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Biomedical subjects

G K Oakes

Publications and source records attributed to G K Oakes.

32 records · Page 2Linked to original sources
PubMed1977

Fetal pancreatic glucagon responses in glucose-intolerant nonhuman primate pregnancy.

Rhesus monkey pancreatic alpha-cell function in streptozotoc-induced glucose-intolerant pregnancy is similar to that in normal primate pregnancy. Specifically, basal maternal and fetal plasma glucagon levels equate, and the fetal alpha cell does not respond to the glucagonogenic stimulus of either intravenous alanine or insulin-induced hypoglycemia. This contrasts with the accelerated maturation of the fetal beta cell in glucose-intolerant pregnancy, and does not support the concept of functional coupling of the pancreatic islet by a common glucose-based process. Fetal plasma glucagon levels do increase after L-dopa injection to the fetus. These data indicate that alpha cell unresponsiveness is a function of the glucagon-releasing mechanism rather than inadequate hormonal synthesis.

Alanine↗
PubMed1977

Plasma renin activity in sheep pregnancy after fetal or maternal nephrectomy.

The effect of bilateral fetal or maternal nephrectomy on basal and diuretic-stimulated plasma renin activity (Pra) was examined in 9 chronically catheterized Dorset sheep. After fetal nephrectomy, there was an initial rapid decrease in fetal PRA with t1/2 of 42-84 min, followed by a slower decrement with 1/2 350-720 min. There was no change in maternal PRA during this period. Similarly, after maternal nephrectomy, the decrease in maternal PRA had two exponential components and there was no associated change in fetal PRA. These data, and the maternal and fetal PRA responses to intravenous furosemide, have demonstrated that renin does not cross the sheep placenta and that fetal PRA is independent of the maternal PRA level.

Animals↗
PubMed1977

24-hour rhythms in uterine and umbilical blood flows of conscious pregnant sheep.

The possibility that 24-hour rhythms exist in uterine blood flow (UtBF) and umbilical blood (UmBF) was investigated in 5 days postoperative chronically instrumented near-term pregnant sheep acclimated to a controlled environment. UtBF, UmBF and pressure measurements were made at 15-min intervals over 24 h beginning at 0800 h. Each data series was examined for the presence of significant rhythms of a 24-hour period using a method of Fourier analysis. UtBF 24-hour rhythms were found in all ewes; UmBF 24-hour rhythms were found in 4 of 5 lambs. A consistent reciprocal phase relationship between UtBF and UmBF was evident within animals. There were no associated rhythms in maternal arterial, fetal arterial, or amniotic fluid pressures. These results indicate that the presence of circadian rhythms must be considered as a possible variable when long-term uteroplacental hemodynamic studies are planned.

Amniotic Fluid↗
PubMed1976

Effect of propranolol infusion on the umbilical and uterine circulations of pregnant sheep.

Propranolol was infused intravenously for 60 minutes to five ewes (4 mug per kilogram per minute) or five fetal sheep (10 mug per kilogram per minute). The umbilical blood flow was significantly decreased by 18 per cent from control at 60 minutes with either maternal or fetal propranolol infusion. Uterine blood flow and maternal and fetal mean arterial pressure did not significantly change. Maternal and fetal heart rates decreased 18 and 9 per cent from control, respectively, during maternal propranolol infusion. With propranolol to the fetus, fetal heart rate decreased 15 per cent and maternal heart rate did not change. During all infusion, maternal and fetal arterial pH, PCO2 and PO2 remained within normal physiologic limits.

Acid-Base Equilibrium↗
PubMed1976

Effect of ritodrine infusion on uterine and umbilical blood flow in pregnant sheep.

The effect of ritodrine upon uterine artery blood flow (UtBF) and umbilical vein blood flow (UmBF) was investigated in near-term chronic sheep preparations. During intravenous ritodrine infusions to the ewe in sequential dose rates from 100 to 800 mug per minute, UtBF fell progressively to 43 per cent below control levels and mean maternal arterial pressure (MMAP) declined 20 per cent. During constant infusions of 100, 400, or 800 mug per minute of ritodrine to the ewe for 120 minutes,, UtBF decreased 10, 37, and 31 per cent, respectively, and MMAP decreased 6, 20 and 25 per cent respectively. Dose-related maternal tachycardia and hyperglycemia occurred. There were no significant changes in UmBF, mean fetal arterial pressure, or fetal heart rate. During all infusions, maternal and fetal arterial pH, PCO2, and PO2 remained within normal physiologic limits. Simultaneous infusion of ritodrine (400 mug per minute) and propranolol (100 mug per minute) blocked the maternal tachycardia, but decreases in UtBF, MMAP, and UmBF were observed. Ritodrine infusions to the fetus (25 mug per minute) resulted in fetal tachycardia and a variable increase in UmBF.

Animals↗
PubMed1976

Uteroplacental blood flow during hyperthermia with and without respiratory alkalosis.

The effect of induced maternal hyperthermia (1.5 degrees C rise over 60 min) on the uterine and umbilical circulations was examined in two groups of chronically instrumented pregnant sheep. Hyperventilation occurred in both groups. In the group in which the resultant respiratory alkalosis was untreated (N = 5), uterine blood flow decreased to 53 +/- 3% (mean +/- SE; P less than 0.01) of control at maximal maternal temperature. Umbilical blood flow also decreased 30 +/- 6% (P less than 0.01) below control levels. In the other group, normocapnia was maintained (N = 4). Uterine blood flow decreased in this group to 75% +/- 2% (P less than 0.01) of control levels, but umbilical blood flow did not significantly change. There was no significant change in maternal or fetal mean arterial pressure during hyperthermia in either group. Thus, maternal hyperthermia per se results in a significant decrease in uterine blood flow primarily through an effect on uterine vascular resistance, but without a concomitant change in umbilical blood flow.

Alkalosis, Respiratory↗
PubMed1976

Effects of hypercapnia on uterine and umbilical circulations in conscious pregnant sheep.

Changes in the uterine and umbilical circulations during induced hypercapnia were studied in nine unanesthetized near-term pregnant sheep. Blood flows were measured with electromagnetic flow transducers and arterial pressures with vascular catheters implanted under anesthesia 2-16 days prior to experiments. Hypercapnia was induced in the fetus alone by giving acetazolamide iv to the fetus, 100-200 mg/kg. Mean fetal arterial Pco2 increased from 49.5 to 63.4 mmHg but no significant changes in umbilical blood flow occurred. Stepwise increases in both maternal and fetal arterial Pco2 were induced by increasing maternal inspired CO2 concentration to a maximum of 12%. No dignificant changes occurred in uterine or umbilical circulations until hypercapnia was severe (maternal arterial Pco2 greater than 60 mmHg, fetal arterial Pco2 greater than 70 mmHg). With severe hypercapnia uterine vascular resistance increased significantly and uterine blood flow decreased despite an increase in maternal arterial pressure; fetal arterial pressure and umbilical blood flow increased significantly, but umbilical vascular resistance did not. We conclude that hypercapnia in conscious pregnant sheep is associated with significant changes in uterine and umbilical circulations, but only when hypercapnia is severe. Carbon dioxide is unlikely to be a factor in normal physiological regulation of the uteroplacental circulation in this species.

Acetazolamide↗
PubMed1976

Lung lecithin biosynthesis in the nonhuman primate fetus: determination of the primary pathway in vivo.

The two pathways for de novo lecithin (phosphatidylcholine) biosynthesis, choline incorporation (1) and phosphatidylethanolamine methylation (II), were examined simultaneously in lung and other tissues of Rhesus monkey fetuses. Cannulation of interplacental fetal vessels permitted studies on the intrauterine fetus without disruption of fetal-placental-maternal-amniotic fluid anatomic integrity. In contrast to observations with indirect techniques in the same species, direct measurement of the incorporation of isotopic precursors (3H-choline and 14C-ethanolamine) into lecithin indicated that pathway I predominates by 100-fold over PE methylation in pulmonary lecithin synthesis. Fetal liver, brain, and kidney also showed 10--70-gold greater choline incorporation that methylation activity. Measurement of lung phosphatidylcholine production via the two pathways in acidemic fetuses (umbilical venous pH less than 7.20) demonstrated marked inhibition of pathway I, but not II. It is concluded that the choline pathway is the major mechanism of lung lecithin synthesis in fetal primates and that this pathway is pH sensitive in vivo.

Acid-Base Equilibrium↗
PubMed1976

Hypothalamic-pituitary control of perinatal prolactin secretion in Macaca mulatta.

Sequential changes in maternal and fetal plasma and amniotic fluid concentrations of prolactin (PRL) and growth hormone (GH) were examined after these intravascular administration of thyrotropin releasing hormone (TRH) or L-dopa alone or combined directly to the near-term Rhesus fetus. The neonatal plasma responses to these same stimuli were also examined. Fetal and neonatal plasma PRL levels increased immediately after TRH injection and remained elevated from baseline levels (102-800%) throughout the 45 min sampling period. Maternal plasma PRL levels also increased markedly. Although amniotic fluid concentrations were more variable, the trend was an increase. After L-dopa injection, fetal and neonatal plasma PRL values declined 26-62% from baseline levels. Maternal plasma PRL concentrations also declined 30-50%, but amniotic fluid PRL concentrations progressively increased. When L-dopa and TRH were administered together, fetal plasma PRL levels declined 14-40% from initial levels, but maternal plasma PRL levels did not change in a consistent manner, and amniotic fluid PRL levels remained stable. There was no change from baseline fetal or neonatal plasma GH concentrations in these experiments. The plasma PRL responses of the primate conceptus to these stimuli are consistent with those found in the adult; the unresponsiveness of plasma GH is not. The direction and magnitude of changes in both maternal plasma and amniotic fluid PRL concentrations provide indirect evidence of placental transfer of TRH and L-dopa in some experiments, and require a biophysical explanation not apparent in others.

Amniotic Fluid↗
PubMed1976

A system for inducing hyperthermia in sheep.

A system that produces hyperthermia in pregnant sheep in a consistent and reproducible manner was designed. The experimental approach was through control of ambient temperature and relative humidity in a closed chamber; the sheep was unable to lose its metabolic heat in an environment of 42 degrees C and 75% relative humidity. This system resulted in a steady and progressive temperature rise of 2.5 degrees C in the maternal abdominal aorta in 120 +/- 17 min.

Animals↗
PubMed1975

Plasma renin activity during ovine pregnancy.

Chronic intravascular catheterization in maternal, fetal, and neonatal sheep was utilized to assess basal plasma renin activity (PRA) and changes in PRA in response to furosemide. Maternal PRA increased from base-line levels during the last trimester of pregnancy and remained elevated for 12 wk postpartum. Fetal basal levels of PRA were variable but usually greater than maternal levels. Intravenous administration of furosemide to pregnant ewes resulted in a prompt and significant increase in maternal PRA with inconsistent changes in fetal PRA. Fetal and neonatal animals with low basal levels showed a significant increase in PRA; maternal PRA did not change. Animals with higher basal levels did not respond to the stimulus, perhaps reflecting a maximum renin secretory rate. These data are consistent with the conclusions that fetal renin originated predominantly from the fetal kidney, that fetal PRA receives no significant contribution from the maternal circulation, and that renin does not cross the ovine placenta.

Animals↗
PubMed1975

Perinatal primate parathyroid hormone metabolism.

Near-term fetal and neonatal parathyroid gland function has been studied in the Rhesus monkey. Fetal serum ionized calcium (Ca++) levels are significantly greater than simultaneously obtained maternal levels. Fetal serum parathyroid hormone (PTH) was undetectable both in the basal state and in association with EDTA-induced fetal hypocalcemia. Induced maternal hypocalcemia was associated with increased maternal serum PTH levels and no change in fetal basal serum Ca++ or PTH levels. Only a minimal decrease in simian neonatal serum Ca++ occurred over the first 48 h of life. Normal adult levels of serum PTH were present as early as 6 h of neonatal life. Induced hypocalcemia at 12 h of age resulted in a significant increase in serum PTH levels.

Animals↗