Fetal insulin and placental 3-O-methyl glucose clearance in near-term sheep

It is difficult, if not impossible, to measure the placental transfer of glucose directly because of placental glucose consumption and the low A-V glucose difference across the sheep placenta. We have approached the problem of quantifying placental hexose transfer by using a nonmetabolized glucose analogue (3-O-methyl glucose) which shares the glucose transport system. We have measured the clearance by using a multisample technique permitting least squares linear computing to avoid the errors implicit in the Fick principle. The placental clearance of 3-O-methyl glucose was measured in the control condition and after the administration of insulin to the fetal circulation. A glucose clamp technique was used to maintain constant transplacental glucose concentrations throughout the duration of the experiment. A control series was performed in which the only intervention was the infusion of normal saline. In these experiments the maternal and fetal glucose concentrations remained constant as did the volume of distribution of 3-O-methyl glucose in the fetus. The maternal insulin concentration remained constant and fetal insulin concentration changed from 11 +/- 2 microU/ml to 355 +/- 51 microU/ml (P less than 0.01). In the face of this large increase in fetal plasma insulin, there was no change in the placental clearance of 3-O-methyl glucose. In the control condition the clearance was 14.1 +/- 1.0 ml/min per kg and this was 13.8 +/- 1.0 ml/min per kg in the high insulin condition. Fetal insulin may change placental glucose flux by decreasing fetal plasma glucose concentrations but does not do so by changing the activity of the glucose transport system.     

Maternal insulin and placental 3-O-methyl glucose transport

The effects of insulin in the maternal circulation on the placental clearance of 3-O-methyl glucose were investigated in 7 animals in the presence of a constant maternal glucose concentration. While maternal insulin concentration changed from 12 +/- 4 to 175 +/- 33 mu Units/ml, the placental clearance remained constant at 16.2 +/- 1.2 (control) and 15 +/- 1.3 ml/min per kg fetus under the influence of the insulin. To test the secondary hypothesis that in the control condition the hexose transport system was saturated, we performed a further series of experiments in 6 fasted animals. In these animals the control maternal plasma insulin concentration was 2 +/- 0.3 mu Units/ml and after the infusion of insulin it increased to 562 +/- 26 mu Units/ml. Under conditions of constant maternal and fetal plasma glucose concentrations, this massive elevation of plasma insulin did not change the placental clearance of 3MeG which was 15.2 +/- 1.6 in the control condition and 13.3 +/- ml/min per kg under the influence of high insulin. We conclude that maternal insulin ranging from 2 mu Units/ml to supraphysiologic doses does not effect a physiologically significant change in placental hexose transfer. Placental glucose transfer can probably therefore, be changed only be changing the concentration of glucose in the maternal and fetal plasma.     

Fetal metabolism and placental transfer of vasoactive intestinal peptide in sheep

Vasoactive Intestinal Peptide (VIP) is a 28-amino-acid putative neurotransmitter that may have a role in the regulation of myometrial blood flow and uterine contractility. The chronically cannulated fetal sheep preparation was used to examine the fetal clearance and placental transfer of VIP. Metabolic Clearance Rate (MCR) and placental transfer of VIP were measured by alternate steady-state infusion of VIP into the mother and fetus. Plasma concentrations of VIP were measured by radioimmunoassay. MCR was similar in the pregnant (45 +/- 10 ml/kg/min) and nonpregnant ewes (35 +/- 5 ml/kg/min). However, compared to both pregnant and nonpregnant ewes, fetal MCR was significantly increased at 77 +/- 15 ml/kg/min, indicating highly developed clearance mechanisms in the fetus. VIP did not cross the placenta in either direction. Both the placenta and fetal liver metabolized VIP and contributed to the elevated fetal clearance of VIP. The results show that VIP in fetal tissue is unlikely to influence maternal uterine activity with any VIP-mediated effects emanating from maternal and/or placental sources.     

Estimation of human fetal-placental unit metabolic rate by application of the Bohr principle

The Bohr Principle via continuous indirect calorimetry was used to estimate human fetal-placental unit metabolic rate in 12 normal women undergoing elective caesarean section under continuous lumbar epidural anaesthesia. Maternal oxygen consumption decreased after umbilical cord clamping and after placental removal. Fetal-placental unit oxygen consumption was 10.7 +/- 1.3 ml/min per kg (mean +/- SEM). Fetal oxygen consumption was 6.8 +/- 1.4 ml/min per kg. Placental oxygen consumption was 37 +/- 12 ml/min per kg. Fetal-placental unit carbon dioxide production was 9.2 +/- 1.2 ml/min per kg. These mean values agree favourably with measurements of uterine and fetal metabolism from other mammalian species. Maternal minute ventilation decreased with removal of the fetal-placental unit, and this decrease was found to be linearly related to the fetal-placental unit carbon dioxide production.     

Transfer of morphine across the human placenta and its interaction with naloxone

The purpose of this investigation was to measure the transfer rate and clearance of morphine across the placenta with and without naloxone. Term human placental cotyledons were perfused in vitro. The placenta was perfused with 50 ng/mL of morphine in the absence (n=4) and presence (n=5) of 100 ng/mL of naloxone. Maternal and fetal samples were collected. Student's t-test or one-way repeated measures ANOVA were used for all comparisons. The maternal-to-fetal morphine transfer rate was 0.73+/-0.44 ng/mL/min in the morphine and 0.69+/-0.26 ng/mL/min in the morphine-naloxone experiments (p=0.89). The clearance of morphine was 0.89+/-0.39 mL/min without naloxone and 0.87+/-0.27 mL/min with naloxone (p=0.92). Final morphine concentrations in the morphine experiments were 9.78+/-6.17 ng/mL (maternal) and 3.43+/-2.14 ng/mL (fetal) and 10.04+/-3.89 ng/mL (maternal) and 4.16+/-1.64 ng/mL (fetal) in the morphine-naloxone experiments. Morphine readily crosses the term human placenta. Naloxone does not alter placental transfer or clearance of morphine, suggesting that transfer across the placental barrier is not altered by changes in vascular resistance. Placental retention of morphine prolongs fetal exposure to morphine.     

Fetal growth and placental diffusing capacity in guinea pigs following long-term maternal exercise

To determine the critical level of maternal exercise which produces effects on fetal weight and placental diffusing capacity, we measured the relationship between increasing levels of exercise and its effect on the fetus. Hartley guinea pigs with dated pregnancies were exercised on a treadmill (9.7 m/min at a 6.5% gradient) at one of five exercise levels (0, 15, 30, 45, and 60 min/day). We measured placental diffusing capacity for carbon monoxide (DPCO) fetal body and organ weights, placental weight, and maternal body and heart weights near term (63-64 days). Fetal body weight, kidney weight, and placental weight decreased as a function of increasing exercise level, decreasing 13, 13, and 21% respectively at 60 min/day exercise. DPCO1 decreased from a control value of 2.92 +/- 0.23 to 2.33 +/- 0.10 ml. min-1 torr-1 kg fetal wt in the 15 min/day exercise group, 2.17 +/- 0.08 in the 30 min/day group 2.16 +/- 0.11 in the 45 min/day group, and 2.65 +/- 0.31 in the 60 min/day exercise group. The decrease in placental weight along with the decrease in DPCO per kg of fetal weight suggests that with progressive maternal exercise the fetus is compromised by a smaller than normal placenta with less diffusing capacity.     

Cardiovascular responses to forskolin in the ovine fetus

Six near-term ewes were instrumented to measure regional blood flows in the maternal and fetal subthoracic structures and allowed to recover for 5 days. Control blood flows were measured and 10(-3) molar forskolin was infused in the fetal hindlimb vein at 1 ml/min. After 10 min of infusion, maternal and fetal regional blood flows were measured. The fetal blood pressure was 44 +/- 3 mmHg in the control state and 40 +/- 4 mmHg after forskolin, P less than 0.056. The fetal renal vascular resistance changed from 24.4 +/- 2.4 to 17.5 +/- 1.7 mmHg.ml-1.min.g, P less than 0.005. The placenta had a control resistance of 27.7 +/- 5.0 and 25.6 +/- 5.1 mmHg.ml-1.min.g after forskolin, P less than 0.05. The placental membranes showed vasodilation: control resistance was 261 +/- 49 and 168 +/- 39 mmHg.ml-1.min.g after forskolin, P less than 0.02. The generalized vasodilation of the fetal circulation was paralleled in the maternal circulation. Forskolin, a lipid soluble diterpene, apparently had a placental clearance close to the theoretical maximum. Vasodilation was seen in the maternal renal, placental and uterine vasculatures. Maternal blood pressure was unchanged. Maternal placental vascular resistance was 47.4 +/- 3.0 mmHg.ml-1.min.g in the control state and 40.6 +/- 3.3 mmHg.ml-1.min.g after forskolin, P less than 0.02. Forskolin is a vasodilator in both the fetal and maternal circulations. The maintenance of a relatively normal blood pressure in the face of regional vasodilation shows that forskolin may have a positive inotropic effect on the fetal heart. These results indicate that neither the fetal nor the maternal ovine placental vasculature is maximally dilated in the control state.     

The effect of zinc levels in fetal circulation on zinc clearance across the in situ perfused guinea pig placenta

Although zinc is essential for normal fetal growth and development, little is known about factors that influence its transfer across the placenta. The in situ perfused guinea pig placenta model was used to study the influence of the zinc concentration of fetal circulation on maternofetal placental zinc transfer. A placenta of the anaesthetized sow was perfused (on the fetal side) with a physiological perfusate via the umbilical vessels, with the fetus excluded. The sow was infused intravenously with 65zinc as a tracer of placental Zn clearance, and with antipyrine as an indirect indicator of maternal placental blood flow. Maternal plasma and placental effluent samples collected at intervals were counted for 65zinc by gamma counter, and the absorbance of nitrosated antipyrine was measured at 350 nm. Varying the mean zinc concentration in the perfusate from 0.176 to 1.87 mg/L had no effect on relative zinc clearance calculated as zinc clearance/antipyrine clearance (mean +/- SEM; 0.085 +/- 0.010 vs. 0.114 +/- 0.018; n = 6; p greater than 0.05). The results suggest that short-term changes in fetal zinc status do not influence placental zinc transfer.     

Forskolin dilates the maternal ovine placental vascular bed

We have examined the placental vascular responses to forskolin in 8 near-term sheep. The drug was administered for 5 min at 1 ml/min of 10(-3) M forskolin via a retrograde uterine arterial catheter. Blood flows were measured with radioactive microspheres. Forskolin increased the nonplacental uterine blood flow from 0.318 +/- 0.031 (SEM) to 0.738 +/- 0.071 ml/min per g of tissue, P less than 0.001. The nonplacental uterine vascular resistance decreased from 308 +/- 26 to 132 +/- 12 mmHg/ml/min per g, P less than 0.001. Forskolin increased the placental blood flow from 1.8 +/- 0.18 to 2.08 +/- 0.16 ml/min per g of tissue, P less than 0.05. The placental vascular resistance decreased from 54.7 +/- 5.1 to 45.9 +/- 3.7 mmHg/ml/min per g, P less than 0.03. In the same animals we then infused angiotensin II at 5 micrograms/min via the jugular vein to induce placental vasoconstriction. In this series, the forskolin increased the nonplacental uterine blood flow from 0.141 +/- 0.016 to 0.485 +/- 0.079 ml/min per g of tissue, P less than 0.001, and the uterine vascular resistance decreased from 968 +/- 104 to 283 +/- 36 mmHg/ml/min per g, P less than 0.001. The placental blood flow increased from 2.08 +/- 0.012 to 2.69 +/- 0.17 ml/min per g of tissue, P less than 0.01 and placental vascular resistance decreased from 61.9 +/- 4.4 to 46.0 +/- 3.7 mmHg/ml/min per g, P less than 0.001.(ABSTRACT TRUNCATED AT 250 WORDS)     

Studies on the growth of the fetal sheep. Effects of surgical reduction in placental size, or experimental manipulation of uterine blood flow on plasma sulphation promoting activity and on the concentration of insulin-like growth factors I and II

The effect of long- and short-term manipulations of uterine blood flow on fetal plasma levels of IGF-I and -II have been studied in sheep at days 125-139 of pregnancy and compared with those in near term rats and guinea pig. The primary objective is to show that both long- and short-term reduction of uterine blood flow is associated with increase in the fetal plasma concentration of IGF-II while that of IGF-I falls. In the pregnant sheep long-term depression of utero-placental blood flow was caused by surgical reduction in placental mass (carunclectomy) prior to conception. This reduced fetal weight to 2.42 +/- 0.49 kg (SD) compared with 3.41 +/- 0.46 in controls; the respective values for uterine blood flow being 1694 +/- 558 and 913 +/- 324 ml/min respectively. This was associated with a fall in fetal plasma IGF-I concentration from 22.6 +/- 3.4 ng/ml to 14.9 +/- 1.31 ng/ml and a rise in IGF-II from 1952 +/- 284 ng/ml to 3360 +/- 914 ng/ml respectively. Similar changes in the plasma concentrations of IGF peptides were observed in fetal rats and guinea pigs in response to uterine artery ligation. Short-term reduction (60 min) of the uterine blood flow was caused either by compression of the common uterine artery to depress flow from 1491 +/- 375 to 648 +/- 216 ml/min or through intraarterial infusion of adrenaline at 35 ug/min to lower flow from 1628 +/- 339 to 1195 +/- 128 ml/min. Such falls in uterine blood flow had no significant effect on fetal plasma IGF-I levels but increased IGF-II levels by 30 to 60%.     

Effect of prostaglandin I2 on ovine placental vasculature.

The response of the placental circulations to prostaglandin I2 (maternal dose 20 microgram/kg, fetal dose 180 microgram/kg) was observed in 10 near-term sheep with chronically implanted vascular catheters. The blood flows before and 90 s after the injection of prostaglandin I2 were measured using radioactive microspheres. The injection of prostaglandin I2 to the mother decreased th blood pressure from 109 +/- 4 to 69 +/- 5 mmHg (P < 0.001) and increased the vascular resistance of the maternal cotyledons from 0.166 +/- 0.018 to 0.209 +/- 0.02 mmHg/(ml/min) (P < 0.001). The vascular bed of the non-cotyledonary uterus vasodilated as the resistance fell from 0.705 +/- 0.02 to 0.266 +/- 0.02 mmHg/(ml/min). (P < 0.001). Prostaglandin I2 caused the fetal arteriovenous pressure to fall from 37.6 +/- 1.35 to 26.0 +/- 1.6 mmHg. There was no significant change in the vascular resistance of the fetal cotyledons. We observed vasodilation in the fetal membranes as vascular resistance fell from 1.06 +/- 0.14 to 0.75 +/- 0.10 mmHg/(ml/min) (P < 0.001). The infusion of prostaglandin I2 significantly depressed the response of the placenta and uterus to norepinephrine. We have not proved that prostaglandin I2 plays a direct role in maintaining placental vascular homeostasis but it may modulate the response of this organ to exogenous vasoactive agents.     

NO supports right ventricular flow dominance and whole body O(2) utilization in midgestation fetal lambs

It is unknown if nitric oxide (NO) modulates the relative levels of left (LV) and right (RV) ventricular output, fetal O2 consumption, or blood flow distribution between the body and placenta at midgestation. To address these questions, six fetal lambs were instrumented at 89-96 days gestation (term 147 days), and blood flows were measured with radioactive microspheres 3-4 days later at baseline and after inhibition of NO synthesis with 10 mg/kg (L-NNA10) and 25 mg/kg (L-NNA25) N(omega)-nitro-L-arginine. LV output fell by 74 +/- 15 ml. min(-1). kg(-1) at L-NNA10 (P < 0.005), whereas RV output decreased by 90 +/- 18 ml. min(-1). kg(-1) at L-NNA10 (P < 0.02) and by a further 80 +/- 22 ml. min(-1). kg(-1) at L-NNA25 (P < 0.05). As a result, RV output exceeded LV output at baseline (P = 0.03) and L-NNA10 (P < 0.02) but not at L-NNA25. Fetal body blood flow fell by 95 +/- 25 ml. min(-1). kg(-1) at L-NNA10 (P < 0.01), but because placental blood flow decreased by 70 +/- 22 ml. min(-1). kg(-1) at L-NNA10 (P < 0.01) and a further 71 +/- 21 ml. min(-1). kg(-1) at L-NNA25 (P < 0.01), the fetal body-to-placental blood flow ratio was near unity at baseline and L-NNA10 but rose to 1.5 +/- 0.3 at L-NNA25 (P < 0.05). In association with these flow changes, fetal O2 consumption declined by 1.4 +/- 0.3 ml. min(-1). kg(-1) at L-NNA10 (P < 0.05) and by a further 1.5 +/- 0.6 ml. min(-1). kg(-1) at L-NNA25 (P < 0.02). These findings suggest that, in midgestation fetal lambs, NO supports an RV flow dominance, whole body O2 utilization, and the maintenance of a near-equal fetoplacental blood flow distribution.     

Muscarinic influences on growth hormone secretion in the fetal and neonatal sheep: pharmacological studies and in vitro binding studies

To investigate the ontogenesis of potential cholinergic influences on growth hormone secretion we administered the cholinesterase inhibitor neostigimine, (120 micrograms/kg) to fetal sheep (n = 16) between 77 and 143 days of gestation and to infant lambs (n = 5). Neostigmine administration was associated with a marked rise in fetal growth hormone concentrations. The integrated release of growth hormone in the hour following fetal neostigmine administration was 2880 +/- 425 ng.min/ml compared to -618 +/- 206 ng . min/ml (P less than 0.001) following saline administration (n = 19). There was no relationship between gestational age and the response to neostigmine. In the infant lamb, neostigmine was associated with a lesser (P less than 0.001) but significant (P less than 0.02) growth hormone response. The integrated release was 704 +/- 410 ng . min/ml (n = 5) compared to -44 +/- 40 ng . min/ml following saline (n = 11). The fetal response to neostigmine was abolished by the administration of atropine (200 micrograms/kg bolus followed by 400 micrograms/kg per h infusion) 5 min prior to neostigmine (n = 4). This demonstrates that the effect of neostigmine was mediated by muscarinic receptors. Atropine itself had no effect on fetal growth hormone release (n = 6). In vitro binding studies with the muscarinic ligand, 1-quinuclidinyl [phenyl-4 (n) -3H] benzilate) were performed on homogenates of fetal (n = 3) and adult (n = 3) pituitaries. Scatchard analysis demonstrated both a high affinity and low affinity binding site. The concentration per mg. of original tissue of each of these binding sites was higher (P less than 0.05) in fetal than adult homogenates.(ABSTRACT TRUNCATED AT 250 WORDS)     

Cardiovascular responses to acute, severe haemorrhage in fetal sheep

In adults, the responses to acute haemorrhage vary greatly depending on the amount of blood lost. While many studies have documented fetal responses to mild haemorrhage, fetal responses to severe haemorrhage are not known. In this study we examined the effect of acute, severe haemorrhage in fetal lambs. Despite the severity of haemorrhage, we found that mean arterial blood pressure was restored within 2 min, and heart rate was restored within 30 min. This restoration of blood pressure and heart rate was facilitated by an increase in peripheral vascular resistance mediated in part by secretion of catecholamines and plasma renin. In addition, about 40% of the shed blood volume was restored within 30 min by fluid from either the fetal interstitium or placenta. The PO2 of umbilical venous blood increased from 33 +/- 9 mmHg to 49 +/- 17 mmHg 2 min post-haemorrhage, and to 47 +/- 15 mmHg 30 min post-haemorrhage. However, this increase was not sufficient to offset the fall in both haemoglobin concentration and umbilical-placental blood flow, so that oxygen delivery decreased from 21.1 +/- 5.5 ml/min per kg to 9.1 +/- 5.2 ml/min per kg 2 min post-haemorrhage, and 14.1 +/- 9.2 ml/min per kg 30 min post-haemorrhage. Because of this decrease in oxygen delivery, oxygen consumption fell and a metabolic acidemia ensued. Nevertheless, oxygen delivery to the heart and brain was maintained because hepatic vasoconstriction diverted more of the well oxygenated umbilical venous return through the ductus venosus. Although the fetus was able to tolerate acute loss of 40% of blood volume, larger volumes of haemorrhage resulted in fetal death.     

Lipopolysaccharide differently affects prostaglandin E2 levels in fetal and maternal compartments of perfused human term placenta

The aim of the present study was to examine the effect of lipopolysaccharide (LPS) on the levels of prostaglandin E(2) (PGE(2)) in the perfusates of the fetal and the maternal compartments of perfused human term placental tissue. Term placentas were perfused for 10h in the absence [control, (n=4)] and presence of LPS [LPS=1 microg/kg perfused placental tissue, (n=4)] in the maternal reservoir. Perfusate samples from the fetal and the maternal circulations were collected every 30 min and examined for PGE(2) levels by radio-immunoassay. PGE(2) levels in the fetal circulation were gradually increased reaching significant peak value of 479+/-159 pg/ml, as compared to PGE(2) levels in the maternal circulation (140+/-146 pg/ml) (p<0.05). After 10 hours of perfusion with control medium, PGE(2) levels in the maternal circulation (347+/-144 pg/ml) were significantly higher as compared to the fetal circulation (150+/-57 pg/ml) (p<0.05). In presence of LPS, PGE(2) levels in the fetal circulation increased reaching a peak value of 1028+/-663 pg/ml after 240 min of perfusion. The levels of PGE(2) in the control group after 240 min of perfusion were significantly lower (156+/-77 pg/ml) (p<0.05). No significant differences were detected in the levels of PGE(2) in the perfusate of the maternal compartment in presence of LPS, as compared to control. Our results suggest that the placenta may play an important role in maintaining high levels of PGE(2) in the fetal circulation and low PGE(2) levels in the maternal circulation during normal pregnancy. Moreover, placental PGE(2) release into the fetal and the maternal circulations may be differently affected in presence of intra-uterine infection/inflammation.     

Chronopharmacology of methotrexate pharmacokinetics in childhood leukemia.

Survival has been shown to improve when maintenance therapy for acute lymphocytic leukemia in children is given at night rather during the day. We examined the possibility that diurnal variation in methotrexate pharmacokinetics may contribute to this improvement. In a crossover study, we determined the pharmacokinetics of intravenous methotrexate at 10:00 and 21:00 h in six children with standard or high-risk leukemia. During the study, children refrained from concomitant drugs (6-mercaptopurine and trimethoprim sulfamethoxazole). There was a significant fall in methotrexate plasma clearance at night (from 5.6 +/- 3 ml/min/kg to 4.7 +/- 2.3 ml/min/kg p < 0.05). Renal clearance of methotrexate tended to decrease at night and unbound renal clearance decreased significantly (from 17.5 +/- 1.7 ml/min/kg to 8.5 +/- 3.6 ml/min/kg p < 0.05). Creatinine clearance did not exhibit diurnal variation, when comparing two 12-h collections, but there was a significant decrease in the nonglomerular clearance of methotrexate (from 14.8 +/- 5.2 to 6 +/- 4 ml/min/kg). Because it is a weak organic acid, the tubular secretion of methotrexate depends on urinary pH. At night urinary pH is more acidic. This may result in more reabsorption and hence reduced renal clearance.     

Restriction of placental size in sheep enhances efficiency of placental transfer of antipyrine, 3-O-methyl-D-glucose but not of urea

When placental growth is restricted, fetal growth is reduced but the fetal to placental weight ratio increases, suggesting that the efficiency of placental transfer may have increased. Therefore, placental transfer of antipyrine, 3-O-methyl-D-glucose and urea was measured in control pregnant sheep and in sheep with restricted placental growth (pre-pregnancy excision of endometrial caruncles). Clearance of each decreased with placental weight but clearance of antipyrine and of 3-O-methyl-D-glucose per kg of placenta increased as placental weight decreased. The small placenta exhibited increased efficiency of flow-determined transfer of antipyrine and of facilitated-diffusion transfer of glucose but not of passive transfer of the hydrophilic substance, urea. These compensatory changes should help to maintain oxygen and glucose to the fetus when the growth of the placenta has been limited by reduction of the number of placental attachment sites.     

Development of insulin-sensitivity at weaning in the rat. Role of the nutritional transition.

This study was undertaken to determine the factors involved in the development of insulin-sensitivity at weaning. Glucose kinetics were studied in suckling rats and in rats weaned on to a high-carbohydrate (HC) or a high-fat (HF) diet, in the basal state and during euglycaemic-hyperinsulinaemic-clamp studies. These studies were coupled with the 2-deoxyglucose technique, allowing a measure of glucose utilization by individual tissues. In the basal state, the glycaemia was higher in HF-weaned rats (124 +/- 4 mg/dl) than in suckling (109 +/- 1 mg/dl) and HC-weaned rats (101 +/- 3 mg/dl). Glucose turnover rates were similar in the three groups of animals (14 mg/min per kg). Nevertheless, basal metabolic glucose clearance rate was 20% lower in HF-weaned rats than in the other groups. During the euglycaemic-hyperinsulinaemic experiments, hepatic glucose production was suppressed by 90% in HC-weaned rats, whereas it remained at 40% of basal value in suckling and HF-weaned rats, indicating an insulin resistance of liver of these animals. Glucose clearance rate during the clamp was 18.3 +/- 0.9 ml/min per kg in suckling rats, whereas it was 35.3 +/- 1.2 ml/min per kg in HC-weaned rats and 27.8 +/- 1.1 ml/min per kg in HF-weaned rats, indicating an insulin resistance of glucose utilization in suckling, and to a lower extent, in HF-weaned rats. The deoxyglucose technique showed that peripheral insulin resistance was localized in muscles and white adipose tissue of suckling and HF-weaned rats. These results indicate that the switch from milk to a HC diet is an important determinant of the development of insulin-sensitivity at weaning in the rat.     

Rate of disappearance of glycerol from plasma of fetal and newborn sheep

The disappearance of glycerol from plasma was studied after a single intravenous injection to estimate its volume of distribution (Vdist), plasma clearance rate, and rate constant for irreversible loss (kd). Studies were repeated before and after birth of the lamb to test whether loss of the placenta could account for rapidly increasing plasma concentrations in the newborn. The disappearance of glycerol was closely described by a double-exponential model in each instance. In fetal sheep Vdist averaged 0.41 +/- 0.15 (SD) 1/kg fetal wt (n = 15). This volume decreased to 0.33 +/- 0.11 l/kg (n = 8) soon after functionally removing the placenta (by snaring the umbilical cord and maintaining the fetus with intrauterine ventilation), but the change was not significant. In newborn lambs 1-3 days of age, Vdist averaged 0.45 +/- 0.11 l/kg (n = 5, NS). Plasma clearance rate also did not change significantly, averaging 7.9 +/- 2.9, 7.9 +/- 3.8, and 9.0 +/- 5.9 ml.min-1.kg-1 in the fetus, after simulated birth, and in the newborn lamb, respectively, kd also was not altered measurably and averaged 0.020 +/- 0.006, 0.024 +/- 0.007, and 0.019 +/- 0.007 min-1 during the same time periods. Similar results were obtained by using three widely different amounts of infused glycerol. The results indicate that removal of glycerol does not depend on placental function to an appreciable extent. It is concluded that plasma glycerol concentration reflects principally glycerol turnover and, hence, lipolysis before and after birth.     

Effect of hyperinsulinemia on amino acid utilization in the ovine fetus

We studied the effect of an acute 4-h period of hyperinsulinemia (H) on net utilization rates (AAUR(net)) of 21 amino acids (AA) in 17 studies performed in 13 late-gestation fetal sheep by use of a novel fetal hyperinsulinemic-euglycemic-euaminoacidemic clamp. During H [84 +/- 12 (SE) microU/ml H, 15 +/- 2 microU/ml control (C), P < 0. 00001], euglycemia was maintained by glucose clamp (19 +/- 0.05 micromol/ml H, 1.19 +/- 0.04 micromol/ml C), and euaminoacidemia (mean 4.1 +/- 3.3% increase for all amino acid concentrations [AA], nonsignificantly different from zero) was maintained with a mixed amino acid solution adjusted to keep lysine concentration constant and other [AA] near C values. H produced a 63.7% increase in AAUR(net) (3.29 +/- 0.66 micromol. min(-1). kg(-1) H, 2.01 +/- 0.55 micromol. min(-1). kg(-1) C, P < 0.001), accounting for a 60.1% increase in fetal nitrogen uptake rate (2,064 +/- 108 mg. day(-1). kg(-1) H, 1,289 +/- 73 mg. day(-1). kg(-1) C, P < 0.001). Mean AA clearance rate (AAUR(net)/[AA]) increased by 64.5 +/- 18.9% (P < 0. 001). Thus acute physiological H increases net amino acid and nitrogen utilization rates in the ovine fetus independent of plasma glucose and [AA].