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J C Zolman

Publications and source records attributed to J C Zolman.

15 recordsLinked to original sources
PubMed1990

Permanent alterations in the hypothalamic-pituitary-thyroid axis in the rat following phenytoin exposure in utero.

Phenytoin exposure in utero results in permanent alterations of the hypothalamic-pituitary-thyroid axis in the rat. The DPH exposed animals have decreased weight gain, thyroxine and triiodothyronine concentrations. In addition, they have blunted thyroid-stimulating hormone responses to thyrotropin-releasing hormone, propylthiouracil challenge or thyroidectomy. The diminished pituitary response in these animals is similar to that reported in neonatal thyrotoxicosis in the rat. This may be due, in part, to structural similarities between phenytoin and the thyroid hormone.

Animals↗
PubMed1989

Effects of phenobarbital on hypothalamic-pituitary-thyroid axis in the rat.

It has been reported that phenobarbital (PB) increases the peripheral clearance of T4 and T3 and decreases serum T4 and T3 concentrations in the rat, but serum TSH remains unchanged. To explore a possible direct effect of PB on TSH secretion at the hypothalamic-pituitary level, adult male rats were given PB 100 mg/kg or vehicle IP for 10 days. No difference in their thyroid weights was observed. In the PB-treated group serum T4 was decreased (PB, 3 +/- 0.2 micrograms/dl vs. control, 3.8 +/- 0.1 micrograms/dl, mean +/- SE, p less than .002), as was serum T3 (PB, 51 +/- 6 ng/dl vs. control, 70 +/- 5 ng/dl, p less than .05), but serum TSH remained unchanged. Pituitary TSH and hypothalamic TRH contents also were unchanged. Further studies were carried out similarly in the thyroidectomized hypothyroid rat to eliminate the effect of PB on serum T4 and T3 levels. PB or vehicle were started two days after thyroidectomy. By postoperative day 12, TSH levels in the PB-treated rats were lower than in the controls (PB, 697 +/- 62 microU/ml vs. control, 891 +/- 53 microU/ml, p less than .05). Pituitary TSH and hypothalamic TRH contents again were similar in both groups. When TRH (500 ng/kg body weight, IV) was given, the increment in serum TSH at 10 minutes was significantly lower in the PB group (PB, 53 +/- 26 microU/ml vs. control, 131 +/- 18 microU/ml, p less than .05).(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1985

Regulation versus modulation in GnRH receptor function.

Serum luteinizing hormone (LH) concentration after exposure to gonadotropin-releasing hormone (GnRH) indicates that an instantaneous increase occurs in the rate of release of LH directly from the anterior pituitary, as measured dynamically during superfusion in vitro. On the other hand, estradiol-17 beta (E2) alone shows no such instantaneous effect on LH release rate (at least for the first four hours), in either physiologic or pharmacologic concentrations. At the same time, brief (ten to 30 minute) exposure of isolated anterior pituitary plasma membranes to physiologic concentrations of E2 significantly alters the binding of a fully biologically active 125I-GnRH to its plasma membrane receptor protein. In order to characterize the effect of E2 on GnRH binding further, we preincubated dispersed bovine anterior pituitary cells for six hours in the presence or absence of physiologic concentrations of E2 (10(-10)M). Following preincubation in the presence of E2, the cell suspension was incubated for 30 minutes with physiologic concentrations (5 X 10(-11) - 5 X 10(-10)M) of a fully biologically active 125I-GnRH. The treatment, at least, doubled the number of biologically important high affinity GnRH binding sites (Kd's = 7.5 X -10(-11) - 4.5 X 10(-10)M), and changed the binding capacity of some of the binding sites up to three fold, which altered the cooperativity of GnRH-receptor interaction. Thus, the interaction of E2 with GnRH at the level of GnRH receptor is mandatory for the short-term pituitary effect of E2 on LH release in vitro and in vivo.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1985

Dynamic model of gonadotropin-releasing hormone receptor function.

Functioning gonadotropin-releasing hormone (GnRH) receptor is visualized as an aggregate of identical subunits (not all always functional) with the aggregate usually transformed into at least four successive structurally distinct receptor assemblies. Receptor protein, hormone molecule(s), and carrier(s) are main components of each functional subunit. During the normal lifespan of the functional subunit, each carrier is responsible for delivery of a unit amount of product, per unit time, to the cell surface (ratio between functional carrier and bound hormone, 1:1). Association of hormone with the receptor protein is essential, not only for the initial formation of the functional subunits but also for subsequent conformational changes that are in turn essential for formation of the aggregate only, or later (in the presence of sufficiently high GnRH concentrations) for a successive formation of a family of receptor assemblies (occurring one at a time). The successive assemblies differ from the aggregate by being more stable and from one another by increasing GnRH binding affinity and apparent capacity. They resist stimulation during protracted decay (desensitization).

Animals↗
PubMed1985

Thyrotropin releasing hormone biosynthesis in neonatal rat pancreas.

We studied the in vitro synthesis of thyrotropin releasing hormone (TRH) by pancreatic cells, using [14C]histidine incorporation and radioimmunoassay. Neonatal pancreases were incubated in Krebs-Ringer-bicarbonate buffer, pH 7.4, at 37 degrees C, under 95% O2-5% CO2. [14C]histidine was added and its incorporation into the tissue proteins was followed up to 4 hours. The methanolic pancreatic extracts obtained were subjected to Sephadex G-200 chromatography. Two peaks of [14C] radioactivity were eluted, one small (20% of total radioactivity) with the void volume and one large in a position identical to synthetic TRH. Both radioactive peaks corresponded to immunoreactive TRH peaks present in the same samples. In pulse-chase experiments, excess cold histidine was used and samples taken showed again similar radioactive peaks with an initial increase and a later decrease in the size of the small peak. We conclude that the neonatal rat pancreas actively synthesizes TRH and/or a higher molecular weight TRH-like protein, which may represent a TRH precursor.

Animals↗
PubMed1984

Localization of putative gonadotrophin releasing hormone receptor protein in the anterior pituitary.

Specific binding of a fully biologically active 125I-gonadotrophin releasing hormone (GnRH) to isolated anterior pituitary cells is time dependent, saturable and the concentration dependent binding curves exhibit positive cooperativity. Binding to intact or solubilized plasma membranes and an affinity purified GnRH receptor protein reveals in all instances multiple high affinity binding sites. Thus, GnRH receptor protein appears to be an intrinsic constituent of the cell membrane, and perhaps, other membranous organelles. To investigate the latter, the binding of 125I-GnRH to various subcellular fractions was studied and its affinity and time requirements determined. GnRH binding to plasma membranes and secretory granules was to multiple high affinity sites, while that to nuclei and microsomes was to a single high affinity site. Binding was 1.83 +/- 0.07, 0.78 +/- 0.04, 0.31 +/- 0.03 and 0.27 +/- 0.03 fmol micrograms-1 protein for isolated plasma membranes, secretory granules, microsomes and nuclei, respectively, after 30 min incubation with 10(-9) M GnRH. The magnitude of binding to microsomes did not change during the incubation period. It did not show any decrease (p greater than 0.05) in isolated nuclei and plasma membranes, except for the 24 h time period, when a significant drop (p less than 0.001) was seen. Binding to the secretory granule fraction culminated at 15 min and then decreased (p less than 0.001) steadily to a non-detectable level at 24 h. Thus GnRH receptor protein or its portion may be an integral part of some membranous particles in the anterior pituitary cells.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1983

Positive co-operativity in the membrane receptor mediated response.

In a hormonal system stimulated by one of the known hypothalamic peptides, gonadotropin releasing hormone (GnRH), the biological response seen, when physiologically called for, may be positively co-operative, showing as well the expected dynamics due to a singular or continuing stimulus. Mediation is by a receptor protein which in purified form shows positively co-operative binding and specifically aggregates in solution in the presence of the hypothalamic peptide. An introduced simple amplification receptor model encompassing both kinetic and structural features is based on known biological and biochemical parameters of the hormonal system. A novel feature of the model is an obligatory presence on a cell, at given times, of characteristic molecular forms (assemblies of various order) which all must be fully occupied as a condition of (1) their nontransient existence and (2) stimulus mediated effects. The model relates temporal changes in assemblies' occupancy, conformation and affinity to ligand concentration. It provides for sigmoid dose response curve.

Animals↗
PubMed1983

Peptide-receptor protein relationships: steroid feedback in GnRH stimulation of the anterior pituitary.

Gonadotropin releasing hormone receptor protein (GnRH.RP) purified to homogeneity has several binding sites for its effector hormone, as evidenced from the concentration dependence binding curve. Binding of a fully biologically active iodinated GnRH decreased (P less than 0.001) by pretreatment of immobilized receptor protein with physiological concentrations of both main sex steroids, testosterone and estradiol 17-beta. With increasing concentrations of the steroids the binding of 125I-GnRH decreased biphasically for testosterone and multiphasically for estradiol 17-beta. The findings are conducive to an allosteric receptor model.

Animals↗
PubMed1981

Effect of calcium and potassium on basal and gonadotrophin-releasing hormone (GnRH) stimulated release of luteinizing hormone (LH).

LH release from the anterior pituitary was studied by the method of an in vitro superfusion of bovine anterior pituitary tissue slices. LH release was stimulated by increased potassium concentration (23 and 59 mM) and by synthetic GnRH (1 and 4 ng/ml). While the potassium effect was completely dependent on extracellular calcium, that of GnRH was only partially dependent. Additive effect was observed when GnRH followed enhanced potassium infusion but not vice versa. This suggested that the mechanism responding to potassium may be contained within the stimulatory pathway of GnRH. There was a difference in the dynamics of the LH response: maximum response was attained in about 10 min of potassium infusion while infusion of GnRH resulted mostly in multiphasic stepwise release of LH reaching a plateau in 60 to 90 min only. It is speculated that the potassium effect involves the K+, Na+ dependent ATPase.

Animals↗
PubMed1980

Gonadotropin-releasing hormone receptor binding in bovine anterior pituitary.

Synthetic gonadotropin-releasing hormone (GnRH) was monoiodinated at a high specific radioactivity with 125I. The iodinated hormone retained full biological activity as assessed by the release of luteinizing hormone in vitro from bovine anterior pituitary tissue slices. Specific binding of 125I-labeled gonadotropin-releasing hormone of high affinity and low capacity was obtained using dispersed bovine anterior pituitary cells. The binding had sigmoid characteristics, compatible with the presence of more than one binding site. The subcellular fraction responsible for binding was identified with the plasma membranes. However, significant binding also occurred in the secretory granules fraction. The plasma membranes were solubilized with sodium dodecyl sulfate. Using gonadotropin-releasing hormone covalently coupled to a solid phase, a protein was purified by an affinity technique from the solubilized plasma membrane preparation which possessed similar binding propperties as plasma membranes, both intact and solubilized. The protein migrated as a single component on polyacrylamide gel in sodium dodecyl sulfate and the estimated molecular weight was 60 000. The character of the gonadotropin-releasing hormone concentration dependence binding as well as association kinetics were multiphasic and suggested the presence of more than one binding site. When analyzed by the Hill plot, the Hill coefficient of all binding curves was always greater than one which is compatible with positive cooperativity. This was further supported by the dissociation studies where the dissociation rate was inversely proportionate to both the gonadotropin-releasing hormone concentration and the time interval during which the gonadotropin-releasing hormone-gonadotropin-releasing hormone receptor protein complex was formed. Using difference chromatography, aggregation of the purified gonadotropin-releasing hormone receptor protein was demonstrated to occur upon its exposure to gonadotropin-releasing hormone. The formed macromolecular complexes bound preferentially 125I-labeled gonadotropin-releasing hormone. It is concluded that a single receptor protein is responsible for gonadotropin-releasing hormone binding in the bovine anterior pituitary. It is a part of the plasma membranes. Its interaction with gonadotropin-releasing hormone provokes transitions of the protein into different allosteric forms and this may be related to the biological effect of gonadotropin-releasing hormone on gonadotropin secretion.

Animals↗
PubMed1978

Luteinizing hormone secretion and gonadotropin releasing hormone binding in heifer and steer anterior pituitaries.

Luteinizing hormone (LH) release and tissue content were studied in explanted bovine pituitaries from heifer and steer during superfusion in vitro. Compared to steer, heifer pituitaries contained significantly more LH and released more LH into the incubation medium. Quantitative response in LH release to gonadotropin releasing hormone (GnRH) stimulation was about equal in heifer and steer, but relative increase was more pronounced in steer. Specific 125I-GnRH binding to dispersed anterior pituitary (AP) cells and isolated plasma membranes was also analyzed. Concentration dependent binding curve of 125I-GnRH to dispersed cells exhibited sigmoid characteristics suggesting positive cooperativity. There was a sex dependent difference, the binding curve for heifer being shifted to the left and reaching saturation at lower GnRH concentrations than that for steer. "Multisigmoid" concentration dependent GnRH binding curves were obtained using isolated plasma membranes, revealing the presence of multiple binding sites. One of the binding sites, present in heifer, was absent in the steer derived materials, but could be induced by pretreatment of the steer plasma membranes with 17-beta estradiol as low as 50 pg/ml. It is concluded that the differences in GnRH binding may be responsible, at least in part, for the observed differences in LH secretion pattern between heifer and steer.

Animals↗
PubMed1978

Binding of gonadotrophin releasing hormone (GnRH) by bovine anterior pituitary plasma membranes and purified GnRH receptor protein (GnRH.R).

[125I]-(GnRH) concentration dependence binding curves using isolated bovine anterior pituitary plasma membranes, intact and solubilized, and purified GnRH receptor protein, are compared. In all instances the concentration dependence binding curves had a stepwise character here interpreted as multisigmoid, with several steep increases and plateaus. These curves are compatible with the existence of several binding sites for GnRH. Purification of the GnRH receptor protein (GnRH.R) resulted in about 500 000-fold increase of binding activity and yielded a single protein species on polyacrylamide gel electrophoresis in SDS, of estimated molecular weight 60 000. Similarity of GnRH binding by the purified protein with that of intact and solubilized plasma membranes suggested that a single protein was responsible for the binding in each instance. Thus heterogeneity of GnRH binding is likely attributable to phase transitions of a single receptor protein into different allosteric forms. The data suggest that positive cooperativity is involved in the studied system.

Animals↗