The Interrelationship of Growth Hormone (GH), Liver

0013-7227/90/1264-1914$02.00/0 Endocrinology Copyright© 1990 by The Endocrine Society

Vol. 126, No. 4 Printed in U.S.A.

The Interrelationship of Growth Hormone (GH), Liver Membrane GH Receptor, Serum GH-Binding Protein Activity, and Insulin-Like Growth Factor I in the Male Rat TOVA BICK, TAMAR AMIT, RONNIE J. BARKEY, PNINA HERTZ, MOUSSA B. H. YOUDIM, AND ZEEV HOCHBERG Rapapaport Family Institute for Research in the Medical Sciences and Department of Pharmacology, Faculty of Medicine, Technion, Haifa, Israel

ABSTRACT. Indirect evidence suggests that the serum GHbinding protein (GH-BP) is related and possibly derived from the GH-receptor. GH, through its specific receptor, is the major regulator of insulin-like growth factor I (IGF-I) synthesis. The present study was undertaken to correlate serum GH-BP activity with liver plasma membrane GH receptors and their effects on serum IGF-I concentration during spontaneous pulsation of rat (r)GH in the normal male rat and after continuous delivery of human (h)GH to hypophysectomized male rats. In the first set of experiments, 45-day-old male rats were decapitated at 15 min intervals for 4 h. Serum GH-BP levels fluctuated with a 60 min lag behind the rGH levels. IGF-I pulsated over a 3-fold concentration range. IGF-I peak levels coincided with one of the rGH peaks, but its periodicity was longer than 3 h. Taken together with our previous studies on the turnover of the GH receptors, we suggest that each GH surge results in individual pulse-related turnover wave of receptor internalization and recycling. This is accompanied by a parallel increase in serum GH-BP activity. The GH and the receptor wave are responsible for an individual secretion pulse of IFG-I. In the second set of experiments male

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SPECIFIC GH binding protein (GH-BP) has been demonstrated in the sera of several mammal species, including rabbit (1), mouse (2, 3), sheep (1), pig, dog (4), and rat (4a). Indirect evidence suggested that the serum GH-BP is related and possibly derived from the GH receptor. The rabbit GH-BP shares immunological (5) and structural (6, 7) determinants with the liver membrane GH receptor, but little is presently known about its physiological role. In the human (h) and rabbit, GH-BP and the membrane-bound GH receptor have similar species specificity for GH (1, 8, 9). We have recently found that the same is true for rat (r) GH-BP and hepatic receptors (4a). Moreover, patients with GHresistant Laron-type dwarfism have been shown to have no GH-BP activity (10, 11), and the African pigmies are

rats were hypophysectomized at 35 days of age. Four days later osmotic minipumps were implanted for continuous delivery of hGH. After 6 days of hGH treatment the rats were killed, blood was collected for hGH, GH-BP, and IGF-I determination, and the livers were removed. Plasma membranes were prepared, and lactogenic and somatogeriic binding of [125I]hGH was evaluated. Removal of endogenous ligand was performed by exposing the membranes to 3 M MgCl2. Continuous administration of hGH induced a dose-dependent increase in liver membrane lactogenic and somatogenic binding. Parallel to that increase, serum GHBP also increased in a dose-dependent manner, and the correlation between serum GH-BP and the liver membrane receptor was significant. Furthermore, hGH induced a dose-dependent increase in IGF-I concentration. There was a close correlation between IGF-I concentration and liver somatogenic receptors. It is concluded that up-regulation of the liver membrane GH receptors is accompanied by increased GH-BP and IGF-I. In both the pulsation experiment and the continuous infusion experiment, GH-BP closely correlated with the liver membrane GH receptor. (Endocrinology 126: 1914-1920,1990)

deficient of hepatic membrane GH receptor as well as the serum GH-BP (12). GH, through its specific receptors, is the major regulator of insulin-like growth factor I (IGF-I)/somatomedin C synthesis. A close correlation was observed between hepatic GH somatogenic binding and serum IGF-I levels in hypophysectomized rats, both being restored by GH treatment (13). The present study was undertaken to correlate serum GH-BP with liver plasma membrane GH receptors and their effects on serum IGF-I concentration during spontaneous pulsation of rGH in the normal male rat and after continuous delivery of hGH to hypophysectomized male rates.

Materials and Methods Materials

Received September 5,1989. Address requests for reprints to: Dr. Zeev Hochberg, Rappaport Family Institute, Technion, P.O.B. 9697, Haifa 31096, Israel.

Recombinant authentic hGH used for injections, for the binding studies and for iodination (2.5 IU/mg) was a gift from 1914

The Endocrine Society. Downloaded from press.endocrine.org by [${individualUser.displayName}] on 27 April 2016. at 02:11 For personal use only. No other uses without permission. . All rights reserved.

GH RECEPTOR AND BINDING PROTEIN Biotechnology General Ltd. (Rehovot, Israel). rGH B6, monkey anti-rGH for serum rGH determination by RIA, and ovine PRL (oPRL NIADDK-18), used as unlabeled hormone in the binding studies, were obtained from the NIDDK Pituitary Hormone Distribution Program (Baltimore, MD). Hormones were dissolved in 0.01 M NaHCO3 to a concentration of 1 mg/ml. L-T4 and dexamethasone were purchased from Glaxo (City, UK) and Teva (City, Israel), respectively. Recombinant IGF-I was obtained from Kabi (Stockholm, Sweden). Rabbit anti IGF-I antibody for serum IGF-I determination by RIA was a gift from Dr. P. Gluckman (Auckland, New Zealand). Na[125I] for iodination was purchased from the Nuclear Research Center-Negev (Beer-Sheva, Israel). Sephadex G-50 and G-100 and dextran T-70 were purchased from Pharmacia (Uppsala, Sweden), BSA (fraction V and RIA grade), polyethylene glycol (mol wt ~6000) and Norit-A charcoal were purchased from Sigma Chemical Co. (St. Louis, MO). All other chemicals were of analytical grade and obtained from local commercial sources. Animals Sprague-Dawley male rats (locally bred) were housed undisturbed under controlled conditions of lighting (12-h light, 12-h dark cycle) and temperature (22 C ± 2 C). They received standard rat chow pellets and tap water until killed. Experimental design In the first set of experiments an attempt was made to synchronize the rats' GH secretion by complete uninterrupted environment with constant cycles of 12-h light, 12-h dark (14). On killing day, after lighting at 0700 h 45-day-old rats were decapitated at 15 min intervals, from 0945 h through 1345 h. This was repeated for 3 days, bringing to 6-8 the number of rats killed at each time point (15). Cervical blood was collected, and the livers were removed and rapidly frozen in liquid N2. After determination of serum rGH levels by RIA (15), unsynchronized rats were excluded. Three rats were selected for each time point by the best fit to the pulsatility of serum rGH. Only two such rats were identified at two time points. Serum GHBP activity, serum IGF-I concentration, and liver membrane GH receptors were determined. In the second set of experiments hypophysectomy was performed through the ear canal under ether anesthesia in 35-dayold male rats (16). Successful procedure was ensured by growth arrest over the next week and by postmortem examination of the sella turcica. The rats received replacement therapy with L-T4 1 Mg/100 g BWday and with dexamethasone 100 Mg/100 g BW • day until they were killed on the 45th day of life. Six days before killing, during ether anesthesia, an osmotic minipump (model 1001, Alza, Palo-Alto, CA) was implanted under the skin of the back. The pumps delivered a continuous flow of hGH (1 jul/h). On the day of death, blood was collected for hGH, GH-BP, and IGF-I determination, and the livers were removed and rapidly frozen in liquid N2 for measurement of somatogenic and lactogenic hGH binding. Antibodies against hGH were determined in all sera by the method of Demura et al. (17). Six rats that developed antibodies against hGH were excluded from the study.

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RIAs Serum levels of rGH were determined by a double antibody RIA (15). The sensitivity of the assay was 0.1 ng. Inter- and intraassay variability were less than 10% and 5%, respectively. Serum hGH was measured by a commercial RIA kit (Cis, Gifsur-Yvette, France). IGF-I was measured by RIA on acid ethanol-extracted serum using a nonequilibrium technique (18). The standard was acid ethanol extract of pooled normal rats' sera, used over the range of 100 fil. This extract was assigned a value of 1 U IGF-I/ml. hGH binding to plasma membranes Somatogenic and lactogenic binding before and after dissociation of the endogenous ligand by 3 M MgCl2 were determined on liver pure plasma membranes (200 ng/tube) using as ligand [125I]hGH (1 ng/tube) as described and validated in a previous study (15). Total specific binding and somatogenic specific binding of [125I]hGH were determined in the absence or presence of excess oPRL (1 /ug/tube), respectively. Lactogenic binding was calculated from the difference between total and somatogenic binding. GH-BP measurement Detailed descriptions of the binding assay and the separation procedure were previously reported (4a). In short, recombinant hGH was iodinated by the chloramine T method (19) and separated on a Sephadex G-50 column. Binding studies were done in a mixture of 0.1 ml serum, 1 ng [125I]hGH with or without 0.1 yug unlabeled hGH in 0.01 M phosphate buffer. Twenty-four hours incubation was performed at 4 C and terminated by adding 1 ml cold dextran-coated charcoal (0.2% dextran T-70-2% charcoal) for 15 min. Specific binding was corrected for the respective endogenous hGH or rGH levels (10).

Results Serum GH-BP pulsatility Serum concentration of immunreactive rGH was measured in cervical blood samples of normal 45-day-old male rats, at 15 min intervals from 0945 h-1345 h. The results are shown in the upper panel of Fig. 1. During the 4 h of the experiment, two peaks of rGH are evident. The first peak occurred at 0945 h and the second one at about 1230 h. As shown in the middle panel of Fig. 1, serum GH-BP activity fluctuated with a 60 min lag behind the rGH level. At the time of the GH peak serum GH-BP activity was minimal. An hour later, when serum rGH was almost undetectable GH-BP activity reached its peak. About 15 min later GH-BP activity decreased gradually reaching its nadir at the time of the next GH peak. Serum IGF-I pulsatility Spontaneous changes in serum IGF-I concentration from the same experiment is shown in the lower panel


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hGH ( n g / m l ) FIG. 2. Induction of serum GH-BP activity after 6 days sc infusion of hGH to hypophysectomized male rats. The specifically bound labeled hormone (B) is expressed as a percentage of the total labeled hormone (T) incubated with 0.1 ml serum

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CLOCK TIMtt FIG. 1. Spontaneous changes in serum rGH levels (upper panel), serum GH-BP activity (middlepanel), and serum IGF-I concentration (lower panel) in normal, 45-day-old male rats that were decapitated at 15 min intervals over 4 h. The specifically bound hormone (B) per 0.1 ml serum (middle panel) is expressed as a percentage of the total labeled hormone (T) incubated. Each circle represents an individual rat.

of Fig. 1. IGF-I reached its peak at 1230 h-1315 h during the rGH peak. Peak values were about 3 times higher than nadir values. Induction of GH-BP by hGH Hypophysectomized rats were given continuous sc infusion of hGH through an osmotic minipump for 6 days. GH-BP activity correlated closely with serum hGH concentration (Fig. 2) (r = 0.85; n = 18; P < 0.001). Serum GH-BP activity increased by about 6-fold as a function of serum hGH.

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FIG. 3. Relationship between serum hGH concentration and liver free (upper panel) and total (lower panel) somatogenic binding sites in hGH-treated 45-day-old hypophysectomized male rats.

Induction of hepatic receptors by hGH Continuous infusion of hGH increased both free and occupied somatogenic and lactogenic hepatic receptors in a dose-dependent manner (Figs. 3 and 4). The upper


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panels of Figs. 3 and 4 show significant positive correlations between serum hGH concentrations and free somatogenic (r = 0.565; n = 14; P < 0.05) or lactogenic (r = 0.758; n = 14; P < 0.01) binding sites. The lower panels of the same figures show the correlation between hGH concentration and somatogenic or lactogenic binding after exposing the membrane preparations to 3 M MgCl2 (r = 0.489; n = 22; P < 0.05 and r = 0.873; n = 19; P < 0.001 for somatogenic and lactogenic binding, respectively). Serum GH-BP and hepatic receptors The GH-BP data of Fig. 2 and the hepatic receptor data of Figs. 3 and 4 have been correlated. Dependence between serum GH-BP activity and somatogenic or lactogenic binding sites before and after 3 M MgCl2 dissociation of the endogenous ligand are shown in Figs. 5 and 6. Significant linear positive correlations were found between GH-BP activity and somatogenic (free + total) and lactogenic (free + total) binding (r = 0.643, n = 15, P < 0.001; r = 0.499, n = 16, P < 0.05; r = 0.806, n = 15, P < 0.001; and r = 0.747, n = 14, P < 0.01 for free somatogenic, total somatogenic, free lactogenic and total lactogenic binding, respectively).

0 5 10 15 hGH BINDING PROTEIN ( % specific binding /O.I ml serum ) FlG. 5. Relationship between serum GH-BP activity and liver free (upper panel) and total (lower panel) somatogenic binding sites in hGH-treated, 45-day-old hypophysectomized male rats. Linear regression was fitted by a computerized best fit analysis. P < 0.01 and P < 0.05 for free and total somatogenic binding sites, respectively.

Serum IGF-I level and hGH In the hypophysectomized rats that were treated with continuous sc infusion of hGH for 6 days, serum IGF-I concentration increased by about 3-fold as a function of serum hGH increment (Fig. 7). The correlation between serum IGF-I and serum hGH concentrations was significant (r = 0.609; n = 50; P < 0.001). Serum IGF-I and hepatic receptors In the hypophysectomized rats, continuous hGH administration induced changes in liver somatogenic (free + total) and lactogenic (free) binding, which correlated positively and significantly with serum IGF-I concentrations (r = 0.675; n = 12; P < 0.05; r = 0.632; n = 12; P < 0.05; r = 0.612; n = 13; P < 0.05 for free somatogenic, free lactogenic and total somatogenic binding), respectively (Figs. 8 and 9). However, there was no correlation between total lactogenic binding and serum IGF-I concentration (Fig. 9, lower panel).

Discussion Results of the present study demonstrate close interrelationship between serum GH levels, plasma mem-



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5 10 15 0 hGH BINDING PROTEIN (% specific binding/O.I ml serum) FIG. 6. Relationship between serum GH-BP activity and liver free (upper panel) and total (lower panel) lactogenic binding sites in hGHtreated, 45-day-old hypophysectomized male rats. Linear regression was fitted by a computerized best fit analysis. P < 0.001 and P < 0.01 for free and total lactogenic binding sites, respectively.

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hGH ( n g / m l ) FIG. 7. Induction of serum IGF-I concentration by hGH after 6 days of hGH infusion to hypophysectomized male rats.

brane GH receptors, serum GH-BP activity, and serum IGF-I concentration in normal and hypophysectomized male rats on chronic hGH infusion. In previous reports we have shown the dynamics of the GH receptor turnover and its response to the spontaneous pulsatility of serum GH levels (15, 20). It was shown that a GH serum peak is followed by an immediate down-regulation of the plasma membrane receptor, which undergoes a process

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20 15 10 I]-hGH BOUND (B/T* 100/mg protein) FIG. 8. Correlation between serum IGF-I concentration and liver free (upper panel) and total (lower panel) somatogenic binding sites in hGH-treated, 45-day-old hypophysectomized male rats. Linear regression was fitted by a computerized best fit analysis. P < 0.05 for free and total somatogenic binding sites. r 125

of internalization. In the normal rat with its 3 h serum GH pulse intervals the [125I]GH bound receptor was found in the Golgi membranes 1 h after the GH pulse. We now show that the serum levels of GH-BP pulsate with the same frequency, with 1 h lag time behind the serum GH peak. The GH-BP peak coincides with the process of receptor internalization. In a previous study we demonstrated that the rat GH-BP is a selective binder of somatogenic hormones (4a). In both human and rabbits GH-BP has identical N-terminal amino-acid sequence with the GH receptor (6, 7). GH-BP has been demonstrated to undergo endopeptidase cleavage from somatogenic receptors of IM-9 lymphocytes (21). Whereas a similar relationship has now been shown to occur in the rat, the present report reveals several indirect indications to a mechanistic link between the GH receptor and the GH-BP. Each pulse of rGH causes a down-regulation of the GH plasma membrane receptor that was followed by up-regulation reaching its maximal level in time for the next pulse. In the process of downregulation, part of the receptor was internalized and located in intracellular organeles before targeted for lysosomal degradation or recycling to the plasma mem-



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50 100 150 [ 1 2 5 I]-hGH BOUND (B/T* 100/mg protein) FIG. 9. Correlation between serum IGF-I concentration and liver free {upper panel) and total (lower panel) lactogenic binding sites in hGHtreated, 45-day-old hypophysectomized male rats. Linear regression was fitted by a computerized best fit analysis. P < 0.05 for free lactogenic binding sites.

brane (22). During this internalization process GH-BP appeared in the circulation. Whether membrane endopeptidases were involved in shedding the GH-BP remains to be investigated. IGF-I, the mediator of GH action, fluctuated over a 3fold concentration range. In our experiment, IGF-I peak levels coincided with the second rGH peak. IGF-I appears to oscillate for longer than a 3-h time period. This asymmetry may result from a long lag time between the GH pulse and the resultant rise in IGF-I. Daughaday and Reeder (23) reported that it takes up to 24 h after GH administration before IGF-I can be measured in hypophysectomized rats. Several other studies suggested a more repid time course for GH action on IGF-I secretion; in perfusion studies of rat liver, bovine GH induced a significant increase in the perfusate's IGF-I concentration within 1 h in both normal and hypophysectomized rats (24). Baxter et al. (25) demonstrated that in chronically cannulated male rats there was a 1 h lag between GH pulses and the following rise in IGF-I, and recently Maiter et al. (26) have demonstrated an increase in serum IGF-I within 6 h after a single sc injection of rGH to hypophysectomized female rats. We interpret the present

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results to indicate that individual GH peaks induce individual IGF-I secretion episodes. However, the lag time between the rGH rise and the IGF-I response cannot be deduced from our results. Continuous administration of hGH to hypophysectomized rats induced a dose-dependent increase in liver membrane lactogenic and somatogenic binding. This is in accordance with several reports that described the induction by hGH of both lactogenic and somatogenic receptors in livers of rats (27-29), rabbits, lambs (30), and swine (31) after long term GH delivery. Parallel to that increase, serum GH-BP also increased in a dosedependent manner and the correlation between serum GH-BP and the liver membrane hGH-receptors was significant, suggesting indirectly that the GH-BP may originate from the GH-hepatic receptor. Recently, Smith and Talamantes (3) have also found a strong temporal relationship between the up-regulation of the GH receptor in the liver and the large increase in the concentration of the serum GH-BP during the mouse pregnancy. In the interrelationship of GH, GH-receptor, and GH-BP the latter awaits to be assigned a biological task. In the meantime it can be used as a simple accessible tool for assessment of liver GH receptors. After the hGH infusion to the hypophysectomized rats, serum IGF-I concentration increased in a dose-dependent manner. GH's ability to stimulate serum IGF-I concentration has been well described. Baxter and Zaltsman (13) demonstrated that GH infusion to hypophysectomized rats restored IGF-I levels (13). Breier et al. (32) found that in young steers serum IGF-I concentrations increase in response to exogenous GH (32). We now show a close correlation between IGF-I concentration and liver somatogenic receptors. Similar phenomena were observed by several studies. Baxter and Zaltsman (13) reported restoration of both GH receptors and IFGI by GH infusion (13). Bryson and Baxter (33) showed that partial restoration of GH receptors after adrenalectomy of diabetic rats that was closely paralleled by an increase in IGF-I. Chung and Etherton (31) showed that after injection of exogenous porcine GH to the pig there was a close relationship between the induction of somatogenic receptors to GH and IGF-I concentration. These data suggest that the GH-receptors are closely involved in the regulation of IGF-I secretion. They also provide indirect evidence that induction of GH receptors is required for IGF-I secretion in the hypophysectomized rat. Taken together with our previous studies on the turnover of the GH-receptors, it is suggested that each GH surge, regulated from the hypothalamus, results in individual, pulse-related turnover wave of receptor internalization and recycling. This is accompanied by a parallel increase in serum GH-BP which seems to be shed from plasma membrane receptors. The GH and the receptor



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wave are responsible for an individual secretion pulse of IGF-I. The data presented here also support the contention that the GH-BP activity may reflect both the membrane GH receptor binding and its effector activity.

17.

References

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1. Ymer SI, Herington AC 1985 Evidence for the specific binding of growth hormone to a receptor-like protein in rabbit serum. Mol Cell Endocrinol 41:153 2. Peeters S, Friesen HG 1977 A growth hormone binding factor in the serum of pregnant mice. Endocrinology 101:1164 3. Smith WC, Talamantes F 1988 Gestational profile and affinity cross-linking of the mouse serum growth hormone-binding protein. Endocrinology 123:1489 4. Daughaday WH, Trivedi B, Lauterio T, Serum 125I-hGH binding protein in non-GH deficient canine and porcine dwarfs. Program of the 69th Annual Meeting of The Endocrine Society Meeting, Indianapolis, IN, 1987, p 97 (Abstract) 4a. Amit T, Barkey RJ, Bick T, Hertz P, Youdim MBH, Hochberg Z, Identification of growth hormone binding protein in rat serum. Mol Cell Endocrinol, in press 5. Baumann G, Shaw MA 1988 Immunochemical similarity of the human plasma growth hormone-binding protein and the rabbit liver growth hormone receptor. Biochem Biophys Res Commun 152:573 6. Spencer SA, Hammonds RG, Henzel WJ, Rodriguez H, Waters MI, Wood WI 1988 Rabbit liver growth hormone receptor and serum binding protein. J Biol Chem 263:7862 7. Leung DW, Spencer SA, Cashianes G, Hammonds RG, Collins C, Henzel WJ, Barnard R, Waters M, Wood W 1987 Growth hormone receptor and serum binding protein: purification, cloning and expression. Nature 330:537 8. Baumann G, Stolar MW, Amburn K, Barsano CP, De Vries BC 1986 A specific growth hormone-binding protein in human plasma: initial characterization. J Clin Endocrinol Metab 62:134 9. Herington AC, Ymer S, Stevenson J 1986 Identification and characterization of specific binding proteins for growth hormone in normal human sera. J Clin Invest 77:1817 10. Daughaday WH, Trivedi B 1987 Absence of serum growth hormone binding protein in patients with growth hormone receptor deficiency (Laron Dwarfism). Proc Natl Acad Sci USA 84:4636 11. Baumann G, Shaw MA, Winters RJ 1987 Absence of the plasma growth-hormone-binding protein in Laron type drwarfism. J Clin Endocrinol Metab 65:814 12. Baumann G, Melisaa MD, Shaw BS, Thomas J, Merimee MD 1989 Low levels of high affinity growth hormone-binding protein in African pygmies. N Engl J Med 320:1705 13. Baxter RC, Zaltsman Z 1984 Induction of hepatic receptors for growth hormone (GH) and prolactin by GH infusion is sex independent. Endocrinology 115:2009 14. Tannenbaum GS, Martin JB 1976 Evidence for an endogenous ultradian rhythm governing growth hormone secretion in the rat. Endocrinology 98:562 15. Bick T, Youdim MBH, Hochberg Z 1989 Adaptation of liver membrane somatogenic and lactogenic growth hormone (GH) binding to the spontaneous pulsation of GH secretion in the male rat. Endocrinology 125:1711 16. Zarrow MX, Yochim JM, McCarthy JL 1964 Experimental Endo-

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crinology: A Source Book of Basic Techniques. Academic Press A.P., New York, p 310 Demura R, Ishiwatari N, Namura T, Demura H, Shizume K, Okuno A, Sarayama K, Fujieda K 1989 Measurement of plasma free and total growth hormone concentrations in patients with growth hormone antibodies developed in response to therapy. Acta Endocrinol (Copenh) 120:161 Daughaday WH, Mariz IK, Blethen SL 1980 Inhibition of access of bound somatomedin to membrane receptor and immunobinding site: a comparison of radioreceptor and radioimmunoassay of somatomedin in native and acid-ethanol extracted serum. J Clin Endocrinol Metab 51:781 Greenwood FC, Hunter WM, Glover JS 1963 The preparation of 131 I-labeled human growth hormone of high specific activity. Biochem J 89:114 Bick T, Youdim MBH, Hochberg Z 1989 The dynamics of somatogenic and lactogenic growth hormone binding: internalization to Golgi fractions in the male rat. Endocrinology 125:1718 Trivedi B, Daughaday WH 1988 Release of growth hormone binding protein from IM-9 lymphocytes by endopeptidase is dependent on sulfydryl group inactivation. Endocrinology 123:2201 Roupas P, Herington AC 1989 Cellular mechanisms in the processing of growth hormone and its receptor. Mol Cell Endocrinol 61:1 Daughaday WH, Reeder C 1966 Synchronous activation of DNA synthesis in hypophysectomized rat cartilage by growth hormone. J Lab Clin Med 68:357 Phillips LS, Herington AC, Karl IE, Daughaday WH 1976 Comparison of somatomedin activity in perfusates of normal and hypophysectomized rat livers with and without added growth hormone. Endocrinology 98:606 Baxter RC, Zaltsman Z, Oliver JR, Willoughby JO 1983 Pulsatility of immunoreactive somatomedin-C in chronically cannulated rats. Endocrinology 113:729 Maiter D, Underwood LE, Maes M, Ketelslegers JM 1988 Acute down-regulation of the somatogenic receptors in rat liver by a single injection of growth hormone. Endocrinology 122:1291 Maiter D, Wunderwood LE, Maes M, Davenport ML 1988 Different effects of intermittent and continuous growth hormone (GH) administration on serum somatomedin-C/insulin-like growth factor I and liver GH receptors in hypophysectomized rats. Endocrinology 123:1053 Baxter RC, Zaltsman Z, Turtle JR 1982 Induction of somatogenic receptors in liver of hypersomatotrophic rats. Endocrinology 111:1020 Baxter RC, Zaltsman Z, Turtle JR 1984 Rat growth hormone (GH) but not prolactin (PRL) induces both GH and PRL receptors in female rat liver. Endocrinology 114:1893 Posner BI, Patel B, Vezinhet A, Charrier S 1980 Pituitary-dependent growth hormone receptors in rabbit and sheep liver. Endocrinology 107:1954 Chung SC, Etherton TD 1986 Characterization of porcine growth hormone (pGH) binding to porcine liver microsomes: chronic administration of pGH induces pGH binding. Endocrinology 119:780 Brier BH, Gluckman PD, Bass JJ 1988 The somatotrophic axis in young steers: influence of nutritional status and oestradiol-17 on hepatic high and low affinity somatotrophic binding sites. J Endocrinol 116:169 Bryson JM, Baxter RC 1986 Adrenal involvement in the diabetic induced loss of growth hormone and prolactin receptors in the livers of female rats. Diabetologia 29:106
