Insulin-like Growth factor-I In Rats Research Articles

Pituitary and hypothalamic insulin-like growth factor-I (IGF-I) and IGF-I receptor expression in food-deprived rats

The present study was designed to evaluate a possible role for the insulin-like growth factor-I (IGF-I) system in mediating the suppression of growth hormone (GH) secretion observed in food-deprived rats by measuring IGF-I mRNA, receptor concentration and receptor mRNA in neuroendocrine tissues (hypothalamus and pituitary). Rats were deprived of food (food-deprived) for 72 h or had free access to food (fed). Tissues were processed for measurement of steady-state levels of: (a) IGF-I and IGF-I receptor mRNA (by solution hybridization/RNase protection assay); (b) IGF-I in serum and tissue extracts (by RIA) and (c) IGF-I displacable [ 125I]IGF-I binding to plasma membrane preparations. Food deprivation resulted in decreased serum and liver levels of IGF-I. Kidney IGF-I mRNA levels were reduced 80% in food-deprived rats with a concomitant increase in IGF-I receptor concentration and mRNA levels. Refeeding of food-deprived rats fully normalized these perturbations. Pituitary IGF-I content was reduced 50% in food-deprived rats while IGF-I mRNA levels were unaffected. A modest increase was seen in pituitary IGF-I receptor concentration; however, IGF-I receptor mRNA levels were not changed. Hypothalamic IGF-I mRNA content was reduced in 72 h food-deprived rats while IGF-I receptor binding capacity and mRNA were unaffected. In conclusion, IGF-I mRNA levels are decreased in liver, kidney and hypothalamus together with a reduction in plasma IGF-I in food-deprived rats but is unaffected in anterior pituitary. IGF-I receptor gene expression and binding capacity are coordinately regulated in kidney and hypothalamus, but not in the pituitary. Thus, nutrient signals appear to differentially influence the pituitary compared to peripheral tissues, suggesting that IGF-I may play a unique role in pituitary GH regulation.

Effects of FSH and testosterone on intratesticular insulin-like growth factor-I and specific germ cell populations in rats treated with gonadotrophin-releasing hormone antagonist

Evidence derived predominantly from a series of in-vitro studies indicates that insulin-like growth factor-I (IGF-I) plays an important role as a paracrine and autocrine regulator within the testis. We investigated the effects of FSH substitution on spermatogenesis and testicular IGF-I content in rats treated with gonadotrophin-releasing hormone (GnRH) antagonist (ANT) and injected with ethane dimethane sulphonate (EDS), a Leydig cell toxin. FSH treatment partially prevented the marked decrease in intratesticular IGF-I in rats treated with only GnRH antagonist but not in GnRH antagonist+EDS-treated animals (controls, 191.0 +/- 4.5; ANT, 80.1 +/- 5.6; ANT+EDS, 81.6 +/- 3.4; ANT+EDS+FSH, 86.3 +/- 1.4; ANT+FSH, 137.7 + 7.3 (S.E.M.) ng/testis). Correlation analysis of testicular IGF-I content with the number of pachytene spermatocytes and round spermatids per cross-section revealed r values of 0.77 and 0.74 respectively (P < 0.001). We then analysed the same parameters in GnRH antagonist-treated rats which, in addition, received daily injections of the anti-androgen flutamide, in order to investigate the potential role of testosterone, as opposed to other Leydig cell products, in the regulation of spermatogenesis and testicular IGF-I. FSH treatment prevented regression of spermatogenesis in rats treated with GnRH antagonist alone but not in GnRH antagonist- and flutamide-treated rats.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of human insulin-like growth factor-binding protein-1 on the half-life and action of administered insulin-like growth factor-I in rats

We have determined the effect of human insulin-like growth factor-binding protein-1 (hIGFBP-1) on the circulating half-life (t1/2) of human insulin-like growth factor-I (hIGF-I) and on hIGF-I-stimulated 2-deoxyglucose uptake in tissues of the cannulated conscious rat. The levels of hIGF-I in rat serum were measured by radioimmunoassay. The assay was carried out in the presence of partially purified rat IGF to remove interference from any IGFBPs not removed by acid-ethanol extraction. An intravenous bolus of 12.5 micrograms hIGF-I, given to 12-h fasted rats, disappeared from the circulation in a double exponential fashion with an initial t1/2 of 1.2 +/- 0.2 min (n = 8), which increased to 5.3 +/- 0.9 min when 100 micrograms hIGFBP-1 was co-administered (n = 5; P < 0.001). The second phase of disappearance of hIGF-I indicated an apparent t1/2 of 35.7 +/- 5.6 min which was not significantly altered by the co-infusion of hIGFBP-1. Circulating hIGFBP-1, measured with a primate-specific radioimmunoassay, disappeared in a single exponential fashion with a t1/2 of 8.8 +/- 0.7 min. A tracer amount of 2-deoxy-[1-3H]glucose was administered intravenously at the same time as the peptide(s) and the rate of tissue uptake and phosphorylation of 2-deoxy-[1-3H]glucose determined. Compared with a control group (n = 4), hIGF-I significantly stimulated hexose uptake into heart, soleus and red quadriceps muscles and hIGFBP-1 partially reversed this effect.(ABSTRACT TRUNCATED AT 250 WORDS)

Increased serum clearance and degradation of 125I-labeled IGF-I in protein-restricted rats.

Dietary protein restriction in young rats stunts growth and decreases serum insulin-like growth factor I (IGF-I) concentrations. To investigate the possibility of a diet-induced accelerated clearance of IGF-I, the clearance of 125I-labeled IGF-I was determined by measuring the radioactivity in serum obtained at different intervals after bolus injection in 5-wk-old rats that had received either a normal 15% protein diet (P15; n = 6) or a 5% protein isocaloric diet (P5; n = 7) for 1 wk. The clearance and volume of distribution of 125I-IGF-I were increased in P5 rats by 58 and 75%, respectively, whereas the terminal elimination half-life was not changed. The calculated IGF-I production rate was decreased by 40% in P5 rats. The intermediate distribution phase (alpha) was twofold faster in P5 than in P15 animals, and 125I-IGF-I was preferentially bound to IGF binding proteins in the 40-kDa complex in P5 rats. These observations might explain the increased clearance and degradation of IGF-I in the P5 rats. Dietary protein restriction appears to lower serum IGF-I concentrations by both decreasing its production rate and increasing its clearance.

Nutrition and Somatomedin: XXVII. Total and Free IGF-I and IGF Binding Proteins in Rats With Streptozocin-Induced Diabetes

Impaired growth in diabetic humans occurs despite increased growth hormone and normal insulinlike growth factor I (IGF-I). Because IGF-I circulates complexed to binding proteins (BPs), we asked whether diabetes-related changes in IGF BPs could be associated with alterations in free unbound IGF-I--presumably the active form. Rats were given streptozocin (STZ) in increasing doses to produce graded severity of diabetes. IGF BP-1 and BP-3 were measured by ligand blotting, total IGF-I was determined by radioimmunoassay after separation from BPs by isocratic high-performance liquid chromatography (HPLC) at pH 3.9, and free IGF-I was estimated operationally as immunoreactivity with molecular weight equal to native IGF-I after HPLC at pH 7. Animals given 36 mg/kg STZ exhibited a glucose level of 9.74 mM and impaired weight gain, with little alteration in IGF BPs or total or free IGF-I. In contrast, animals given 72 mg/kg STZ (glucose level 24.64 mM and weight loss) had insignificant changes in total IGF-I and BP-3 but a 300% increase in BP-1 and a 50% fall in free IGF-I (both P less than 0.005). With 144 and 288 mg/kg STZ, animals had further metabolic decompensation and weight loss, with progressive fall in BP-3 and rise in BP-1; total and free IGF-I fell to 10-20% of control (both P less than 0.001).(ABSTRACT TRUNCATED AT 250 WORDS)

Metabolic Effects of IGF-I in Diabetic Rats

Insulinlike growth factor I (IGF-I) stimulates glucose utilization (GU) in nondiabetic rats. We compared the effects of IGF-I and insulin on glucose metabolism in control (fed plasma glucose 7.7 +/- 0.1 mM, n = 30) and partially (90%) pancreatectomized diabetic (plasma glucose 18.4 +/- 0.8 mM, n = 30) awake unstressed rats. IGF-I was infused at 0.65 or 1.96 nmol.kg-1.min-1 and insulin at 22 or 29 pmol.kg-1.min-1 in combination with [3-3H]glucose while euglycemia was maintained by a variable glucose infusion. In controls, GU during the 0.65- and 1.96-nmol.kg-1.min-1 IGF-I infusions (127 +/- 7 and 168 +/- 4 mumol.kg-1.min-1, respectively) was similar to rates observed during the 22- and 29-pmol.kg-1.min-1 insulin infusions (121 +/- 2 and 156 +/- 5 mumol.kg-1.min-1). Whole-body glycolytic rate (3H2O generation) and muscle glycogen synthetic rate were identical during insulin and IGF-I infusions. In diabetic rats, GU was reduced by 30% versus control rats (P less than 0.01) during both the low-dose (88 +/- 7 vs. 121 +/- 7 mumol.kg-1.min-1) and higher-dose (109 +/- 4 vs. 156 +/- 5 mumol.kg-1.min-1) insulin clamps. The defect in insulin action involved both muscle glycogen synthesis and glycolysis. In diabetic rats, IGF-I elicited rates of GU similar to controls (115 +/- 10 and 164 +/- 12 mumol.kg-1.min-1 during the 0.65- and 1.96-nmol.kg-1.min-1 infusions, respectively) and corrected the intracellular defects in glycogen synthesis and glycolysis.(ABSTRACT TRUNCATED AT 250 WORDS)

Plasma clearance and tissue distribution of labelled insulin-like growth factor-I (IGF-I), IGF-II and des(1–3)IGF-I in rats

Incubation of 125I-labelled insulin-like growth factor-I (IGF-I) with rat plasma at 4 degrees C led to the transfer of approximately half the radioactivity to 150 kDa and smaller complexes with IGF-binding proteins. The extent of association was greater with labelled IGF-II and essentially absent with the truncated IGF-I analogue, des(1-3)IGF-I. A greater degree of binding of IGF peptides with binding proteins occurred after i.v. injection of the tracers into rats, but most of the des(1-3)IGF-I radioactivity remained free. Measurement of the total plasma clearances showed the rapid removal of des(1-3)IGF-I compared with IGF-I and IGF-II; the mean clearances were 4.59, 1.20 and 1.34 ml/min per kg respectively. The mean steady-state volume of distribution was larger for des(1-3)IGF-I than for IGF-I and IGF-II (461, 167 and 181 ml/kg respectively), probably because of the differences in plasma protein binding. With all tracers, radioactivity appeared in the kidneys to a greater extent than in other organs. The amount of radioactivity found in the adrenals, brain, skin, stomach, duodenum, ileum plus jejunum and colon was in rank order, des(1-3)IGF-I greater than IGF-I greater than IGF-II. Since this ranking is the opposite of the abilities of the three IGF peptides to form complexes with plasma binding proteins, we propose that the plasma binding proteins inhibit the transfer of the growth factors to their tissue sites of action. Moreover, we suggest that IGF analogues that are cleared rapidly from blood may have greater biological potencies in vivo.

Effect of Puberty on Initial Kidney Growth and Rise in Kidney IGF-I in Diabetic Rats

Prepubertal subjects have a low incidence of diabetic nephropathy compared with duration-matched postpubertal subjects. At puberty, there is an increase in insulinlike growth factor I (IGF-I) levels, and because IGF-I has been implicated in the early kidney enlargement of experimental diabetes, we studied the development of kidney enlargement and kidney IGF-I levels in prepubertal (aged 5 wk) and postpubertal (aged 13 wk) Sprague-Dawley rats during the 7 days after induction of diabetes with streptozocin. Kidney weight in postpubertal diabetic animals was significantly greater than in postpubertal controls by day 2 (1.46 +/- 0.06 vs. 1.16 +/- 0.09 g, P less than 0.05), and by day 7, kidney weight had increased by 36% (1.61 +/- 0.07 vs. 1.18 +/- 0.08 g, P less than 0.001). Despite comparable blood glucose levels in the prepubertal and postpubertal diabetic rats, kidney weight in prepubertal diabetic animals was significantly greater than in prepubertal controls by 14% on day 7 only (0.84 +/- 0.01 vs. 0.73 +/- 0.03 g, P less than 0.05). Kidney IGF-I content was significantly elevated in diabetic postpubertal rats, peaking on day 1 (diabetic vs. control, 1082 +/- 156 vs. 543 +/- 21 ng/g, P less than 0.001) and day 2 but not in prepubertal diabetic rats. Thus, prepubertal diabetic rats have reduced and retarded kidney growth and attenuated kidney IGF-I levels, suggesting that local IGF-I accumulation may play an important role in diabetes-associated kidney enlargement.

Insulin-Like Growth Factor-I (IGF-I) Infusion into Hypophysectomized or Protein-Deprived Rats Induces Specific IGF-Binding Proteins in Serum*

The insulin-like growth factors (IGFs) are small peptides that are present in serum and extracellular fluids and stimulate the growth of many cell types. In extracellular fluids the IGFs are bound to carrier proteins that are believed to modify the biological actions of the IGFs. At least three structurally distinct IGF-binding proteins (IGF-BPs) have been identified, and the serum concentrations of one of these has been shown to be regulated by pituitary GH. We report here that this GH responsive protein [39,000-45,000 mol wt (Mr)] can be induced (7-fold) by infusion of IGF-I in hypophysectomized rats or (3.5-fold) in protein-deprived rats, whereas two other forms of IGF-BP (e.g. 31,000-34,000 and 24,000 Mr) showed no change in the hypophysectomized animals and minimal increases in the protein-deprived animals. Likewise, GH injections in hypophysectomized animals resulted in a 7-fold increase in the 39,000-45,000 Mr form and no change in the 31,000-34,000 and 24,000 Mr forms. The protein-deprived animals showed a 3.2-fold increase in the 39,000-45,000 Mr and 2.4- to 1.8-fold increases in the 31,000-34,000 and 24,000 Mr forms, respectively. Changes in the larger Mr IGF-BP in these experimental models are paralleled by changes in serum IGF-I, suggesting that the GH dependence of the former protein is mediated at least partially via IGF-I. Our findings also suggest that the secretion of IGF-I and at least one IGF-BP may be linked, providing a mechanism by which their extracellular fluid concentrations are coordinated. Because IGF-BPs are present in extracellular fluids and can modulate IGF-I-receptor interaction, induction of this protein may be an important mediator of IGF action.

Nerve regeneration and serum levels of insulin-like growth factor-I in rats with streptozotocin-induced insulin deficiency

Peripheral nerve regeneration was studied in female Sprague-Dawley rats with streptozotocin-induced insulin deficiency. Nerve regeneration was provoked by a crush lesion on the sciatic nerve 21 days after the streptozotocin injection. The regeneration was assessed by a pinch test at different time-points after injury. The rate ofregeneration in insulin-deficient animals, 2.5 mm/day, was significantly lower than in control animals, 2.9 mm/day(P < 0.05). There was no difference in the initial delay, i.e. the period before regeneration attains a constant velocity. One group of insulin-deficient rats was treated with insulin during the regeneration period by means of implanted osmotic mini-pumps. This treatment prevented the decrease in regenerationsw. After 6 days the sciatic nerves of insulin-deficient rats had regenerated 12.3 ±0.3mm(mean±S.E.M.), while the corresponding value for insulin-treated rats was 15.7 ±0.6 mm (P > 0.01). The streptozotocin-treated rats were found to have a 39% reduction in the serum level of insulin-like 1 growth factor-I (IGF-I)_compared to control rats (0.33 ± 0.22 μg/ml and 0.54 ± ml respectively, (P < 0.001). Insulin treatment 1830 1732 during the regeneration period completely restored the IGF-I level back to normal.

Differential effects of estrogen and growth hormone on uterine and hepatic insulin-like growth factor I gene expression in the ovariectomized hypophysectomized rat.

The acute and chronic effects of 17 beta-estradiol (E2) and GH on uterine and hepatic insulin-like growth factor I (IGF-I) gene expression in ovariectomized hypophysectomized (ovx-hypox) rats were examined. Six hours after a single injection of E2 (5 micrograms/100 g BW), uterine IGF-I gene expression was increased 22.5 +/- 5.4-fold (P less than 0.005) above that in untreated rats. In the same experiment E2 alone had no significant effect on hepatic IGF-I gene expression. Similarly, in chronic experiments uterine IGF-I in ovx-hypox rats receiving 0.1 or 1 microgram/rat.day E2 for 10 days was significantly increased compared to that in ovx-hypox rats that did not receive E2 [5.38 +/- 0.79 vs. 1.10 +/- 0.15 (P less than 0.005) and 6.64 +/- 0.28 vs. 0.93 +/- 0.06 (P less than 0.005), respectively]. While administration of human GH alone significantly increased uterine IGF-I expression (3.76 +/- 1.61-fold compared to that in untreated rats; P less than 0.05), a significant and reproducible attenuation of E2-induced IGF-I expression was seen in the two acute experiments where GH reduced the E2-induced response by 36 +/- 3.7% and 53 +/- 19.4%. While chronic administration of E2 to ovx-hypox rats resulted in uterine growth, a significant decrease in body weight was seen in rats treated with 1 microgram/day E2 compared to that in untreated ovx-hypox controls (-4.3 +/- 1.5 vs. 2.5 +/- 0.6 g; P less than 0.0005). E2 treatment also significantly decreased the GH-induced increase in weight gain at each GH dose by approximately 40%. GH-induced hepatic IGF-I gene expression and serum IGF-I concentration were similarly reduced by chronic E2 administration. In contrast, in acute experiments where E2 alone had no effect on hepatic IGF-I expression, it acted in a synergistic fashion with GH and resulted in significantly greater accumulation of IGF-I mRNA. From these observations we conclude that 1) both E2 and human GH are potent stimulators of IGF-I gene expression in appropriate target tissues; and 2) in addition to any effects E2 has on GH secretion and IGF-I action, the growth-retarding effect of estrogen in the rat involves inhibition of GH-dependent hepatic IGF-I expression.

&lt;I&gt;In vivo&lt;/I&gt; Effects of Insulin-Like Growth Factor I in Rats

The scarcity of somatomedin/insulin-like growth factor (SM/IGF) has prevented investigation of the mechanism of SM/IGF action. Recently insulin-like growth factor I (IGF-I) has been synthesized by recombinant DNA technology. With the availability of large quantities of the biosynthetic IGF-I, we undertook a study of biological effects of IGF-I in vivo in rats. 120μg/day of IGF-I was administered continuously for 7 days via subcutaneous implanted osmotic minipump to hypophysectomized (hypox), normal, and diabetic rats. The body weights and tibial epiphyseal widths after 7 day treatment of IGF-I in both hypox and normal rats were significantly greater than those for untreated rats. Blood urea nitrogen levels in both hypox and normal rats treated with IGF-I were significantly lower than those for untreated rats, suggesting that IGF-I has anabolic action in vivo. In contrast to hypox and normal rats, in diabetic rats, IGF-I treatment did not stimulate growth in vivo, but, had a little anabolic and insulin-like effects in vivo.This study demonstrates that IGF-I has growth promoting, anabolic and insulin-like effects in vivo and suggests that IGF-I might be useful in the treatment of growth retardation but not that for diabetes mellitus.