control group (P less than 0.05). Both long- and short-term hypoxia produced a similar reduction in fetal body weight. The adrenal glands were significantly heavier in the hypoxic fetuses than in controls. Placental weight was not effected by hypoxia, but exposure from 30 days gestation reduced the average size of cotyledons (P less than 0.05). It is concluded that the fetal sheep increases its ability to acquire and transport oxygen in response to chronic hypoxia, but this compensation is not sufficient to prevent growth retardation or changes to the pattern of tissue growth.     

Effect of hypoxia on the initiation of secondary wool follicles in the fetus

The development of secondary wool follicles in single fetal sheep subjected to hypobaric hypoxaemia was studied. One group of pregnant ewes were exposed to 57.1 kPa from 30 to 135 days gestation. Fetal weights (mean +/- s.d.) for the hypoxaemic group (3.35 +/- 0.53 kg; n = 4) were significantly lower than for the controls (4.19 +/- 0.31 kg; n = 3, P less than 0.05). At 110 days gestation, a second group had arterial and venous catheters surgically implanted into the ewe and fetus and skin samples were taken from the fetus. At 120 days gestation (10 days after surgery) these animals were subjected to hypoxia for 20 days, at a level to maintain fetal carotid pO2 between 1.47 and 1.87 kPa (mean carotid pO2 for the control fetuses was 2.84 +/- 0.28 kPa). Fetal weight at 140 days was not significantly different in the hypoxaemic and control groups. Morphometric analysis revealed that the secondary to primary follicle ratio (S:P) was less in both groups of hypoxaemic fetuses than in their respective controls. Although hypoxia for 20 days did not significantly alter fetal weight, it produced a low S:P ratio similar to the longer-term hypoxaemic animals. It is concluded that hypoxia has a marked effect in reducing the initiation of secondary follicles in the last third of gestation.     

Fetal pulmonary vasodilation and vasoreactivity during metabolic acidaemia

We studied the pulmonary vascular response to progressive metabolic acidaemia and to an abrupt increase in oxygen tension during metabolic acidaemia in 8 chronically-prepared fetal sheep. Left pulmonary artery blood flow was measured by electromagnetic flow transducer. Two and a half hour infusion of NH4Cl into the fetal inferior vena cava caused pH to fall to 6.94 +/- 0.01 from 7.37 +/- 0.01 (P less than 0.001). During this period of progressive metabolic acidaemia, left pulmonary artery blood flow increased from a baseline value of 60 +/- 8 to 105 +/- 14 ml.min-1 (P less than 0.002). Pulmonary artery pressure did not change significantly and calculated pulmonary vascular resistance fell indicating fetal pulmonary vasodilation. PO2 rose significantly (19.8 +/- 0.7 to 24.1 +/- 1.8 torr; P less than 0.03) and oxygen saturation fell (54.6 +/- 2.8% to 38.9 +/- 3.5%; P less than 0.001) confirming a rightward shift of the oxyhaemoglobin dissociation curve. During acidaemia, administration of 100% oxygen to the ewe further increased fetal PO2 to 37.9 +/- 2.3 torr within 10 min (P less than 0.001) and this increase in PO2 was accompanied by an increase in left pulmonary artery blood flow (P less than 0.001), a fall in pulmonary artery pressure (P less than 0.03) and a decrease in pulmonary vascular resistance (P less than 0.001) indicating further vasodilation. The response of the fetal pulmonary circulation to a 2-h period of increased oxygen tension was qualitatively similar in acidaemic and non-acidaemic fetuses. We conclude that the progressive metabolic acidaemia imposed by these experimental conditions increases pulmonary blood flow likely through an increase in fetal PO2 and that metabolic acidaemia does not block the normal vasodilatory response to an increase in oxygen tension.     

In vivo brown fat response to hypothermia and norepinephrine in the ovine fetus

The goal of this study was to assess the response of fetal brown fat in vivo to hypothermia and norepinephrine infusion. In 10 unanaesthetized, chronically-prepared fetal sheep (133 +/- 2 days of gestation) cold water was passed through tubing encircling the fetus in utero and plasma glycerol concentration was measured as an indicator of brown fat activity. Following cooling for 60 min, amniotic fluid temperature fell 7.79 degrees C to 31.66 +/- 1.73 degrees C (n = 8, P less than 0.001) and maternal temperature fell 0.63 degree C to 38.63 +/- 0.08 degrees C (n = 9, P less than 0.001). Eight of the fetuses were subjected to a second experiment in which norepinephrine was infused intravenously for 15 min. During infusion fetal arterial temperature fell 0.38 degrees C to 39.05 +/- 0.25 degrees C (n = 7, P less than 0.05). Amniotic fluid temperature (n = 7, NS) and maternal arterial temperature (n = 7, NS) remained constant. Glycerol concentration during the infusion increased from 0.73 to 1.27 mg/dl, a 74% increase over control (n = 8, P less than 0.001). Although clearly detectable, these glycerol responses to hypothermia and norepinephrine stimulation are one-third or less of those achieved after birth, indicating that thermogenesis remains quiescent in the near-term fetal sheep, despite powerful stimuli for activation.     

Changes in hypoxanthine and lactate during and after hypoxia in the fetal sheep with chronically-implanted vascular catheters

The effect of intra-uterine hypoxia on the hypoxanthine and lactate concentration in fetal sheep with catheters chronically implanted was investigated. Experiments were conducted on five fetuses. Sixty-four blood samples from nine hypoxic and recovery periods were analysed. A significant increase of hypoxanthine and lactate occurred in parallel with the fall of arterial oxygen saturation (SaO2) and arterial oxygen pressure (PaO2) during the first 20 min of hypoxia. The elevations in plasma hypoxanthine and lactate were significantly greater during more severe hypoxia than mild hypoxia, as judged from the amount of low oxygen gas mixture given to the ewe (7 or 9%). There were no difference in PaO2 and only minor difference in SaO2 between the two groups. The increase in lactate over 20 min was the same throughout the one-hour period of hypoxia, while the increase of hypoxanthine was less pronounced at the end of the period. This might be due to the fact that hypoxanthine was cleared from fetal plasma at a fairly rapid rate, half of the excess concentration being eliminated after 25 +/- 21 min compared to 85 +/- 47 min for lactate in six experiments post hypoxia. Linear regression analysis revealed a highly significant correlation between hypoxanthine and SaO2, pH and lactate (P less than 0.001). These three variables explained 77% of the variance of hypoxanthine, when calculated by multiple regression analysis.     

Effect of thyroidectomy on cardiovascular responses to hypoxia and tyramine infusion in fetal sheep

Fetal sheep were thyroidectomized at 80 days' gestation and reoperated at 118-122 days for insertion of vascular catheters. The effects of hypoxaemia and intravenous tyramine infusion on plasma catecholamine concentrations, blood pressure and heart rate were then determined in experiments at 125-135 days' gestation. Age matched intact fetuses were also studied. Thyroidectomy was associated with increased concentrations of noradrenaline, adrenaline and dopamine in some thoracic and abdominal organs, increased noradrenaline concentrations in the cerebellum, and decreased adrenaline concentrations in the hypothalamus, cervical spinal cord, and superior cervical and inferior mesenteric ganglia. Arterial pressure was significantly lower in the thyroidectomized fetuses (34.0 +/- 0.15 mmHg) than in intact fetuses (44.7 +/- 0.2 mmHg; p less than 0.001). In contrast, plasma noradrenaline concentrations were significantly higher in the thyroidectomized fetuses (2.04 +/- 0.25 ng/ml) compared to the intact fetuses (0.99 +/- 0.08 ng/ml; P less than 0.001). In the intact fetuses there was a significant increase in plasma noradrenaline concentration and blood pressure during hypoxaemia, and bradycardia at the onset of hypoxaemia. In contrast, in the thyroidectomized fetuses hypoxaemia did not cause significant change in plasma catecholamine concentrations, blood pressure or heart rate. Infusion of tyramine produced a 1.9-fold increase of plasma noradrenaline in thyroidectomized fetuses compared to a 9.2-fold increase in the intact fetuses (P less than 0.05). Tyramine infusion caused a similar proportional increase of blood pressure in both thyroidectomized and intact fetuses. Heart rate decreased during the tyramine-induced hypertension in the intact fetus, but increased in the thyroidectomized fetuses.(ABSTRACT TRUNCATED AT 250 WORDS)     

Changes in the PR, RR intervals and ST waveform of the fetal lamb electrocardiogram with acute hypoxemia.

The PR and RR intervals and T wave amplitude of the fetal lamb electrocardiogram were studied during acute hypoxemia produced by reduction of the maternal placental blood flow. Five chronically-instrumented fetal lambs (124 to 143 days of gestation) were subjected to acute hypoxemia (observations = 13) through complete occlusion of the maternal aorta for 60 s. The fetuses responded to the occlusion with a fall in oxygen tension (2.18 +/- 0.12 kPa to 1.11 +/- 0.14 kPa, SEM, P < 0.001) and oxygen saturation (48 +/- 4% to 19 +/- 4%, P < 0.001). Modest changes of pH (7.37 +/- 0.05 to 7.35 +/- 0.01, p), carbon dioxide tension (5.79 +/- 0.15 kPa to 6.17 +/- 0.14 kPa, P < 0.001) and plasma lactate concentration (2.1 +/- 0.6 mmol/l to 2.2 +/- 0.6 mmol/l, ns) occurred. The PR interval showed a triphasic pattern following occlusion. Initially, and simultaneously with the onset of the RR interval lengthening, a prolongation of the PR interval occurred (P < 0.01) with a peak value after 41 +/- 3 s after occlusion. Following this transient prolongation, the PR interval shortened concurrently with a maximum lengthening of the RR interval (P < 0.001) 2 +/- 3 s after the end of the occlusion. A maximum PR shortening (P < 0.001) occurred 27 +/- 5 s after occlusion followed by a prolongation of the PR interval (P < 0.001) with a peak value 203 +/- 21 s after release of the occlusion.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effects of naloxone on the breathing, electrocortical, heart rate, glucose and cortisol responses to hypoxia in the sheep fetus

Continuous infusions of naloxone HC1 (0.5 mg/kg or 3.8 mg/kg) or saline were given intravenously to fetal sheep at 119 to 137 days of gestation during a one hour period of air administration and a one hour period of hypoxia induced by having ewes breathe 9% O2, 3% CO2 and 88% N2. Fetal carotid PaO2 fell to 13.0 +/- 0.5 mmHg during hypoxia with no change in pH. During hypoxia, plasma cortisol concentration increased significantly more in naloxone-infused fetuses than controls. Ewes, whose fetuses received naloxone, showed a significant increase in cortisol during hypoxia whereas no increase was observed in controls. There were no significant differences between saline and naloxone-infused fetuses during hypoxia in fetal breathing incidence, amplitude, frequency, number of deep inspiratory efforts per hour, heart rate, electrocortical activity or in the rise in plasma glucose caused by hypoxia. Results suggest that endogenous opiates may have a role in modulating cortisol production in the ewe and fetus during hypoxia but do not have a role in mediating the decrease in incidence of breathing activity or rise in plasma glucose. During air administration, naloxone significantly increased fetal breath amplitude, fetal and maternal plasma glucose, fetal heart rate, and the number of electrocortical changes per hour. This suggests endogenous opiates may have a more important role in the normoxic pregnant ewe and fetus.     

Effect on maternal and fetal renal function and plasma renin activity of a high salt intake by the ewe

The effect on renal function of replacing maternal drinking water with a solution containing 0.17 M NaCl was studied in 9 ewes and their chronically catheterised fetuses over a period of 9 days. Maternal sodium intake increased from control values of 2.19 +/- 0.09 mmol/h to 44.3 +/- 7.4 (P less than 0.001) and 46.3 +/- 6.5 mmol/h (P less than 0.001) on the 3rd and 6th days of salt ingestion. Maternal plasma sodium levels were not affected, but the urinary sodium/potassium ratio increased from 0.15 +/- 0.07 to 2.26 +/- 0.34 (P less than 0.001) after 6 days and plasma renin activity fell from 2.87 +/- 0.76 to 1.00 +/- 0.25 ng/ml per h (P less than 0.05). The changes in maternal sodium intake had no effect on fetal plasma sodium levels nor on fetal plasma renin activity. Sodium excretion and fetal urinary sodium/potassium ratio did not change. However, 3 days after the ewes returned to drinking water fetal plasma renin activity was significantly higher than it was prior to maternal ingestion of 0.17 M NaCl. Fetal plasma renin activity was inversely related to fetal plasma sodium levels (P less than 0.01). The results show that changes in maternal sodium intake had no long term effect on fetal plasma sodium levels nor on fetal renal sodium excretion. The fall in maternal plasma renin activity in the absence of any change in the fetal renin activity, indicates that the fetal renin angiotensin system is controlled by factors other than those influencing the maternal renin angiotensin system. Since fetal urinary sodium/potassium ratios remained unchanged it would suggest that fetal sodium excretion is not influenced by maternal levels of aldosterone.     

The value of urine osmolality as an index of stress in the ovine fetus

In ovine fetuses, during 100-130 days of gestation, urine osmolalities less than 175 mosmol/kg water were associated with plasma immunoreactive adrenocorticotrophin (ACTH) concentrations below 40 pg/ml in 40/41 samples. In 18/29 fetuses with urine osmolalities greater than 175 mosmol/kg water plasma ACTH was significantly elevated. In 38 samples of fetal blood there was a significant correlation between plasma ADH and ACTH concentrations. By least squares regression the equation to the line was [ACTH] = 5.06 + 3.70 [ADH] (r = 0.62, P less than 0.001). In 50 samples from fetuses of gestational ages 100-140 days, with urine osmolalities of 302 +/- 86 mosmol/kg (mean +/- SD) the blood pH, pO2 and pCO2 values were not significantly different from those in 50 samples from fetuses with urine osmolalities of 125 +/- 22 mosmol/kg. It is proposed that the measurement of fetal urine osmolality provides a good index of fetal stress. A fetus with a urine osmolality less than 175 mosmol/kg is almost invariably in the optimum, unstressed condition.     

Relationship between corpora lutea or fetal number and plasma concentrations of progesterone and testosterone in mice

Blastocysts (1-14) were transferred unilaterally into 63 pseudopregnant mice which were killed on Day 17. Plasma progesterone concentrations were significantly (P less than 0.05) lower in animals with one fetus than in those with 2-5 or 9-14 fetuses. Plasma testosterone concentrations were correlated with fetal number in mice with 1-13 fetuses (P less than 0.001). The total placental content of chorionic gonadotrophin in 13 litters varied directly with the number in the litter (1-6), and was 1.67 +/- 0.15 ng/placenta. The number of corpora lutea per mouse was negatively correlated with mean CL volume per mouse (P less than 0.001), and the number of conceptuses was positively correlated with mean CL volume per mouse (P less than 0.001). The effect of conceptuses on the ovary was systemic. The relationship between plasma testosterone concentration and conceptus number may be due to gonadotrophins acting on the ovary, or androgens produced by the placenta or fetus.     

Effect of physical training on the capacity to secrete epinephrine

Epinephrine responses to hypoglycemia and to identical relative work loads have been shown to be higher in endurance-trained athletes than in untrained subjects. To test the hypothesis that training increases the adrenal medullary secretory capacity, we studied the effects of glucagon (1 mg/70 kg iv), acute hypercapnia (inspired O2 fraction = 7%), and acute hypobaric hypoxia (inspired Po2 = 87 Torr), respectively, on the epinephrine concentration in arterialized hand vein blood in eight endurance-trained athletes [T, O2 uptake = 66 (62-70) ml.min-1.kg-1] and seven sedentary males [C, O2 uptake = 46 (41-50)]. In response to identical increments in glucagon concentrations, plasma epinephrine increased more in T than in C subjects [0.87 +/- 0.11 vs. 0.38 +/- 0.14 (SE) nmol/l, P less than 0.05]. In response to hypercapnia [arterial PCO2 = 56 +/- 0.7 Torr (T) and 55 +/- 0.4 (C), P greater than 0.05], the increment in epinephrine was significant in T (0.38 +/- 0.11 nmol/l) but not (P less than 0.1) in C subjects (0.22 +/- 0.11). Hypoxia [arterial PO2 = 42 +/- 2 Torr (T) and 41 +/- 2 (C), P greater than 0.05] increased epinephrine in T (0.22 +/- 0.10 nmol/l, P less than 0.05) but not in C subjects (0.01 +/- 0.07). The plasma norepinephrine concentration never changed, whereas heart rate always increased, the increase being higher (P less than 0.05) in T than in C subjects only during hypercapnia. The results indicate that training increases the capacity to secrete epinephrine.     

Plasma adenosine and cardiovascular responses to dipyridamole in fetal sheep.

The effects of dipyridamole infusion on fetal arterial plasma adenosine level, [ADO], and the systemic cardiovascular system were studied in 10 fetal sheep at 130-135 days gestational age. Dipyridamole (0.25 mg/kg) was infused into the fetuses intravenously during normoxia and hypoxia. Plasma [ADO] was measured using high-performance liquid chromatography, (HPLC), and fetal heart rate and arterial blood pressure were monitored throughout the study. These studies were performed in the absence and presence of theophylline, an adenosine receptor antagonist. During normoxia (PO2, 23.8 +/- 2.0 Torr), dipyridamole infusion increased fetal plasma [ADO] from 0.82 +/- 0.10 microM to 1.41 +/- 0.16 microM within 1 min (P < 0.01) and fetal heart rate from 157 +/- 6 bpm to 174 +/- 7 bpm (P < 0.01), but did not change mean blood pressure. Fetal plasma [ADO] and fetal heart rate returned to basal levels quickly. Treatment with theophylline did not alter the elevation of plasma [ADO] after dipyridamole infusion, but abolished responses of fetal heart rate to dipyridamole infusion. After 15 min of hypoxia with an average arterial PO2 of 15.4 +/- 1.1 Torr, fetal plasma [ADO] increased to 1.15 +/- 0.14 microM (P < 0.01). Dipyridamole infusion then further raised fetal plasma [ADO] to 1.67 +/- 0.27 microM (P < 0.01). The duration of the increase of fetal plasma [ADO] after dipyridamole infusion was no longer in hypoxia than in normoxia, however there was no significant change in the pattern of transient fetal bradycardia and persistent hypertension.(ABSTRACT TRUNCATED AT 250 WORDS)     

Erythropoietin responses to progressive blood loss over 10 days in the ovine fetus

Long-term loss of fetal blood can occur with fetomaternal hemorrhage, vasoprevia, or placental previa. Our objective was to determine the effects of progressive fetal blood loss over 10 days on fetal plasma erythropoietin (EPO) concentration and its relationship to arterial PO(2), hematocrit, and the volume of blood loss. Late-gestation fetal sheep (n = 8) were hemorrhaged daily at a rate of 1 ml/min over 10 days. The extent of hemorrhage differed in each fetus and ranged from 30 to 80 ml/day, with the cumulative volume removed ranging from 78 to 236 ml/kg estimated fetal weight. Four fetuses served as time controls. EPO concentration measurements were by radioimmunoassay. Statistical analyses included regression, correlation, and analysis of variance. We found that EPO and arterial PO(2) were unchanged until the cumulative hemorrhage volume exceeded 20-40 ml/kg. Once this threshold was exceeded, plasma EPO concentration increased progressively throughout the study and averaged 14.3 +/- 3.2 times basal values on day 10. EPO concentration, arterial PO(2), and hematocrit changes were related curvilinearly to cumulative hemorrhage volume (P < 0.01), whereas the relationship between plasma EPO and arterial PO(2) was log linear (P < 0.001). We conclude that 1) fetal plasma EPO concentration and arterial PO(2) are insensitive to a slow, mild-to-moderate blood loss over several days; 2) unlike the rapid return of EPO to normal within 48 h after acute hemorrhage, fetal EPO concentration undergoes a progressive increase with moderate-to-severe blood loss over several days; 3) the long-term hemorrhage-induced changes in EPO are best correlated with arterial PO(2); and 4) the fetal EPO response to hemorrhage does not appear to be limited by the fetus's ability to produce EPO.     

Cortisol induces perinatal hepatic gluconeogenesis in the lamb.

To examine the influence of a prenatal increase in plasma cortisol concentration on perinatal initiation of hepatic gluconeogenesis, we infused cortisol into seven fetal sheep at 137-140 days gestation. 14C-Lactate provided tracer substrate for estimation of gluconeogenesis. We measured hepatic blood flow using radionuclide-labeled microspheres. After delivery, fetal arterial blood glucose concentration (1.33 +/- 0.4 mmol/l) increased transiently, but returned to fetal levels within 1 h after delivery. Substantial hepatic gluconeogenesis was induced in the fetus after cortisol infusion, averaging 23.4 +/- 12.2 mumol/min/100 g liver (7.8 +/- 4.4 mumol/min/kg fetal weight). Fetal hepatic glucose output was 44.4 +/- 17.7 mumol/min/100 g liver. Hepatic glucose output did not change after delivery; estimated gluconeogenesis decreased immediately, then increased by 6 h after delivery. Lactate supply to the liver fell substantially, from 1.1 +/- 0.4 mmol/min/100 g in the fetus to 0.24 +/- 0.09 at 1 h after delivery. Lactate flux across the liver decreased from 75.3 +/- 23 mumol/min/100 g in the fetus to 20.2 +/- 15.7 at 1 h after delivery. Hepatic lactate flux was significantly related to gluconeogenesis (r = 0.734, P = 0.0001). We conclude that cortisol induces substantial hepatic gluconeogenesis in fetal sheep near term. After delivery, there appears to be a transient decline in gluconeogenesis from lactate, which may be secondary to limited hepatic oxygen and substrate supply. Onset of gluconeogenesis in the fetus fails to sustain increases in either fetal or postnatal blood glucose concentrations.     

Epinephrine-induced changes in muscle carbohydrate metabolism during exercise in male subjects

Epinephrine increases glycogenolysis in resting skeletal muscle, but less is known about the effects of epinephrine on exercising muscle. To study this, epinephrine was given intraarterially to one leg during two-legged cycle exercise in nine healthy males. The epinephrine-stimulated (EPI) and non-stimulated (C) legs were compared with regard to glycogen, glucose, glucose 6-phosphate (G6P), alpha-glycerophosphate (alpha-GP), and lactate contents in muscle biopsies taken before and after the 45-min submaximal exercise, as well as brachial arterial-femoral venous (a-fv) differences for epinephrine, norepinephrine, lactate, glucose, and O2 during exercise. During exercise the arterial plasma epinephrine concentration was 4.8 +/- 0.8 nmol/l and the femoral venous epinephrine concentrations were 10.3 +/- 2.1 and 3.9 +/- 0.6 nmol/l, respectively, in the EPI and C leg. During exercise the a-fv difference for lactate was greater (-0.41 +/- 0.14 vs. -0.21 +/- 0.14 mmol/l; P less than 0.001), and the a-fv difference for glucose was smaller (0.07 +/- 0.12 vs. 0.24 +/- 0.12 mmol/l; P less than 0.01) in the EPI than in the C leg, but the a-fv differences for O2 were similar. Muscle glycogen depletion (137 +/- 63 vs. 99 +/- 43 mmol/kg dry muscle; P less than 0.1) and the muscle concentrations of glucose (P less than 0.05), alpha-GP (P less than 0.1), G6P (P greater than 0.1), and lactate (P greater than 0.1) tended to be higher in the EPI than the C leg after exercise. These findings suggest that physiological concentrations of epinephrine may enhance muscle glycogenolysis during submaximal exercise in male subjects.     

Characterization of glucose-insulin responsiveness and impact of fetal number and sex difference on insulin response in the sheep fetus

GSIS is often measured in the sheep fetus by a square-wave hyperglycemic clamp, but maximal β-cell responsiveness and effects of fetal number and sex difference have not been fully evaluated. We determined the dose-response curve for GSIS in fetal sheep (0.9 of gestation) by increasing plasma glucose from euglycemia in a stepwise fashion. The glucose-insulin response was best fit by curvilinear third-order polynomial equations for singletons (y = 0.018x(3) - 0.26x(2) + 1.2x - 0.64) and twins (y = -0.012x(3) + 0.043x(2) + 0.40x - 0.16). In singles, maximal insulin secretion was achieved at 3.4 ± 0.2 mmol/l glucose but began to plateau after 2.4 ± 0.2 mmol/l glucose (90% of maximum), whereas the maximum for twins was reached at 4.8 ± 0.4 mmol/l glucose. In twin (n = 18) and singleton (n = 49) fetuses, GSIS was determined with a square-wave hyperglycemic clamp >2.4 mmol/l glucose. Twins had a lower basal glucose concentration, and plasma insulin concentrations were 59 (P < 0.01) and 43% (P < 0.05) lower in twins than singletons during the euglycemic and hyperglycemic periods, respectively. The basal glucose/insulin ratio was approximately doubled in twins vs. singles (P < 0.001), indicating greater insulin sensitivity. In a separate cohort of fetuses, twins (n = 8) had lower body weight (P < 0.05) and β-cell mass (P < 0.01) than singleton fetuses (n = 7) as a result of smaller pancreata (P < 0.01) and a positive correlation (P < 0.05) between insulin immunopositive area and fetal weight (P < 0.05). No effects of sex difference on GSIS or β-cell mass were observed. These findings indicate that insulin secretion is less responsive to physiological glucose concentrations in twins, due in part to less β-cell mass.     

The effects of brain-stem section on the breathing and behavioural response to morphine in the fetal sheep

In the unanesthetized fetal sheep the administration of morphine causes initial apnoea followed by hyperpnoea. We thought that a section of the brain at midcollicular level might separate these two effects. Therefore we sectioned the brain stem of five fetuses at 132 +/- 1 (SEM) days of gestation and compared their responses to morphine (17 experiments) with that observed in seven intact fetuses at similar gestational ages (15 experiments). Brain stem sections were confirmed morphologically and histologically. Morphine, 1 mg/kg was injected in the fetal jugular vein during low-voltage electrocortical activity (ECoG). We measured ECoG, eye movements, diaphragmatic activity, blood pressure and amniotic pressure. Sectioned fetuses before the administration of morphine had a complete dissociation between ECoG and breathing activity. With the administration of morphine we found: (i) the length of the apnoea was 139.8 +/- 15.5 min in sectioned fetuses and 17.0 +/- 5.8 min in intact fetuses (P less than 0.01); and (ii) there was no hyperpneic response in the sectioned fetus whereas the length of hyperpnoea in the intact group was 99.1 +/- 11.8 min (P less than 0.001). The results support the idea of two central distinct areas of action of morphine in the fetal brain. The absence of hyperpnoea in the sectioned fetuses suggests that neurons inhibiting the 'respiratory neurons' are located rostrally to the mid-collicular line.     

Effect of morphine on breathing and behavior in fetal sheep

To define the dose response of apnea and breathing to morphine we studied 12 fetuses at 116-141 days of gestation using our window technique. We instrumented the fetus to record electrocortical activity (ECoG), eye movements (EOG), diaphragmatic activity (integral of EMGdi), heart rate, carotid blood pressure, and amniotic pressure. Saline and morphine in doses of 0.03, 0.1, 0.5, 1, and 3 mg/kg were injected in random order in the jugular vein of the fetus during low-voltage ECoG. Fetuses were videotaped for evaluation of fetal behavior. We found 1) that saline did not elicit a response; 2) apnea, associated with a change from low- to high-voltage ECoG, increased from 2.2 +/- 1.5 (SE) min in two fetuses at a dose of 0.03 mg to 20 +/- 6.3 min in seven fetuses at 3 mg/kg (P less than 0.005); 3) the length of the breathing responses, associated with a change from high- to low-voltage ECoG, were 15 +/- 1.8 and 135.9 +/- 18.1 min (P less than 0.0005); 4) integral of EMGdi X frequency, an index equivalent to minute ventilation, increased from 1,763 +/- 317 arbitrary units to 10,658 +/- 1,843 at 1.0 mg/kg and then decreased to 7,997 +/- 1,335 at 3.0 mg/kg. These changes were related to a steady increase in integral of EMGdi, whereas frequency decreased at 3 mg/kg. There was an increase in breathing response to morphine plasma concentrations or morphine doses.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effect of reduced uterine blood flow on fetal and maternal cortisol

We have measured the changes in fetal and maternal plasma concentrations of cortisol in relation to blood gases and percent oxygen saturation during 2- and 4-h episodes of reversibly reduced uterine blood flow in sheep between 120 days gestation and term. During that period of reduced uterine blood flow there was a significant decrease in fetal arterial percent oxygen saturation (SaO2), PO2 and pH. Fetal SaO2 decreased from 59.5 +/- 3.2% to 31.8% +/- 2.8% by 15 min, 32.9 +/- 2.9% by 60 min, and 33.5 +/- 2.9% by 120 min. Fetal PO2 decreased from 3.2 +/- 0.1 KPa to 2.0 +/- 0.2 KPa by 15 min, 2.2 +/- 0.2 KPa by 60 min and 2.3 +/- 0.1 KPa by 120 min. Fetal pH decreased from 7.36 +/- 0.01 to 7.30 +/- 0.03 by 15 min, 7.27 +/- 0.02 by 60 min and 7.25 +/- 0.03 by 120 min. During the period of reduced uterine blood flow, fetal plasma concentrations of cortisol increased from 37.1 +/- 10.8 nmol/l to 53.3 +/- 9.2 nmol/l by 15 min, 49.2 +/- 11.4 nmol/l by 60 min and 43.3 +/- 9.0 nmol/l by 120 min. The greatest percentage increase in fetal plasma concentrations of cortisol occurred in fetuses of 126-139 days gestation. There was no significant change in maternal blood gases, SaO2 or plasma concentrations of cortisol. These experiments demonstrate that there is a significant increase in fetal plasma concentrations of cortisol in response to reductions in uterine blood flow from as early as 120 days gestation.