Search PubMed⌕ Search

Biomedical subjects

D L Foster

Publications and source records attributed to D L Foster.

At least 109 records · Page 6Linked to original sources
PubMed1985

Can the transition into anoestrus in the ewe be accounted for solely by insufficient tonic LH secretion?

It has been proposed that a seasonal increase in oestradiol negative feedback elicits anoestrus by preventing a key step in the preovulatory sequence of endocrine events, namely a sustained increase in tonic LH secretion. In the present study we compared the patterns of serum LH, FSH, oestradiol and progesterone after regression of the last corpus luteum of the breeding season, with their respective patterns during an ovulatory cycle in the late breeding season (samples obtained every 4 h from eight ewes). After regression of the last corpus luteum of the breeding season, serum LH and oestradiol showed distinct deviations from their respective late breeding season patterns. The rise in tonic LH secretion was curtailed. Further, there were no marked increases in oestradiol, despite a distinct, although brief, tonic LH rise; thus there were no gonadotrophin surges. If the hypothesis that the transition into anoestrus is caused solely by insufficient tonic LH secretion were correct, the brief increase in LH should have induced a transient rise in oestradiol. Since this was not the case, these results suggest that a decreased ovarian response to LH may also contribute to the termination of oestrous cyclicity at the transition to anoestrus.

Anestrus↗
PubMed1984

Morphology of proteoliposomes reconstituted with purified lac carrier protein from Escherichia coli.

Proteoliposomes reconstituted with purified lac carrier protein from Escherichia coli were ultra-rapidly frozen and examined by freeze-fracture-etch electron microscopy. The proteoliposomes are greater than 95% unilamellar, and the majority are 30-150 nm in diameter. Fracture faces of proteoliposomes (at a protein:lipid molecular ratio of about 1:2500) display 7.0-nm diameter globular intramembrane particles uniformly distributed on convex and concave surfaces. Calculations of particle composition suggest that each intramembrane particle probably contains one or two molecules of the 46.5-kDa transmembranous lac carrier protein, depending on the correction factor for the thickness of the metal deposited to form the platinum/carbon replicas. Etched surfaces of the proteoliposomes are smooth. Incubation of the proteoliposomes with monoclonal antibody 4B1, which binds to an epitope in the lac carrier on the exterior of the proteoliposomes, dramatically alters the intramembrane particle distribution. After incubation with antibody, the convex (inner monolayer) fracture faces are nearly devoid of intramembrane particles, and an overall 4-fold reduction in the total number of intramembrane particles is observed.

Cell Membrane↗
PubMed1984

Hourly administration of luteinizing hormone induces ovulation in prepubertal female sheep.

This investigation examined the effects of repeated injections of LH on ovarian function in the immature sheep approximately 12 weeks before the time of the first expected spontaneous ovulation. The frequency of endogenous LH pulses during the pretreatment period was approximately one pulse each 3 h. The first experiment determined that rapid injection (iv) of 15.5 micrograms LH replicated the amplitude of endogenous pulses. Hourly injection of this dose for 48 h to simulate the rapid LH pulse frequency of the follicular phase of the postpubertal female induced a LH surge and ovulation in two of three lambs. By contrast, administration of 33% of the dose over the 48-h period did not [5.2 micrograms/h (three lambs) or 15.5 micrograms each 3 h (three lambs)]. The second experiment (seven lambs) determined the time course of the preovulatory estradiol rise produced in response to hourly LH pulses (15.5 micrograms/injection), as well as the length of the luteal phase after the induced LH surge. Four lambs produced a sustained estradiol rise, a LH surge, and ovulation. The luteal phase was normal (13 days) in one and short in three lambs (6-11 days). In the remaining three prepubertal females that did not ovulate in response to 48-h injections of LH, the estradiol rise was not sustained. Circulating estradiol in five untreated control lambs exhibited only transient increases during the course of the study. The results indicate that hourly administration of physiological quantities of LH over a relatively brief period (48 h) can produce a follicular phase culminating in first ovulation in the immature lamb. In the context of the mechanism proposed for puberty in the female sheep, the findings are consonant with the hypothesis that the hypothalamus, through its modulation of LH pulse frequency, governs the initiation of ovulation.

Animals↗
PubMed1984

Preovulatory gonadotropin surge system of prepubertal female sheep is exquisitely sensitive to the stimulatory feedback action of estradiol.

The sensitivity of the LH surge mechanism to estradiol-positive feedback was examined in immature lambs (19 weeks of age) several weeks before first ovulation (31 weeks of age). To minimize differences in basal estradiol concentrations, lambs were pretreated with a low level of estradiol (2 pg/ml; Silastic capsule) for 4 days and were ovariectomized before they were challenged with one of five levels of estradiol (n = 5 lambs/level). The estradiol increments produced after insertion of the second set of estradiol implants ranged from 2-11 pg/ml. All estradiol increments, even those as low as 2 pg/ml, produced LH surges; in the absence of an estradiol increment (second implant not inserted; n = 5 lambs), a LH surge did not occur. The similarity of the dose-response curve for estradiol-induced LH release in the lamb to that which we previously reported for the mature female indicates that the lamb is equally sensitive to estradiol stimulatory feedback action long before the age of first ovulation. This suggests that low estradiol secretion, rather than reduced sensitivity of the surge mechanism to estradiol-positive feedback, is responsible for the anovulatory condition of the immature female sheep.

Animals↗
PubMed1983

Structure of the lac carrier protein of Escherichia coli.

Circular dichroic measurements on the lac carrier protein purified from the cytoplasmic membrane of Escherichia coli indicate that 85 +/- 5% of the amino acid residues comprising this integral membrane protein are arranged in helical secondary structures. Analysis of the sequential hydropathic character of this protein by the method of Kyte and Doolittle (J. Mol. Biol. (1982) 157, 105-132) indicates that the protein is composed of at least 12 hydrophobic segments with a mean length of 24 +/- 4 residues/segment. Approximately 70% of the 417 amino acids in the lac carrier are found in these domains. The hydropathic profile, together with the circular dichroic measurements, suggest that the 12 hydrophobic segments are largely in a helical conformation. If the segments are assumed to be alpha-helical, the mean length of each domain approximates the thickness of the most hydrophobic portion of the lipid bilayer. Based on these considerations, it is proposed that the lac carrier protein consists of at least 12 alpha-helical segments that traverse the membrane in a perpendicular sense, i.e. in a fashion similar to bacteriorhodopsin.

Circular Dichroism↗
PubMed1983

Response to estradiol inhibition of tonic luteinizing hormone secretion decreases during the final stage of puberty in the rhesus monkey.

This investigation was conducted to determine if a decrease in responsiveness to estradiol inhibition of tonic LH secretion occurs in the female rhesus monkey during the late stages of sexual maturation. The study was conducted during the several month period between menarche and first ovulation. Serum LH was measured in postmenarchial females (n = 8) before and after ovariectomy and insertion of estradiol implants that achieved subadult concentrations (10-30 pg/ml) of the steroid. Removal of the ovaries 1-4 months after menarche (31 +/- 1 months) produced a prompt 3- to 5-fold rise in circulating LH. This was prevented by estradiol replacement therapy beginning at ovariectomy. Similarly, insertion of estradiol implants after the castration response was established decreased the high concentrations of circulating LH to preoperative levels where they remained for several months. At 42 +/- 1 months of age, and in the face of unchanging peripheral estradiol, circulating LH rose severalfold in the ovariectomized females (n = 7). At a similar age (44 +/- 2 months, n = 4), ovulations began in untreated females. The results indicate that subadult concentrations of peripheral estradiol which are capable of suppressing LH secretion before the age of first ovulation are ineffective in this regard afterward. This provides compelling evidence for a decrease in estradiol inhibition of tonic LH secretion during puberty in the primate female.

Age Factors↗
PubMed1983

A role for estradiol in enhancing luteinizing hormone pulse frequency during the follicular phase of the estrous cycle of sheep.

Experiments were conducted to test the hypothesis that the increased frequency of LH pulses during the follicular phase of the sheep estrous cycle can be explained by the withdrawal of progesterone. This steroid imposes a profound inhibition of LH pulse frequency in the luteal phase. Experimental ewes were ovariectomized in the late luteal phase of the estrous cycle and divided into three groups: 1) no estradiol provided; 2) basal estradiol maintained at 1-2 pg/ml by small sc Silastic estradiol implants; and 3) peak estradiol of 5-6 pg/ml provided by larger estradiol implants. Control ewes had intact ovaries; their follicular phases were synchronized by insertion and subsequent removal of progesterone implants. LH pulses were monitored beginning 24 h after ovariectomy of experimental ewes or progesterone implant removal from intact control ewes. In the follicular phase controls, LH pulse frequency increased 3- to 4-fold after progesterone withdrawal, reaching up to two pulses per h. When estradiol was not provided after ovariectomy of experimental ewes, LH pulse frequency also increased, but not to the extent seen in the follicular phase control. This high frequency was achieved, however, in experimental ewes treated with either basal or peak estradiol. Both estradiol treatments also reduced LH pulse amplitude. These results fail to support the hypothesis that the high frequency of LH pulses in the follicular phase is solely a consequence of progesterone withdrawal at luteolysis. Rather, they suggest that estradiol (but not necessarily rising estradiol) also contributes to the high frequency pulses of LH that occur in the ewe at this time.

Animals↗
PubMed1982

Lactose-proton symport by purified lac carrier protein.

The lac carrier protein of Escherichia coli was purified by an improved procedure and its activity assayed by a rapid filter method. Following reconstitution of the carrier by octyl glucoside dilution, proteoliposomes were concentrated by filtration on a microporous filter. Lactose accumulation by adsorbed or entrapped proteoliposomes is driven by an artificially imposed pH gradient (interior alkaline), by a membrane potential (interior negative), or by a combination of both forces. Activity is almost completely abolished by the protonophore carbonyl cyanide m-chlorophenylhydrazone or by the competitive inhibitor thiodigalactoside. Addition of lactose to proteoliposomes under appropriate conditions results in alkalinization of the external medium. This effect is not observed with liposomes devoid of lac carrier or in the presence of proton conducting agents. The results provide a strong indication that the lac gamma gene product is the only protein in the cytoplasmic membrane of Escherichia coli required for lactose-proton symport.

Biological Transport↗
PubMed1982

Stoichiometry of subunits in the H+-ATPase complex of Escherichia coli.

The H+-ATPase (F1F0) of Escherichia coli was purified from cells labeled with either [35S]sulfate or [U-14C-D] glucose, and the molar ratio of subunits in the complex determined. The molar ratio was calculated from the radioactivity incorporated into each subunit, using either the subunit sulfur content or subunit molecular weight. These labeling experiments confirm an alpha 3 beta 3 gamma 1 delta 1 epsilon 1 ratio of subunits in F1, and indicate a chi 1 psi 2 omega 10 ratio of subunits in F0. The chi, psi, and omega designations used here refer to the subunits of F0 in order of decreasing molecular weight. Staining with Coomassie brilliant blue gave a reliable indication of the molar ratio of subunits in F1, but very erroneous values for each of the subunits of F0. We attempted to estimate the ratio of subunits in the native membrane, since the stoichiometry determined for the purified complex could be an anomaly of purification. These estimates were made after labeling cells with [35S]sulfate during amplification of the ATPase genes carried on a lambda transducing phage. The subunit ratios in the native membrane were reasonably close to those obtained with purified F1F0. We conclude that the stoichiometry determined reflects the composition of F1F0 in the native membrane. The most surprising conclusion from this study is that there are 10 +/- 1 omega ("proteolipid") subunits in each F1F0 complex. This is considerably more than had been assumed previously.

Adenosine Triphosphatases↗
PubMed1981

Purification and reconstitution of functional lactose carrier from Escherichia coli.

The lactose carrier protein of Escherichia coli was purified by a simple procedure employing differential solubilization and ion-exchange chromatography and reconstituted into liposomes by octylglucoside dilution. The proteoliposomes exhibited both membrane potential-driven lactose transport and lactose counterflow. Furthermore, the purified protein was identified as the product of the lac y gene. These and other results demonstrate that the lactose carrier is the only polypeptide species essential for energy-coupled lactose transport and counterflow.

Amino Acids↗
PubMed1981

The endocrine basis of the synergistic suppression of luteinizing hormone by estradiol and progesterone.

The basis of the synergism between estradiol and progesterone in suppressing tonic (pulsatile) LH secretion was examined in the ewe, making use of the observation that progesterone exerts its inhibition selectively on LH pulse frequency, while estradiol decreases only pulse amplitude. To accomplish this, we analyzed changes in LH pulse patterns produced by a low level of progesterone in ovariectomized ewes treated with Silastic estradiol implants from the time of gonadectomy. A serum progesterone level of about 1 ng/ml was chosen because it inhibits LH only in the presence of estradiol. Under these circumstances, a decrease in LH pulse amplitude during progesterone treatment would suggest that progesterone increased the response to the existing level of estradiol; a decrease in pulse frequency would suggest that estradiol increases the effectiveness of progesterone. It was found that the low level of progesterone produced a decrease in LH pulse frequency in estradiol-treated ovariectomized ewes without altering pulse amplitude. These results are consistent with the hypothesis that the synergism of these two steroids reflects, at least in part, an estradiol-induced increase in the sensitivity of the central nervous system to the negative feedback action of progesterone.

Animals↗