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Biomedical subjects

A M Walker

Publications and source records attributed to A M Walker.

At least 325 records · Page 18Linked to original sources
PubMed1978

Dissociation of the porcine anterior pituitary: the kinetics of luteinizing hormone release in response to luteinizing hormone-releasing hormone.

A method has been developed for disaggregating porcine anterior pituitary tissue and for providing dissociated preparations which have good, stable viability in culture. The secretory capacity for these preparations in terms of their ability to release luteinizing hormone in response to luteinizing hormone-releasing hormone has been documented in detail. In following the short-term kinetics of LH secretion a biphasic pattern of release has been demonstrated in which a sharp initial peak, maximal at 2 min, is followed by a second, more prolonged phase of release reaching a maximum between 10 and 20 min.

Animals↗
PubMed1978

Calcium as a second messenger in the stimulation of luteinizing hormone secretion.

In cells dissociated from porcine anterior pituitary glands and maintained in culture for 48 h the specific secretagogue luteinizing hormone-releasing hormone (LH-RH) induces a biphasic pattern of luteinizing hormone (LH) release. A biphasic pattern of release is also induced by 57 X 10(-3) M K+ and the ionophore A-23187. By reducing the availability of Ca2+, either by omission from the medium, chelation or interfering with Ca2+ transport across the plasma membrane, it is shown that LH release stimulated by LH-RH is much less dependent upon the availability of extracellular Ca2+ than that stimulated by either high K+ or A-23187. Nevertheless, by using a lanthanum displacement protocol to follow the influx of 45Ca2+ it is shown that LH-RH stimulation does induce an influx of extracellular Ca2+. Parallel experiments in which the stimulated 45Ca2+ efflux from preloaded cells is followed confirm the influx data but suggest, in addition, that when the influx of extracellular Ca2+ is inhibited, the peptide is able to mobilize Ca2+ from an intracellular location. It is thus concluded that while LH release can be initiated by an increase in the intracellular level of Ca2+, and although LH-RH stimulation does increase the permeability of the plasma membrane to Ca2+, the stimulation of LH release by LH-RH is not dependent upon extracellular Ca2+.

Animals↗
PubMed1978

Sympathetic and parasympathetic control of heart rate in unanaesthetized fetal and newborn lambs.

Development of autonomic nervous control of basal heart rate was studied in unanesthetized fetal lambs (93 days to term) and newborn lambs (2--29 days), using atropine and/or propranolol blockade. Fetal lambs showed a progressive increase in parasympathetic restraint of heart rate; vagal influence in the newborn lamb was similar to the term fetus. Sympathetic stimulation of fetal heart rate declined toward term, possibly due to the strongly increasing parasympathetic influence. Sympathetic influence in the newborn was similar to the early-gestation fetus. Intrinsic heart rate was about 185 beats/min throughout the fetal and newborn life span studied. Thus changes in basal heart rate resulted from a different balance of the sympathetic and parasympathetic components of the autonomic nervous outflow.

Adrenergic beta-Antagonists↗
PubMed1977

Effects of salbutamol and isoxsuprine on uterine and umbilical blood flow in pregnant sheep.

The effects of salbutamol and isoxsuprine upon uterine artery blood flow (UtBF) and umbilical vein blood flow (UmBF) were investigated in near-term, nonlaboring chronic sheep preparations. During both intravenous salbutamol and isoxsuprine infusions to the ewe, UtBF and mean maternal arterial pressure decreased significantly. Also, dose-related maternal tachycardia, hyperglycemia, and relative acidemia occurred. There were no significant changes in UmBF, mean fetal arterial pressure, or fetal heart rate (FHR) during salbutamol infusions, but UmBF and FHR increased during isoxsuprine infusions. During the 120 minute postinfusion recovery period, UtBF rose significantly after the salbutamol infusions but not after the isoxsupine infusions. The effects and structure-activity relationship of these two drugs are comparable to those of ritodrine and fenoterol, two other beta-adrenergic agonists.

Albuterol↗
PubMed1977

Effect of fenoterol (Th1165a) infusion on uterine and umbilical blood flow in pregnant sheep.

The effect of fenoterol (Th1165a) upon uterine artery blood flow (UtBF) and umbilical vein blood flow (UmBF) was investigated in near-term, nonlaboring chronic sheep preparations. During intravenous fenoterol infusions to the ewe in either incremental doses from 0.025 to 0.200 microng per kilogram per minute or constant infusions of 0.025 microng per kilogram per minute for 120 minutes. UtBF and UmBF did not change significantly. Dose-related maternal tachycardia, hyperglycemia, and relative acidemia occurred, but there were no significant changes in mean maternal and fetal arterial pressures or fetal heart rate. The simultaneous infusion of propranolol (2 microng per kilogram per minute) with fenoterol (0.200 microng per kilogram per minute) blocked the maternal tachycardia but resulted in a significant decrease in UmBF and a significant increase in UtBF. In all of the maternal infusions. UtBF significantly rose and plateaued up to 14 per cent above the control level during the 120 minute recovery period after infusion. A non-beta-adrenergic effect of fenoterol is suggested as the cause of this UtBF increase.

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↗
PubMed1975

Lung liquid production and composition in the "in utero" foetal lamb.

Experiments were carried out on foetal lambs in utero from 103 to 133 days' gestation, to measure the daily flow and solute composition of liquid from the foetal lung. The mean daily flow from foetuses with a tracheal fistula was 231 ml/day (9.6 ml/h), while the mean hourly flow in foetuses with a tracheal loop was 5.1 ml/h. The pattern of flow of lung liquid was intermittent and irregular, with short periods of rapid flow interspersed with longer periods of slow and no flow. The Na+, Ca2+, Mg2+, Cl- and urea concentrations showed no significant change with increasing gestational age, but the K+ concentration showed a significant rise. It is concluded that the lung of the foetal lamb behaves as an exocrine gland, secreting a substantial volume of liquid of a relatively constant composition.

Animals↗
PubMed1975

Effect of ventilation on pulmonary blood volume of the fetal lamb.

Blood volume changes in the fetal lung following the onset of ventilation were studied by isotopic measurement of red blood cell and plasma volume in rapidly frozen lungs of ten near term fetal lambs. Total pulmonary blood volumes of fetal lambs ventilated with 3% O2 and 7% CO2 in nitrogen (so that blood gas levels were little changed from fetal values), or with air, were compared with measurements in unventilated lambs. Regional correlations of blood volume and blood flow (measured with isotope-labeled microemboli) within the lungs were also examined. Total pulmonary blood volume averaged 5.6 ml/kg body weight in unventilated fetal lambs and was approximately 43% greated in fetal lambs after 5-20 min of air ventilation, but not significantly different in lambs ventilated with 3% O2 and 7% CO2 in nitrogen. Thus it is ventilation with air, rather than the introduction of gas into the alveoli, which enlarges the fetal pulmonary vascular bed. Regional pulmonary blood volume and blood flow were correlated, though poorly, in air-ventilated lungs, but not in lungs ventilated with 3% O2 and 7% CO2 in nitrogen; this suggests that a common factor may operate to increase both blood flow and blood volume in the fetal lung following the introduction of air.

Animals↗