Ability of growth hormone fragments to compete with I-iodinated human growth hormone for specific binding to isolated adipocytes of hypophysectomized rats

Several noncovalent complexes of large fragments of human GH, which are less active than native human GH in stimulating glucose metabolism in adipose tissue of hypophysectomized rats, were tested for their ability to compete with ¹²⁵I-iodinated human GH for specific binding to isolated adipocytes of hypophysectomized rats. The complexes tested were A (residues 1–134 + residues 141–191; S-carbamidomethylated), B (residues 1–134 + residues 135–191; S-carbamidomethylated) and C (residues 1–134 + residues 135–191; S-carboxymethylated). When compared to native human GH, the complexes were less active in competing with ¹²⁵I-iodinated human GH for specific binding to adipocytes, and their order of potency in the binding assay (A > B > C) was similar to that of their respective activities in stimulating glucose metabolism in isolated adipose tissue of hypophysectomized rats.     

Growth Hormone Treatment of Hypophysectomized Rats Increases Catecholamine-induced Lipolysis and the Number of β-Adrenergic Receptors in Adipocytes: No Differences in the Effects of Growth Hormone on Different Fat Depots

Growth hormone (GH) has a lipolytic effect in adipose tissue but this effect may differ in adipose tissue from various fat depots. This latter possibility was investigated in the present study, in which the effects of GH in vivo on catecholamine-induced lipolysis and the number of β-adrenergic receptors in isolated adipocytes from different fat depots of hypophysectomized rats were investigated. Female and male Sprague-Dawley rats were hypophysectomized or sham-operated at 45 days of age. One week after the operation, hormonal replacement therapy with L-thyroxine and hydrocortisone acetate was given. In addition, groups of rats were treated with GH (1.33 mg/kg per day, given as two daily subcutaneous injections). After 1 week of hormonal treatment, adipocytes were isolated from the parametrial, epididymal and inguinal fat pads, and glycerol release after catecholamine-stimulation and ¹²⁵I-cyanopindolol binding were measured. Hypophysectomy resulted in a marked decrease in the lipolytic response to catecholamines. GH treatment significantly increased catecholamine-induced lipolysis with similar effects in adipocytes from parametrial or epididymal and inguinal fat depots in both female and male rats. There were no differences between norepinephrine compared with isoproterenol-induced responses. ¹²⁵I-cyanopindolol binding was reduced after hypophysectomy and normalized by GH treatment, without differences between parametrial and inguinal adipose tissue regions. We conclude that the lipolytic effects of GH in the rat may partly be mediated by a stimulatory effect on β-adrenergic receptors in adipocytes. In addition, GH exerted similar effect on catecholamine-induced lipolysis and β-adrenergic receptors in adipocytes from parametrial, epididymal and inguinal fat depots.     

Human pituitary growth hormone: immunochemical investigations of biologically active fragments

Guinea pig antisera to human growth hormone were tested for their ability to recognize the two biologically active fragments of the hormone produced by human plasmin digestion and a synthetic active fragment. A precipitin line was obtained with native human growth hormone, plasmin-treated human growth hormone, and its NH₂-terminal fragment (residues 1–134). In the microcomplement-fixation and radioimmunoassay experiments, the NH₂-terminal plasmin fragment (residues 1–134) showed a greater immunoreactivity than the COOH-terminal plasmin fragment (residues 141–191). This, in turn, was more active than the synthetic fragment (residues 95–136).     

Growth hormone receptor structure in adipocytes from normal and growth hormone-deficient rats

Using cross-linking techniques, we compared the properties of the growth hormone (GH) receptor in freshly isolated adipocytes from normal rats, from GH deficient rats, and in preincubated adipocytes from normal rats. Bound [125I]iodo-hGH was cross-linked to adipocytes with disuccinimidyl suberate, and membrane proteins labelled with [125I]iodo-hGH were visualized using sodium dodecyl sulfate polyacrylamide gel electrophoresis and autoradiography. All of the adipocytes tested exhibited a prominent Mr = 134,000 band and additional less intense bands in the presence of reductant. No significant differences in the overall banding pattern of membrane proteins were evident in reducing or nonreducing gels, using adipocytes from rats made GH deficient by hypophysectomy or by treatment with antibodies against rat GH, or in fresh and preincubated cells from normal rats. Taken together with binding studies, these findings suggest that differences in the ability of GH to stimulate glucose oxidation in rat adipose tissue probably involve differences distal to the GH receptor.     

Effects of hypophysectomy and acute growth hormone treatment upon glucose metabolism in adipose tissues and isolated adipocytes of rats.

Glucose oxidation and incorporation into lipid were measured in epididymal adipose tissues and isolated adipose cells of normal and hypophysectomized rats in an effort to determine whether the acute hypoglycemic effect of a systemic growth hormone (GH) injection was related to alterations in the glucose metabolism of adipose tissue. The rats were fed rat chow or a high sucrose diet and received 100 mug GH intraperitoneally 30 minutes or three and one-half hours before sacrifice. Hypophysectomized rats showed a lower plasma glucose as compared with normal rats on both diets. Thirty minutes after a GH injection there was a further decrease of the plasma glucose which, however, was not present in those rats receiving GH three and one-half hours before sacrifice. Adipose tissues from hypophysectomized rats fed the high sucrose diet showed a blunted insulin sensitivity as compared with normal rats on a similar diet. The insulin sensitivity of these tissues was further decreased 30 minutes after a GH injection. Basal glucose metabolism of isolated adipocytes from hypophysectomized rats, as compared with normal rats, was depressed if they were fed rat chow, was at normal levels if they were fed the high sucrose diet and was increased if they were fed the sucrose diet and received triiodothyronine and cortisone supplements. No manipulations of diet or hormonal treatments made the isolated adipocyte from hypophysectomized rats sensitive to insulin either 30 minutes or three and one-half hours after a GH injection. Since basal glucose utilization is not enhanced by GH injection and both the blunted insulin sensitivity of adipose tissue and the absent insulin sensitivity of adipopocytes would be expected to produce hyperglycemia rather than hypoglycemia, it is concluded that immediate systemic effects of a GH injection on carbohydrate metabolism are not related to changes in glucose metabolism of the peripheral adipose tissues.     

Demonstration of in vitro actions of growth hormone on isolated rat adipocytes

Previous studies have shown that adipocytes isolated from normal rats possess specific receptors for growth hormone (GH). Thus, we have now investigated the direct effects on such cells of GH added in vitro. The biological actions of GH determined were the stimulation of 14C-glucose oxidation to 14CO2 and conversion to 14C-lipid. As in adipose segments, hGH stimulation of these parameters required a 3-4 hr preincubation period in GH-free medium. The effect of hGH or bGH was dose-dependent with maximal effects at 1-2 micrograms/ml (CO2 55 +/- 13% stimulation above basal n = 9; lipid 33 +/- 4% n = 21, mean +/- SEM). The magnitude of the effect in isolated adipocytes was lower than that seen in adipose segments from similar groups of animals (lipid 67 +/- 26%, n = 4). The induction of responsiveness by preincubation was accompanied by a parallel increase in 125I-hGH binding. These studies have demonstrated that adipocytes from normal rats not only possess specific receptors for GH but also are metabolically responsive to GH added in vitro. These data suggest that the isolated rat adipocyte should be a useful model for further investigation of the relationship between GH receptor and post-receptor events.     

Allosteric effects of monoclonal antibodies on human growth hormone

We have previously shown that a monoclonal antibody (MAb) recognizing the human growth hormone (hGH) antigenic domain left exposed after binding to lactogenic receptors enhanced hGH binding probably through allosteric effects on the hormone binding site. Since receptors displaying different specificities would not recognize exactly the same hGH region, we explored whether some of our MAb could affect hGH binding to somatogenic receptors from rabbit liver and to human liver hGH-specific receptors.The effect of MAbAE5, AC8 and F11 on hGH binding was measured by determining the formation of¹²⁵I-MAb:hGH:receptor complexes using two different experimental approaches. Results from procedure A, which involved the previous binding of the hormone to microsomes before adding¹²⁵I-MAb, indicated that the hGH domain defined by epitopes AE5, AC8 and F11 is uncovered in the various hormone:receptor complexes.Procedure B was devised to reveal any alteration in the hGH molecule induced by the MAb. In this case preformed¹²⁵I-MAb:hGH complexes were added to microsomes. Data showed that¹²⁵I-MAb AE5:hGH complexes bound better to the various receptors than¹²⁵I-MAb AE5 to hGH:receptor complexes. On the contrary, hGH previously bound to¹²⁵I-MAb AC8 or¹²⁵I-MAb F11 was less recognized by the receptors than the free hormone. Furthermore, binding of MAb AE5 or MAb F11 to hGH 20 K (a natural hGH variant lacking residues 32–46) also enhanced its affinity to the various receptors whereas MAb AC8 did not inhibit hGH 20 K binding.Results indicated that MAb recognizing the hGH antigenic area that remains unmasked after binding to different membrane-bound receptors are able to affect hormone binding site. MAb would induce either positive or negative allosteric changes in the hormone region involved in its binding to lactogenic, somatogenic and hGH-specific receptors.     

In vitro insulin-like actions of the growth factor from the tapeworm, Spirometra mansonoides

In vitro actions of purified plerocercoid growth factor (PGF) were compared with those of insulin and human growth hormone (hGH) in adipose tissue from normal male rats. Insulin-like effects were measured by the ability of PGF, insulin, or hGH to stimulate oxidation of [U-14C]glucose to 14CO2, to stimulate lipogenesis, and to inhibit epinephrine-induced lipolysis. PGF and insulin stimulated significant increases in glucose oxidation and lipogenesis in adipose tissue that had not been preincubated as well as in tissue that had been preincubated. hGH stimulated insulin-like effects only in tissue that had been preincubated for 3 hr. Insulin, hGH, and PGF inhibited epinephrine-induced lipolysis of preincubated (3 hr) adipose tissue. hGH produced a dramatic lipolytic response in tissue freshly removed from normal rats but no dose of PGF was lipolytic. PGF did not displace 125I-insulin from its receptors on adipocytes but did competitively inhibit 125I-hGH binding to adipocytes. These results suggest that PGF has direct insulin-like actions which are initiated by binding a GH receptor, but PGF had no anti-insulin action and the insulin-like activity of PGF was unaffected by refractoriness of adipose tissue to GH.     

Growth hormone binding to cultured preadipocytes from obese fa/fa rats increases during cell differentiation

Preadipocytes from 7 day-old obese Zucker rats and from their lean littermates were grown in primary culture. Specific binding of human growth hormone (hGH) to the cells is demonstrated. There is no change in hGH binding to preadipocytes from lean animals before (day 5 of the culture) and after adipose conversion (day 12). Moreover, addition of insulin to the culture medium does not affect the specific binding of 125I-hGH to the cells. On the contrary, when 1 nM insulin is added to the culture medium of preadipocytes from obese rats, the specific binding of 125I-hGH is enhanced during the cell differentiation, with a 3-fold increase in the number of binding sites. The functional significance of these additional GH sites in differentiated preadipocytes from obese rats has to be examined.     

Receptor binding properties and insulin-like effects of human growth hormone and its 20 kDa-variant in rat adipocytes

The natural 20 kDa-variant of human growth hormone (hGH) binds with high affinity to IM-9 human lymphocyte receptors, in agreement with its potency in biological assays for growth promoting and lactogenic activities. In contrast, 20 kDa-hGH has only 3% of the potency of 22 kDa-hGH in binding to the receptors of normal and hypophysectomized rat adipocytes. In agreement with the binding potency, 20 kDa-hGH is only 3% as potent as 22 kDa-hGH in stimulating lipogenesis in normal rat adipocytes preincubated for a few hours in hGH-free medium. The 20 kDa-hGH is also much weaker than 22 kDa-hGH in stimulating lipogenesis in adipocytes from hypophysectomized rats. These data strongly support the concept that the rat adipocyte receptor, which mediates the insulin-like effects of growth hormone, is different from the receptor found on human IM-9 lymphocytes. Preincubation of rat adipocytes with hGH induces a refractoriness to subsequent activation of lipogenesis by hGH but does not abolish the response to insulin, while preincubation with insulin slightly potentiates the hGH response and does not change the insulin response. Additivity studies and a detailed comparison of the lipogenic effects of insulin and hGH suggest that hGH shares only a subset of the metabolic pathways activated by insulin.     

Growth hormone promoted tyrosyl phosphorylation of growth hormone receptors in murine 3T3-F442A fibroblasts and adipocytes

Because many growth factor receptors are ligand-activated tyrosine protein kinases, the possibility that growth hormone (GH), a hormone implicated in human growth, promotes tyrosyl phosphorylation of its receptor was investigated. 125I-Labeled human GH was covalently cross-linked to receptors in intact 3T3-F442A fibroblasts, a cell line which differentiates into adipocytes in response to GH. The cross-linked cells were solubilized and passed over a column of phosphotyrosyl binding antibody immobilized on protein A-Sepharose. Immunoadsorbed proteins were eluted with a hapten (p-nitrophenyl phosphate) and analyzed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and autoradiography. The eluate from the antibody column contained an Mr 134,000 125I-GH-receptor complex. A similar result was obtained when the adipocyte form of 3T3-F442A cells was used in place of the fibroblast form. O-Phosphotyrosine prevented 125I-GH-receptor complexes from binding to the antibody column, whereas O-phosphoserine and O-phosphothreonine did not. In studies of GH-promoted phosphorylation in 3T3-F442A fibroblasts labeled metabolically with [32P]Pi, GH was shown to stimulate formation of a 32P-labeled protein which bound to immobilized phosphotyrosyl binding antibodies. The molecular weight of 114,000 obtained for this protein is similar to that expected for non-cross-linked GH receptor. The Mr 114,000 phosphorylated protein could be immunoprecipitated with anti-GH antibody, indicating that GH remained noncovalently bound to this protein during absorption to and elution from the immobilized phosphotyrosyl binding antibody. Phosphoamino acid analysis after both limited acid hydrolysis and extensive base hydrolysis of the Mr 114,000 phosphoprotein confirmed the presence of phosphotyrosyl residues.(ABSTRACT TRUNCATED AT 250 WORDS)     

Temporal relationship of the prolactin-dependent LH-induced LH receptor to the LH stimulus

The time course for LH induction of luteinizing hormone (LH) receptors as reflected in binding of ¹²⁵l-labeled hCG was investigated in hypophysecto-mized adult male rats. A low dose of oLH (10 μg) was administered to hypophysectomized adult male rats following pretreatments with prolactin, follicle-stimulating hormone (FSH), growth hormone (GH), or saline. Testicular binding of hCG was determined at different times following the LH injection using Leydig cell membrane preparations from a testicular homogenate. Seven days after hypophysectomy, hCG binding was at a nadir of 19 ± 7% (mean ± SD) of control values. Pretreatment with prolactin (100 μg/day) for 7 days was associated with a nonsignificantly different hCG binding that was 30 ± 5% of control values. Prolactin pretreatment plus a single 10 μg LH i.p. injection increased ¹²⁵l hCG binding up to 56 ± 10% of control values within 30 minutes of the LH injection. Luteinizing hormone-induced hCG binding persisted at a high level (51 ± 4% of control values) for 2 hours but returned to hypophysectomized control levels 6 hours after the i.p. LH injection. Seven days pretreatment with FSH or GH at 100 μg/day plus 10-μg LH injections was also tested. Neither FSH nor GH had a statistically significant effect on hCG binding nor could they mimic the ability of prolactin to allow for LH induction of hCG binding in the hypophysectomized adult male rats. These studies suggest that the induction or “up-regulation” of Leydig cell hCG binding by ovine LH is rapid and specifically dependent upon pre-exposure to prolactin.     

Biological characterization of purified native 20-kDa human growth hormone

Because of the propensity of the 20-kDa variant of human growth hormone (GH) to aggregate with itself and with 22-kDa human GH, it has been difficult to prepare monomeric 20-kDa GH in highly purified form. This has been a major complicating factor in determining whether 20-kDa GH has a biological activity profile distinct from that of 22-kDa GH. In the present study, native 20-kDa GH was isolated from a human GH dimer concentrate and purified by a procedure that included column electrophoresis in agarose suspension as a final separation step. This procedure yielded highly purified monomeric 20-kDa GH, which was contaminated to an extent of less than 1% with 22-kDa GH, and which exhibited only a small degree of dimerization upon storage. The native 20-kDa GH was quite active in stimulating growth in hypophysectomized rats, when growth was assessed by body weight gain, longitudinal bone growth, the stimulation of sulfation of cartilage, and the elevation of serum IGF-1 level. However, in all of these growth assays, the 20-kDa GH was somewhat less active than the native 22-kDa GH to which it was compared; e.g., in the body weight gain and longitudinal bone growth assays, it had an estimated potency of 0.6 relative to the 22-kDa GH. The 20-kDa GH exhibited substantial diabetogenic activity when tested for the ability to raise fasting blood glucose concentration and to impair glucose tolerance in ob/ob mice. Also, the native 20-kDa GH had significant in vitro insulin-like activity, although its potency was approximately 20% that of the native 22-kDa GH to which it was compared. Thus, the biological activity profile of native 20-kDa GH differs from that of 22-kDa GH primarily in that insulin-like activity is markedly attenuated.     

Isolation and somatomedin activity of bovine growth hormone fragment 87–124

Various bovine growth hormone (GH) fragements were prepared and tested for somatomedin-like activity in vitro. Cyanogen bromide cleavage followed by reduction and alkylation yielded three fragments which were identified as GH (6–124), GH (150–179) and GH (125–149). No consistent effect was found when these preparations were tested for their ability to stimulate in vitro sulfate and thymidine uptake by rat costal cartilage and to compete with [¹²⁵I]iodoinsulin for insulin-binding sites on placenta membranes. A fourth peptide was isolated by cleavage of the tryptophanyl and methionyl bonds of bovine GH using anhydrous heptofluorobutyric acid and cyanogen bromide. In addition to significant amounts of non-specific cleavage products, a peptide having a molecular weight of about 4800 was isolated. The amino terminal residue was leucine and the carboxyl terminal was homoserine. These data, in addition to the amino acid composition, suggested that the peptide corresponded to residues 87–124. Fragment GH (87–124) stimulated sulfate (minimum effective concentration, 5 · 10^?8 M) and thymidine (minimum effective concentration, 10^?8 M) uptake by rat costal cartilage. It also cross-reacted, albeit weakly, with insulin-binding sites on placenta membrane. Maximum displacement was 35% of non-specific binding. These observations demonstrate that somatomedin-like activity can be generated from the growth hormone molecule which is inherently devoid of such activity.     

Effects of Growth Hormone on the Function of β‐Adrenoceptor Subtypes in Rat Adipocytes

Objective: The influence of growth hormone (GH) on the regulation of lipolytic response to specific agonists to β‐adrenoceptors and several post‐receptor steps in the lipolytic cascade were investigated. Research Methods and Procedures: Adipose tissues from rats were incubated with or without GH (1.38 nM). After a 24‐hour incubation, isolated adipocytes were prepared for different assays. Rats were hypophysectomized. One week after operation, l‐thyroxine and hydrocortisone acetate was given to hypophysectomized rats. One group of rats was treated with GH (1.33 mg/kg, daily). After 1 week of hormonal treatment, adipose tissues were removed for different studies. Results: GH treatment increased both basal lipolysis and lipolytic sensitivity to dobutamine and CGP 12177 in adipocytes. The lipolytic sensitivity to terbutaline was not influenced by GH treatment. GH treatment increased the maximal lipolytic response to dobutamine and CGP 12177, but not to terbutaline as determined with absolute values of lipolysis. Forskolin‐induced lipolysis was increased by addition of GH to tissues. Moreover, GH treatment resulted in enhanced expression of hormone‐sensitive lipase. GH treatment in hypophysectomized rats influenced neither the expressions of Gαs protein and cholera toxin‐catalyzed adenosine diphosphate‐ribosylation of Gαs protein, nor cholera toxin‐induced 3′, 5′‐cyclic adenosine monophosphate accumulation. However, the expression of Gαi protein was decreased after GH treatment. Discussion: These and previous results suggest that GH increases lipolysis in rat adipocytes partly through the β‐adrenergic system, including increases in both β₁‐ and β₃‐adrenergic receptor function, and partly through enhanced adenylate cyclase function, and expression of hormone‐sensitive lipase, perhaps via a decrease in Gαi protein expression.     

Modification by bovine growth hormone of liver plasma membrane enzymes, phospholipids, and circular dichroism

Liver plasma membrane phospholipid distribution, protein conformation, and 5′-nucleotidase, Mg²⁺-adenosine triphosphatase and (Na⁺ + K⁺)-adenosine triphosphatase specific activities, were shown to depend on pituitary status and treatment with bovine growth hormone.In whole liver homogenates, hypophysectomy produced a decrease in the proportion of phosphatidyl serine, lysophosphatidyl choline, and phosphatidic acid and diphosphatidyl glycerol and an increased proportion of phosphatidyl ethanolamine. The phospholipid distribution in liver plasma membranes was the same for normal and hypophysectomized rats. Plasma membranes obtained from bovine growth hormone-treated hypophysectomized rats had approximately 50%, more phosphatidyl serine than membranes obtained from untreated hypophysectomized or normal rats.Plasma membranes from hypophysectomized rats had 75% of the 5′-nucleotidase, the same level of (Na⁺ + K⁺)-adenosine triphosphatase, and twice the Mg²⁺-adenosine triphosphatase of membranes from normal rats. Twelve hours after administration of bovine growth hormone to hypophysectomized rats, (Na⁺ + K⁺)-adenosine triphosphatase had almost doubled and Mg²⁺-adenosine triphosphatase decreased by 50%. 5′-Nucleotidase remained unchanged. Twenty-four hours after bovine growth hormone administration, both (Na⁺ + K⁺)-adenosine triphosphatase and 5′-nucleotidase had increased. Mg²⁺-adenosine triphosphatase was 23% of the baseline level of untreated hypophysectomized rats. Treatment for 3 days or 5 days increased the 5′-nucleotidase 2-fold.Circular dichroism spectra of liver plasma membranes isolated from hypophysectomized rats consistently showed greater negative ellipticity in the far ultraviolet range (250-190 nm) than those from normal rats or rats treated with bovine growth hormone.     

Two contiguous thrombin fragments of human somatotropin form a functionally active recombinant, but the two homologous fragments from sheep hormone do not

Two thrombin fragments of reduced-carbamidomethylated human somatotropin representing the full primary structure of the native hormone (residues 1-134 and 135-191) have been found to form a recombinant molecule with properties similar to those of reduced-carbamidomethylated human somatotropin as shown by circular dichroism spectroscopy, two receptor-binding assays, and radioimmunoassay. In contrast, the homologous thrombin fragments of reduced-carbamidomethylated sheep hormone (residues 1-133 and 134-191) do not undergo recombination. Furthermore, neither the reduced-alkylated nor the reduced and nonalkylated C-terminal thrombin fragment of sheep hormone is able to interact with the reduced-carbamidomethylated N-terminal thrombin fragment of human hormone, under conditions which favor the recombination of the two human somatotropin fragments.     

In vitro insulin-like actions of human growth hormone: a study with an insulin antibody

When polyclonal insulin antibodies were preincubated with either adipose tissue from hypophysectomized rats or adipocytes from normal rats, human growth hormone failed to stimulate glucose oxidation. Removal of insulin from adipocytes through incubation with pyruvate at pH 7.0, followed by washing three times, also abolished subsequent in vitro insulin-like action of hGH. Administration of the same insulin antibody to hypophysectomized rats 30 minutes prior to injection of hGH did not inhibit the insulin-like activity of the hGH as measured by its ability to decrease serum glucose and non-esterified fatty acid levels. It is concluded that the in vitro promotion of glucose oxidation by hGH requires insulin. Because of the uncertainty of complete removal of insulin in intact animals, such a conclusion cannot be made regarding in vivo insulin-like action of hGH.     

Endocrine Regulation of Fetal Adipose Tissue Metabolism in the Pig: Interaction of Porcine Growth Hormone and Thyroxine

HAUSMAN, D.B., G.J. HAUSMAN, AND R.J. MARTIN. Endocrine regulation of fetal adipose tissue metabolism in the pig: interaction of porcine growth hormone and thyroxine. Obes Res. 1999;7:76–82. Objective : This study tested the hypothesis that combined treatment of thyroxine (T₄) and growth hormone (GH) could normalize cellular and metabolic aspects of adipose tissue development of hypophysectomized fetal pigs. Research Methods and Procedures : On day 70 of gestation, pig fetuses were hypophysectomized by microcauterization or remained intact. Hypophysectomized fetuses remained untreated or were treated from day 90 to day 105 of gestation with T₄, GH, or a combination of both hormones. Results : Body weights were unaffected by hypophysectomy or hormone treatment. De novo lipogenesis in subcutaneous adipose tissue was increased 10-fold by hypophysectomy, consistent with our previous results. This increase was abolished by GH treatment in the hypophysectomized fetuses. In contrast, T₄ treatment of the hypophysectomized fetuses resulted in a 12-fold further increase in adipose tissue lipogenesis, an effect that was negated by concomitant administration of GH. Lipolytic response to isoproterenol was decreased by hypophysectomy, unaffected by GH treatment, and restored to intact values by T₄ or by T₄+GH treatment in the hypophysectomized fetuses. Discussion : In contrast to T₄, GH does not influence serum insulin-like growth factor-I or adipose tissue lipolysis, but decreases lipogenesis in the fetal pig. However, replacing both T₄ and GH normalized hypophysectomized fetuses to a greater extent than either GH or T₄ alone. Thus, any influence of thyroid hormones on stimulating adipose tissue lipogenesis in the developing fetal pig may be normally counterregulated by pituitary-derived growth hormone.     

Binding of gonadotropin releasing hormone agonist to rat ovarian granulosa cells

We have previously shown that gonadotropin releasing hormone (GnRH) and its agonists inhibit ovarian functions by a direct action on ovarian granulosa cells $\text{in vitro}$. A labeled GnRH agonist, [des-Gly¹⁰, D-Ser (TBu)⁶, Pro⁹-NHEt]GnRH, was used here to examine the possibility that these inhibitory actions of GnRH were mediated through specific receptors which recognize GnRH. Ovarian membrane fractions obtained from immature, hypophysectomized diethylstilbesterol-treated rats were incubated with the ¹²⁵I-GnRH agonist and specific binding was determined by a filtration assay. Stereospecific, high affinity binding was detected in the ovarian membranes; the dissociation constant for the labeled GnRH agonist was determined to be 0.84 ± 0.33 × 10^?10 M and the binding capacity was calculated to be 12.9 fmol/mg protein, or 0.142 fmol/μg DNA. The binding affinity for the GnRH decapeptide was 3.3 times lower than that of the GnRH agonist whereas two GnRH partial peptides did not compete for the ¹²⁵I-agonist binding. After sequential treatment with FSH, LH and prolactin to the hypophysectomized female rats, the ovarian GnRH binding capacity increased per ovary, but decreased per mg ovarian protein.Furthermore, ovarian granulosa cells were isolated and their binding capacity was determined to be 25.2 fmol/mg protein, or 0.133 fmol/μg DNA, suggesting that the granulosa cells contain GnRH binding sites. Thus, this report demonstrates the presence of stereospecific, high affinity GnRH binding sites in the rat ovarian granulosa cells.