A new phosphorescence imaging method (Rumsey et al. Science Wash. DC 241: 1649-1651, 1988) has been used to continuously monitor the PO2 in the blood of the cerebral cortex of newborn pigs. A window was prepared in the skull and the brain superfused with artificial cerebrospinal fluid. The phosphorescent probe for PO2, Pd-meso-tetra(4-carboxyphenyl)porphine, was injected directly into the systemic blood. The phosphorescence of the probe was imaged, and the lifetimes were measured using flash illumination and a gated video camera. The PO2 in the blood of the veins and capillary beds of the cortex was calculated from the lifetimes. Systemic blood pressure was continuously monitored while the systemic arterial PCO2, PO2, and blood pH were measured periodically. The PO2 in the blood was quantitated for 60- to 200 microns2 regions within the image (from a total field of approximately 3 mm diam). The PO2 in the microvasculature was not uniform across the viewing field but increased or decreased in each region independently of the other regions. Thus at any point in time the PO2 in a region could be substantially above or below the average value. During hyperventilation, which lowered arterial PCO2 and increased pH of the blood, the average PO2 decreased in proportion to the decrease in arterial PCO2. For example, hyperventilation, which decreased arterial PCO2 from its normal value of 40 Torr to 10 Torr, caused a rapid (within 5 min) decrease in PO2 in the blood of capillaries and veins to approximately one-third of normal.

Net ovine uteroplacental glucose consumption (Ro,up) and transfer rates to the fetus (Rf,up) were measured at different concentrations of maternal (GA) and fetal (Ga) arterial plasma glucose that were set and maintained independently by a glucose clamp procedure. Five GA/Ga combinations were studied: 70/15, 70/20, 70/30, 50/14, and 50/24 mg/dl. Rf,up was inversely related to Ga both at GA = 70 and GA = 50. Linear regression analysis of Rf,up vs. Ga for the GA = 70 and GA = 50 groups of observations revealed similar slopes (-0.286 +/- 0.012 vs. -0.217 +/- 0.028 dl.min-1.kg fetus-1) but a significantly higher intercept for the GA = 70 group (10.3 +/- 0.12 vs. 5.5 +/- 0.47 mg.min-1.kg fetus-1). In contrast, Ro,up increased significantly in response to an increase of Ga and had no significant dependence on GA. These results indicate that uteroplacental glucose metabolism occurs primarily in tissues that have direct access to glucose molecules carried by the umbilical circulation and that the glucose transport capacity of the placental barrier is greater on its fetal than its maternal surface. Uteroplacental glucose metabolic rate and its dependence on fetal glucose concentration are major factors that determine the magnitude and variability of the glucose concentration gradient (and thus the rate of net glucose transfer) between maternal and fetal plasma.

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ABSTRACT. Glucose and insulin clamp experiments were performed in vivo in chronically catheterized, late-gestation fetal lambs to quantify the effects of glucose and insulin on fetal glucose metabolism. Fetal glucose uptake from the placenta via the umbilical circulation (umbilical glucose uptake) was measured by application of the Fick principle, and fetal glucose utilization rate (GUR) was measured using [U-'4C]glucose tracer. Fetal plasma insulin concentrations ranged from 2 to 119 fiU.ml-' and fetal blood glucose concentrations ranged from 7.3 to 62.6 mg.dl-I. GUR varied from 2.82 to 15.12 mg/min/kg and the exogenous glucose entry rate (umbilical glucose uptake + glu- tration and plasma insulin concentration of 20.6 mg/dl and 10 wU/ml, respectively. According to this model, the glucose and insulin effects were additive. Furthermore, change in GUR was not proportionate to change in glucose concentration, accounting for a decreasing metabolic clearance rate at higher glucose concentrations. These results demonstrate the three-dimensional nature of the simultaneous additive effects of glucose and insulin on glucose utilization. These results also serve to emphasize that comparative studies of insulin and glucose metabolism in fetal lambs must be conducted at similar concentrations of glucose to avoid inaccurate estimates of the magnitude of insulin effect on glucose metabolism. (Pediatr Res 23: 381-387, 1988) Abbreviations 38Recently, we demonstrated that the maximum response of the fetal lamb to insulin infusion was a doubling of fetal glucose utilization rate (4). These studies were camed out using a glucose clamp technique in which individual fetuses were studied at "euglycemia," defined as the control (preinsulin) level of glucose. These glucose concentrations covered the normal range of glucose concentrations for well-fed Columbia-Rambouillet sheep, 15-25 mg/dl whole blood. Over this range of glucose concentration, glucose utilization rate was positively correlated with glucose concentration both at normal and high levels of insulin. These results strongly suggested that ovine fetal glucose metabolism could be more fully characterized according to effects of both insulin and glucose concentrations. To date, however, there has been little direct experimental evidence provided to complete such characterization of fetal insulin and glucose effects. As a result, the response of fetal glucose metabolism to changes of fetal glycemia and fetal insulinemia, produced by changes in maternal glucose concentration that occur with important clinical problems such as diabetes mellitus or fasting-induced hypoglycemia, have not been adequately quantified.Therefore, the present studies were conducted to measure glucose metabolism in the fetal lamb at different levels of plasma insulin and blood glucose concentration in order to provide a more complete description of how insulin and glucose act together to regulate fetal glucose metabolism. In these studies, glucose and insulin clamp experiments were performed in vivo i...

ABSTRACT. We studied ten normoglycemic [maternal glucose (GA) = 70 mg/dL] and six insulin-induced hypoglycemic (GA = 22 mg/dL) pregnant sheep to test the hypothesis that development of fetal glucose production (GPR) could help maintain fetal glucose concentration, limit uteroplacental-fetal glucose transfer (UPGT), and sustain uteroplacental glucose consumption (UPGC). Compared with the normoglycemic group, the hypoglycemic group demonstrated the following values: fetal glucose concentration (G,) was 9.8 + 0.8 mg/dL (51% lower, p < 0.01), uterine glucose uptake (UtGU) was 16.7 2 1.4 mg/min (54% lower, p < 0.01), UPGT was 3.1 + 0.6 mg/min (81% lower, p < 0.001), and UPGC was 13.6 2 1.4 mg/min (30% lower, p < 0.05). The reduction in UPGC was significantly less ( p < 0.05) than the reductions in UPGT and UtGU. Fetal glucose utilization rate (CUR) was decreased 20% ( p < 0.05) to 3.99 + 0.35 mg/min/kg. A further decrease in CUR was prevented by the appearance of fetal GPR of 2.82 + 0.32 mg/min/kg ( p < 0.05) compared with negligible GPR in the normoglycemic group. UPGT and UPGC in both groups were not influenced by maternal or fetal insulin infusions as long as G, did not change; however, fetal glucose infusion that increased G, increased UPGC in both groups. We conclude that, during chronic maternal hypoglycemia, increased fetal GPR limits the simultaneous decrease in fetal CUR and glucose concentration. By sustaining G, fetal GPR limits UPGT to a significantly greater extent than UtGU, diverting UtGU to UPGC. Thus, fetal GPR promotes placental as well as fetal metabolic autonomy when the maternal supply of glucose is reduced. (Pediatr Res 25:429-434, 1989) Abbreviations GPR, fetal glucose production GA, maternal glucose concentration G,, fetal glucose concentration GIR, glucose infusion rate UGU, umbilical glucose uptake rate (equal to UPGT) UPGT, uteroplacental-fetal glucose transfer UPGC, uteroplacental glucose consumption GUR, glucose utilization rateIn experiments designed to measure the effect of a reduced supply of glucose on fetal glucose metabolism, we observed in Received October 13, 1988; accepted December 28, 1988 fasted, pregnant sheep that, at rates of net fetal glucose uptake less than half of normal, fetal glucose utilization significantly exceeded umbilical glucose uptake (1, 2). Similar preliminary studies showed the progressive development of a positive fetal hepatic vein-umbilical vein glucose concentration difference after three d of fasting-induced maternal and fetal hypoglycemia (3). We interpreted these observations as evidence of GPR by the fetus itself. As previous studies (4, 5) found negligible rates of fetal GPR during maternal normal glycemia, an increased fetal GPR presumably serves to maintain the rate of fetal glucose utilization when maternal glucose supply decreases. It is not obvious, however, how fetal GPR might also affect placental glucose flux. This is an important concern because in recent experiments using fetal glucose and insulin infusions we observed that changes in f...

Glucose and insulin clamp experiments were performed in late-gestation fetal lambs to quantify the separate and combined effects of physiological concentrations of fetal glucose (G; 7.3-62.6 mg/dl) and insulin (I; 2-119 uU/ml) on fetal glucose metabolism and O2 consumption. Fetal glucose utilization rate (GUR) varied from 2.82 to 15.12 mg.min-1.kg-1. Fetal CO2 production from fetal glucose carbon oxidation (CO2Pr) varied from 32 to 234 mumol.min-1.kg-1 and was directly related to G and I [CO2Pr = -0.00868 + 0.00578 (G) + 0.000901 (I) - 0.0000619 (G)2, r = 0.88] and to GUR (CO2Pr = 0.0159 GUR - 0.0130, r = 0.89). CO2Pr accounted for 54.7% of the mean GUR and for 35.9% of the mean umbilical O2 uptake (UO2U), ranging from 26.0% in the control studies to 36.5% in hyperinsulinemic-euglycemic studies and to 45.1% in hyperinsulinemic-hyperglycemic studies. UO2U varied from 0.200 to I [UO2U = 0.303 + [0.000813 (G)] + [0.0000461 (I)], r = 0.89] and to GUR (UO2U = 0.0098 GUR + 0.275, r = 0.91). These results define independent (additive) effects of G and I on glucose oxidation in the late gestation fetal lamb and demonstrate the necessity for considering the levels of both G and I when studying these aspects of fetal metabolism.

Alterations in brain cell membrane structure and function during cerebral hypoxia were investigated by measuring Na⁺, K⁺-ATPase activity and levels of lipid peroxidation products in brain cell membranes obtained from newborn piglets following exposure to 60 min of hypoxic hypoxia in vivo. Cerebral hypoxia was documented as a decrease in the ratio of phosphocreatine to inorganic phosphate (PCr/Pi) using ³¹P-NMR spectroscopy. During hypoxia (FiO₂ 0.07-0.11), PCr/Pi decreased 28–47% compared to the corresponding baseline value without a decrease in cerebral ATP levels. No change in brain cell membrane Na⁺, K⁺-ATPase activity was observed for changes in PCr/Pi of < 30%. When PCr/Pi was at least 30% lower than baseline, Na⁺, K⁺-ATPase activity decreased linearly as a function of the decrease in PCr/Pi (r = 0.95). Levels of lipid peroxidation products (conjugated dienes and fluorescent compounds) increased significantly as PCr/Pi decreased. These data suggest that below a critical threshold value of oxidative metabolism there are progressive changes in brain cell membrane structure and function during cerebral hypoxia, and demonstrate that membrane alterations occur prior to changes in cellular ATP levels.

To determine the separate effects of changes in fetal glucose and insulin concentrations on uteroplacental glucose transfer (UPGT) and consumption (UPGC) we studied 24 late-gestation pregnant sheep during fetal insulin infusions alone and with simultaneous glucose clamp. Insulin infusion alone increased fetal glucose utilization rate (GUR) by 45% (P less than 0.001), decreasing fetal glucose concentration by 40% (P less than 0.01) and thereby increasing fetal glucose clearance (Clglu) by 150% (P less than 0.001). Maternal-fetal glucose gradient also increased, resulting in a 40% increase (P less than 0.02) in UPGT [measured as umbilical glucose uptake (UGU)] and a 30% decrease (P less than 0.05) in UPGC. Addition of a fetal glucose clamp returned fetal glucose concentration to base line and restored UPGC and UGU to control values with a further 2.25-fold increase in fetal GUR. Clglu did not change, as the increase in GUR was proportional to the increase in fetal glucose concentration. Similarly, in animals receiving an insulin infusion plus glucose clamp throughout, maternal glucose concentration, fetal glucose concentration, UPGC, and UGU did not change, whereas GUR and Clglu increased approximately 1.9-fold. These changes were noted at constant maternal glucose concentration and uterine glucose uptake. Thus variation in fetal glucose concentration rather than fetal insulin concentration directly regulates uteroplacental glucose transfer and consumption, whereas both fetal insulin and glucose affect, in separate ways, fetal glucose utilization and clearance.