Circulating levels of somatomedins in man and rats are reduced by fasting and restored by refeeding. To determine the mechanisms for these alterations in somatomedin levels, the affinities and binding capacities of ¹²⁵I-labelled bovine GH (somatogenic binding sites) and of ¹²⁵I-labelled ovine prolactin (lactogenic binding sites) were assessed in liver homogenates from fasted and refed rats. Correlations were made with plasma immunoreactive somatomedin-C (Sm-C) and plasma insulin.

During fasting and refeeding there was a close temporal relationship between the fall and the rise of plasma levels of Sm-C and insulin, and the number of hepatic GH binding sites. After fasting for 1 day, plasma Sm-C dropped by 68% and plasma insulin by 76% when compared with values before fasting. At the same time, GH binding capacity was significantly reduced (7·3 ± 2·8 (s.e.m.) pmol/liver v. controls, 20·3 ± 2·1 pmol/liver; P < 0·01). Refeeding for 24 h normalized plasma insulin levels and restored GH binding capacity to values before fasting (13·2 ± 2·4 v. 20·3 ± 2·1 pmol/liver; P > 0·05). Plasma Sm-C rose significantly with refeeding and returned to initial values at day 4 of refeeding (0·82 ± 0·10 v. 0·77 ± 0·07 units/ml; P > 0·05).

In contrast, changes in prolactin binding sites correlated poorly with changes in plasma Sm-C. There was a modest decline with fasting significant only after 72 h (6·8 ± 0·6 v. 13·9 ± 3·2 pmol/liver; P < 0·05), and refeeding for 24 h did not restore prolactin binding (5·7 ± 1·2 pmol/liver).

Since a number of reports suggest that fasting induces a state of tissue insensitivity to GH, our findings suggest that the reduction in hepatic GH binding capacity might be a mechanism for the fasting-induced reduction in Sm-C. The reduction in plasma insulin, which accompanies fasting, might play a permissive role in the intracellular metabolic events involved in Sm-C and GH receptor regulation.

ABSTRACT

Administration of GH complexed with monoclonal antibodies (MABs) potentiates the in vivo actions of the hormone. In particular, growth and serum IGF-I concentrations of GH-treated hypophysectomized rats are increased by concomittant injection of anti-GH MABs. Among 37 anti-bovine GH (bGH) MABs, we selected one MAB with the most potentiating effects to investigate the mechanisms responsible for this phenomenon. Hypophysectomized rats were killed 18 h after a single s.c. injection of bGH (100 μg/rat), alone or complexed with increasing doses of MAB (4, 40, 400 μg/rat; MAB:bGH molar ratio: 0·005, 0·05, 0·5). IGF-I was measured by radioimmunoassay in acid–extracted sera and livers, whereas liver IGF-I mRNA was quantified by Northern blot hybridization. The in vivo occupancy of liver somatogenic (GH) receptors was derived from the determinations of total and free ¹²⁵I-labelled bGH binding to liver homogenates treated with 4 mol MgCl₂/l or water. Injection of MAB–bGH complexes enhanced body weight gain and raised serum IGF-I, liver IGF-I and liver IGF-I mRNA more than bGH alone (1·6-, 6-, 10- and 7-fold increases at the highest dose of MAB, compared with bGH alone; P < 0·001). These potentiating effects of the MAB were dose-dependent and significant potentiation of the growth response was already observed with the lowest dose of MAB. In vivo occupancy of liver GH receptors was markedly higher 18 h after injection of MAB–bGH complexes than after bGH alone, and this effect was also dose-dependent (receptor occupancy of 28%, 37% and 83% after 4, 40 and 400 μg of MAB respectively compared with 6% after bGH alone; P < 0·05, 0·05 and 0·001 respectively). In contrast, the in vitro binding of ¹²⁵I-labelled bGH to liver homogenates was decreased in the presence of high doses of MAB.

We conclude that low amounts of MABs complexed with bGH potentiate the stimulation by the hormone of liver IGF-I synthesis and secretion in a dose–dependent manner. These effects are mediated, at least in part, through changes in hormone-receptor interaction in vivo, leading to enhanced and/or prolonged binding of bGH to its somatogenic receptors.

Journal of Endocrinology (1993) 139, 383–393

Abstract

Zinc depletion attenuates growth and decreases circulating IGF-I. To investigate the mechanisms responsible for the IGF-I decline, we determined the effects of dietary zinc (Zn) deficiency on body and organ growth, serum IGF-I, serum GH-binding protein (GHBP), liver GH receptors and liver expression of their corresponding gene. After 1 week of adaptation to a normal zinc diet, a zinc-deficient diet (ZD; Zn, 0 p.p.m.) or a zinc-normal diet (CTR; Zn, 75 p.p.m.) was available ad libitum to 4-week-old Wistar rats for 4 weeks. Pair-fed animals (PF) received the zinc-normal diet in the same absolute amount as that consumed the day before by the ZD group. The food intake of ZD and PF rats was reduced by 32% (P<0·001) compared with the CTR group. Zinc depletion specifically reduced body weight gain (−22%, P<0·05), serum IGF-I concentrations (−52%, P<0·001), hepatic GH receptors (−28%; P<0·05) and serum GHBP levels (−51%; P<0·05), compared with the PF group. GH concentrations were reduced in ZD animals compared with CTR rats (P<0·01). The caloric restriction of PF animals also decreased body weight gain (−50%, P<0·001), serum IGF-I concentrations (−21%, P<0·05), liver GH receptors (−38%, P<0·001) and serum GHBP levels (−38%, P<0·01), when compared with the CTR group. Both ZD and PF groups had reduced liver IGF-I and GH receptor/GHBP mRNA levels in comparison with the CTR group (P<0·01). However, only liver IGF-I mRNA levels were specifically reduced by zinc deficiency (ZD vs PF rats; P<0·05). Our observations suggest that beside the decline of GH secretion, decreased hepatic GH receptors and/or GHBP concentrations might be responsible for the decline of circulating IGF-I in ZD animals.

Journal of Endocrinology (1995) 144, 449–456

ABSTRACT

To define the mechanism(s) for the decrease of somatomedin concentrations in acute protein malnutrition, we have assessed the relationships between serum immunoreactive somatomedin-C/insulin-like growth factor-I (Sm-C/IGF-I), serum immunoreactive GH and total (MgCl₂-treated homogenates) as well as free (water-treated homogenates) liver somatogenic (GH) binding sites in growing rats fed a 5% protein diet for 12 or 24 h and given an s.c. injection(s) of rat GH (rGH) or saline. Control rats were fed a 15% protein diet and injected with rGH or saline.

After 12 and 24 h of protein restriction, body weight was 6·9 and 8·2% below controls respectively (P < 0·001), while Sm-C/IGF-I concentrations were reduced by 58 and 66% respectively (P < 0·001 vs controls). Serum GH concentrations were not affected by the low protein intake. Furthermore, injection(s) of 50–100 μg rGH failed to raise serum Sm-C/IGF-I concentrations in the protein-deficient animals. The number of total and free GH-binding sites was modestly (15–20%) decreased at 12 and 24 h in the protein-restricted rats. Serum Sm-C/IGF-I concentrations correlated weakly with free and total binding sites (r = 0·48 and 0·38 respectively). Affinity constants of GH-binding sites were not changed by protein restriction.

The profound reduction in Sm-C/IGF-I concentrations within a few hours of beginning protein restriction, and the discordance between this reduction and the small decline in somatogenic binding sites, suggests that, in addition to GH receptor loss, a postreceptor defect may participate in the GH resistance occurring in the early stages of protein deficiency.

J. Endocr. (1988) 118, 113–120

Dietary zinc deficiency in rats causes growth retardation associated with decreased circulating IGF-I concentrations. To investigate the potential role of low IGF-I in this condition, we attempted to reverse the growth failure by administration of exogenous IGF-I. Rats were fed for 4 weeks a zinc-deficient diet (ZD, Zn 0 ppm) or were pair-fed a zinc-normal diet (PF, Zn 75 ppm). We compared the anabolic action of recombinant human (rh) IGF-I infused at the dose of 120 microg/day for the last experimental week in ZD, PF and freely fed control (CTRL) rats. Zinc deficiency caused growth stunting (weight gain 47% of PF; P<0.001), decreased circulating IGF-I (52% of PF; P<0.01) and liver IGF-I mRNA (67% of PF; P<0.01). Serum insulin-like growth factor-binding protein-3 (IGFBP-3) assessed by ligand blot was also reduced in ZD rats (65% of PF; P<0. 01). While exogenous IGF-I increased body weight in CTRL (+12 g; P<0. 01) and PF (+7 g; not significant) animals, growth was not stimulated in ZD rats (-1.5 g) in comparison with the corresponding untreated groups. However, circulating IGF-I and IGFBP-3 levels were restored by IGF-I infusion to levels similar to those in untreated CTRL rats. In conclusion, restoration of normal circulating levels of IGF-I and IGFBP-3 by rhIGF-I infusion fails to reverse the growth retardation induced by zinc deficiency. These results suggest that growth retardation related to zinc deficiency is not only caused by low serum IGF-I concentrations, but also by inhibition of the anabolic actions of IGF-I.

ABSTRACT

The serum concentration of insulin-like growth factor-I (IGF-I) is reduced in growing rats fed a low-protein diet, and this decrease is age-dependent, being more pronounced in younger animals. To determine whether this decrease in serum IGF-I is related to a decrease in IGF-I mRNA, growing female rats were given free access to either a 15% protein-sufficient or a 5% protein-deficient diet for 1 week. Protein restriction in 4-week-old rats decreased body weight gain by 44% (P < 0·001) compared with 4-week controls), serum IGF-I concentration by 67% (P < 0·001) and liver IGF-I mRNA abundance by 51% (P < 0·001). During week 6, protein restriction for 1 week resulted in a 20% increase in food intake with no change in weight gain, a 38% reduction in serum IGF-I (P < 0·001 compared with 6-week controls) and a 39% decrease in liver IGF-I mRNA (P < 0·001). The serum IGF-I concentration was highly correlated (r = 0·80; P < 0·001) with the hepatic IGF-I mRNA concentration. Skeletal muscle IGF-I mRNA abundance was also decreased significantly by protein restriction (37% at week 4, P<0·001, and 24% at week 6, P < 0·01) and was closely correlated (r = 0·71; P < 0·001) with body weight gain. Liver GH-binding protein and GH receptor mRNA abundance were reduced by 1 week of protein deprivation at week 6 but not at week 4. We conclude that the reduced serum IGF-I of young rats fed a low-protein diet is due, in part, to reduced liver IGF-I mRNA, and that these changes are not dependent on GH binding. Decreased skeletal muscle IGF-I mRNA during protein restriction is consistent with an autocrine/paracrine action of IGF-I in muscle.

Journal of Endocrinology (1991) 130, 305–312

Abstract

Endothelin-1 (ET-1) is a major vasoconstrictor peptide, first found in endothelial cells, and later in many other tissues, including the thyroid gland. We analysed the expression of the ET-1 gene in the rat thyroid gland and changes in ET-1 mRNA and peptide levels in goiter development and involution, two circumstances characterised by vascular changes.

Thyroid hyperplasia was induced in adult Wistar rats by feeding a low iodine diet (LID) supplemented with 0·25% thiouracil for 10 days, and LID alone for 2 further days (H.12 group). Involution was induced by injecting 100 μg iodide and refeeding a normal diet during 6 h, 12 h, and 24 h (I.6h, I.12h, 1.24 h groups). Rats fed a normal iodine diet were used as controls.

A specific 488 bp cDNA corresponding to the known sequence of pre-pro ET-1 was found by RT-PCR from RNA extracts in all thyroid experimental groups, as well as in lung and kidney which were used as positive controls. RP-HPLC analysis showed that ET-1 immunoreactivity eluted similarly as mature ET-1

During hyperplasia, ET-1 mRNA and peptide levels were increased 3·5- and 5-fold respectively. The relative volume of the vascular bed was more than doubled. During iodide-induced involution, the glandular ET-1 mRNA level remained elevated. The concentration of ET-1 peptide increased and was significantly greater at 12 h involution than in the H.12 group. At this time, the capillary reticulum reverted to individual capillaries and the vascular bed was significantly reduced.

These data demonstrate that the ET-1 gene is expressed in the rat thyroid gland and that the ET-1 mRNA and peptide levels are increased during thyroid hyperplasia and remain elevated during a phase of rapid iodide-induced involution. These data suggest that changes in ET-1 production may play a role in control of thyroid gland trophic regulation and vascularity.

Journal of Endocrinology (1994) 143, 65–74