Total Insulin-like Growth Factor I mRNA Research Articles

Resolving the growth-promoting and metabolic effects of growth hormone: Differential regulation of GH–IGF-I system components

Growth hormone regulates numerous processes in vertebrates including growth promotion and lipid mobilization. During periods of food deprivation, growth is arrested yet lipid depletion is promoted. In this study, we used rainbow trout on different nutritional regimens to examine the regulation of growth hormone (GH)-insulin-like growth factor-I (IGF-I) system elements in order to resolve the growth-promoting and lipid catabolic actions of GH. Fish fasted for 2 or 6 weeks displayed significantly reduced growth compared to their fed counterparts despite elevated plasma GH, while refeeding for 2 weeks following 4 weeks of fasting partially restored growth and lowered plasma GH. Fish fasted for 6 weeks also exhausted their mesenteric adipose tissue reserves. Sensitivity to GH in the liver was reduced in fasting fish as evidenced by reduced expression of GH receptor type 1 (GHR 1) and GHR 2 mRNAs and by reduced (125)I-GH binding capacity. Expression of GHR 1 and GHR 2 mRNAs also was reduced in the gill of fasted fish. In adipose tissue, however, sensitivity to GH, as indicated by GHR 1 expression and by (125)I-GH binding capacity, increased after 6 weeks of fasting in concert with the observed lipid depletion. Fasting-associated growth retardation was accompanied by reduced expression of total IGF-I mRNA in the liver, adipose and gill, and by reduced plasma levels of IGF-I. Sensitivity to IGF-I was reduced in the gill of fasted fish as indicated by reduced expression of type 1 IGF-I receptor (IGFR 1A and IGFR 1B) mRNAs. By contrast, fasting did not affect expression of IGFR 1 mRNAs or (125)I-IGF-I binding in skeletal muscle and increased expression of IGFR 1 mRNAs and (125)I-IGF-I binding in cardiac muscle. These results indicate that nutritional state differentially regulates GH-IGF-I system components in a tissue-specific manner and that such alterations disable the growth-promoting actions of GH and promote the lipid-mobilizing actions of the hormone.

Reduced Serum Insulin-Like Growth Factor (IGF) I Is Associated with Reduced Liver IGF-I mRNA and Liver Growth Hormone Receptor mRNA in Food-Deprived Cattle

Nutritional deprivation decreases blood insulin-like growth factor I (IGF-I) concentrations in a variety of species. In this study, we explored the underlying mechanism by determining the effects of food deprivation on the levels of total IGF-I mRNA and total growth hormone receptor (GHR) mRNA, as well as the levels of individual IGF-I mRNA variants and GHR mRNA variants in the liver of steers. Food deprivation for nearly 3 d decreased the levels of serum IGF-I by 63% (P < 0.01), and this decrease was associated with a 75% decrease (P < 0.01) in total IGF-I mRNA in the liver. The food deprivation-induced decrease in liver total IGF-I mRNA was associated with an equivalent decrease in the levels of both class 1 and class 2 IGF-I mRNA. In addition to IGF-I mRNA, food deprivation also decreased the levels of total GHR mRNA in the liver (P < 0.05), and this decrease was associated with a decrease in the liver expression of GHR mRNA variants 1C3 (P < 0.05) and 1A (P = 0.08). Food deprivation did not affect the levels of two other major GHR mRNA variants, 1B and 1C2, in the liver. These results demonstrate that the food deprivation-induced decrease in circulating IGF-I in steers is associated with a coordinate decrease in the expression of different IGF-I mRNA variants and a specific decrease in the expression of GHR mRNA variants 1C3 and 1A in the liver.

Control of ovine hepatic growth hormone receptor and insulin-like growth factor I by thyroid hormones in utero.

By use of RNase protection assays, hepatic growth hormone receptor (GHR) and insulin-like growth factor I (IGF-I) mRNA abundances were measured in sheep fetuses after experimental manipulation of fetal plasma thyroid hormone concentrations by fetal thyroidectomy (TX) and exogenous infusion of triiodothyronine (T(3)) and cortisol. TX abolished the normal prepartum rise in hepatic GHR abundance but had little effect on hepatic GHR gene expression at 127-130 days (term 145 +/- 2 days). By contrast, it upregulated basal IGF-I expression in immature fetal liver by increasing both Class 1 and Class 2 transcript abundance but had no further effects on IGF-I gene mRNA levels at 142-145 days. Raising plasma T(3) to prepartum values by exogenous infusion of either T(3) or cortisol into immature intact fetuses prematurely raised hepatic GHR and IGF-I mRNA abundances to values similar to those seen in intact fetuses at 142-145 days. In TX fetuses, cortisol infusion increased hepatic GHR mRNA but not total IGF-I mRNA abundance at 127-130 days. These findings show that thyroid hormones have an important role in the regulation of hepatic GHR and IGF-I gene expression in fetal sheep during late gestation and suggest that T(3) mediates the maturational effects of cortisol on the hepatic somatotropic axis close to term.

Reduced growth hormone receptor (GHR) messenger ribonucleic acid in liver of periparturient cattle is caused by a specific down-regulation of GHR 1A that is associated with decreased insulin-like growth factor I.

GH receptor (GHR) messenger RNA (mRNA) is transcribed from at least three different promoters within the liver of cattle. The first promoter (P1) is liver specific and alternatively splices exon 1A onto the GHR mRNA (GHR 1A mRNA). The second and third promoters (P2 and P3) have constitutive activity in many tissues and alternatively splice exons 1B and 1C onto the GHR mRNA (GHR 1B and GHR 1C mRNA). The total amount of GHR in the liver partially determines liver insulin-like growth factor I (IGF-I) synthesis in response to GH. Two studies were conducted to characterize the changes in GHR 1A mRNA, alternatively spliced GHR mRNA, and IGF-I mRNA during late pregnancy and early lactation in dairy cattle. Liver RNA was isolated from pregnant Holstein cattle (Bos taurus) on days -14, 0, and 21 relative to parturition (study 1) or days -14, 0, 15, 30, 60, and 90 relative to parturition (study 2). Ribonuclease protection assays were used to quantify total GHR (all GHR variants) as well as liver-specific GHR 1A and alternatively spliced GHR mRNA. Likewise, total IGF-I as well as alternatively spliced IGF-I mRNA (class 1 and class 2 transcripts) were measured. A decrease in total GHR mRNA at parturition (P < 0.01) was associated with a specific decrease in GHR 1A mRNA (P < 0.001). The amount of alternatively spliced GHR mRNA (including GHR 1B and GHR 1C mRNA) did not change at parturition (P > 0.10). Total liver IGF-I mRNA and blood IGF-I concentrations were also decreased at parturition (P < 0.05 and P < 0.01, respectively). However, a decrease in IGF-I mRNA was observed for both class 1 and class 2 IGF-I transcripts (P < 0.01 and P < 0.05, respectively). We conclude that the reduced amount of GHR mRNA during early lactation is caused by a specific down-regulation of GHR 1A mRNA that was associated with decreased liver IGF-I mRNA and decreased blood IGF-I concentrations. These data provide evidence for independent regulation of GHR mRNA by mechanisms that discriminate between GHR P1 (transcribes GHR 1A) and alternative promoters that transcribe constitutive GHR mRNA.

Insulin-like growth factor–I in women with hydatidiform mole and in normal pregnancy

To compare molar and placental tissue insulin-like growth factor-I (IGF-I) levels and IGF-I messenger RNA (mRNA) expression and maternal serum IGF-I levels in molar and normal pregnancies of the same gestational length. Tissue and serum total IGF-I were measured by radioimmunoassay, and tissue IGF-I mRNA expression was measured by a solution hybridization technique in samples from seven patients with hydatidiform mole in gestational weeks 8 through 14 and from 12 women with normal placentas undergoing induced abortion during weeks 8 through 12. Tissue and serum total IGF-I levels and tissue IGF-I mRNA expression were significantly lower in molar pregnancies than in pregnancies with normal placenta. The lower levels of both IGF-I mRNA and of the IGF-I protein itself in hydatidiform mole indicates that the regulation of expression of IGF-I is on the transcriptional level. This finding may reflect a decreased production of the placental growth hormone variant in molar tissue.

Transcription initiation of the rat insulin-like growth factor-I gene in hepatocyte primary culture.

Transcription initiation in the insulin-like growth factor-I (IGF-I) gene is complex, involving multiple sites in two exons. While most transcripts are initiated in exon 1 in vivo, critical regulatory mechanisms are difficult to assess in intact animals. To examine the impact of insulin and growth hormone (GH) under more controlled conditions, we have studied the utilization of different exon 1 and exon 2 transcription-initiation sites in normal rat hepatocytes in primary culture. Normal rat hepatocytes were cultured for 48 h in serum-free medium, with insulin at 10(-6) or 10(-11) M, and with or without human GH 200 ng/ml. Relative abundance of IGF-I transcripts was evaluated by the RNase-protection assay, using a probe which permitted identification of initiation in exon 1 (site 1 (-380 bp from the 3' end of exon 1), site 2 (-343 bp), site 3 (-242 bp), sites 1 and 2 spliced, and site 4 (-32 bp)), as well as in exon 2. After normalization of signal intensity to adjust for differences in length of protected probe, the utilization of initiation sites in vitro was remarkably similar to that in vivo: 1, 14, 6, 23, 19 and 37% for sites 1, 2, 3, 1 and 2 spliced, 4 and exon 2 respectively in the cultured hepatocytes, compared with 1, 12, 18, 21, 18 and 40% for these sites in normal liver. Insulin alone increased transcripts initiated from exon 1, site 2 by over 3 times, and both sites 1 and 2 spliced and exon 2 transcripts by about 5 times. GH alone had similar effects, producing a 4-5 times increase in transcripts from these initiation sites. Addition of both insulin and GH had additive effects, increasing transcripts from exon 1, sites 2, 3 and 4 by 4-6 times, and from exon 1, sites 1 and 2 spliced, and exon 2 by over 8 times. Of the total IGF-I mRNA transcripts, 37% were initiated from sites 2 and/or sites 1 and 2 spliced, and 37% from exon 2. Analysis of the relative contribution of individual initiation sites revealed hormone-induced increases which were statistically significant only for exon 2, in the presence of insulin alone and in combination with GH. In conclusion, in cultured hepatocytes, insulin or GH alone produced a coordinated increase in all exon 1 transcripts, and the effect of the combination of insulin and GH was additive for these transcripts. Exon 2 appeared to be more sensitive to insulin alone, and to GH in the presence of insulin, than exon 1. Since utilization of initiation sites in hepatocytes mimics that found in liver, this in vitro system should be useful for examining underlying transcriptional regulatory mechanisms.

Osteogenic protein-1-mediated insulin-like growth factor gene expression in primary cultures of rat osteoblastic cells.

Osteogenic protein-1 (OP-1) is a member of the bone morphogenetic protein family and has been shown to induce new bone formation in vivo. In the present study, we determined whether the expression of the IGF system, a significant growth factor system in bone, was altered by OP-1 in primary cultures of fetal rat calvarial cells. Levels of messenger RNA (mRNA) encoding insulin-like growth factor I (IGF-I), IGF-II, IGF-I receptor, and IGF-binding proteins (IGFBP-1 to -6) were determined after OP-1 treatment. The level of total IGF-I mRNA was elevated in an OP-1 concentration (0-1000 ng/ml)-dependent manner, with maximal stimulation of IGF-I mRNA of 2- to 3-fold apparent 24 h after treatment. The increase in the IGF-I mRNA level involved a preferential stimulation of transcripts initiated at start site 2 in the exon 1 promoter. The level of IGF-II mRNA also increased by approximately 2-fold in OP-1 treated cells in a concentration-dependent manner. The level of IGF-I receptor mRNA was not altered by treatment. Whereas IGFBP-1 mRNA was not detected in these cells, IGFBP-2 mRNA was expressed, but the expression was not changed after treatment for 48 h in the concentration range (0-1000 ng/ml) tested. The IGFBP-3 mRNA level was increased slightly 48 h after OP-1 treatment in a concentration-dependent manner. The IGFBP-4, -5, and -6 mRNA levels decreased dramatically in an OP-1 concentration-dependent manner. In addition, coincubation of antisense oligonucleotides corresponding to IGF-I or -II mRNA sequence with OP-1 reduced the OP-1 induced elevation in alkaline phosphatase activity. The present results suggest that the differentiation of rat osteoblastic cells in response to OP-1 is mediated in part by increased IGF-I -II gene expression and alterations in the gene expression of different IGFBPs.

Regulation of porcine insulin-like growth factor I and growth hormone receptor mRNA expression by energy status.

Regulation of insulin-like growth factor I (IGF-I) and growth hormone (GH) receptor mRNA in liver and muscle by energy status was assessed in 2-mo-old pigs by altering thermoregulatory demand and energy intake over a 5-wk period to produce a range of plasma IGF-I concentrations from 3.5 +/- 0.7 to 28.9 +/- 6.2 nmol/l. These values were related directly to growth rates (0.06 +/- 0.02 to 0.44 +/- 0.01 kg/day) and total hepatic IGF-I mRNA levels. Increased growth rates were accompanied by an increase in hepatic class 1 and class 2 IGF-I mRNA levels and an increase in the ratio of class 2 to class 1 IGF-I mRNA in liver, suggesting a distinct role for class 2 expression in the endocrine growth response. High levels of class 1 transcripts and a virtual absence of class 2 transcripts characterized all muscle tissues examined, and there was no correlation with plasma IGF-I levels. This suggests that growth promotion in response to increased energy status is regulated via endocrine hepatic IGF-I rather than via a paracrine response. The levels of GH receptor mRNA were positively correlated with overall growth rate (P < 0.005) in liver and negatively correlated (P < 0.05) in muscle, indicating distinct tissue-specific effects of energy status.

Differential regulation of transcription initiation from insulin-like growth factor-I (IGF-I) leader exons and of tissue IGF-I expression in response to changed growth hormone and nutritional status in sheep.

In the mammalian insulin-like growth factor-I (IGF-I) gene, exons 1 and 2 are differentially spliced to exon 3 producing alternate class 1 and class 2 transcripts. The aim of this study was to investigate the tissue expression of these leader exons in lambs growing at different rates as a result of chronic manipulation of nutritional and GH status. Riboprobes were developed so that leader exon-specific and total IGF-I gene expression could be determined using a single RNase protection assay. Lambs were fed a diet containing high or low protein content either ad libitum or at a restricted intake; within these dietary groups they were treated with either saline or GH for 10 weeks. Total hepatic IGF-I messenger RNA (mRNA) transcripts were significantly increased by GH (P = 0.004), protein (P = 0.002), and energy (P < 0.001) status as were circulating IGF-I concentrations (GH, P < 0.001; protein, P = 0.026; energy, P < 0.001). Exons 1 and 2 were expressed in liver but to a variable extent. Increased dietary energy and protein induced increased expression from both class 1 and 2 transcripts, but the percent increases was at least 5-fold greater for class 2 than for class 1 mRNA. GH treatment only stimulated significant increases in expression from class 2 transcripts. In the low protein, energy-restricted, saline-treated lambs exon 1 transcripts accounted for approximately 70% of total class 1 and 2 transcripts, and this proportion declined significantly as class 1 and 2 transcripts, and this proportion declined significantly as GH and nutritional status increased to only 30% in the high protein, ad libitum-fed, GH-treated animals; class 2 transcripts therefore displayed the opposite pattern of expression. These data indicate that exon 2 may be far more sensitive than exon 1 in intact animals which have been stimulated within normal physiological limits. Muscle IGF-I gene expression was at least 20-fold less than that for the liver and consisted mainly of class 1 transcripts. Muscle total IGF-I mRNA was insensitive to changed nutritional status or to GH treatment, even though significant increases in muscle growth occurred in response to ad libitum intake and GH, indicating that hepatically derived endocrine IGF-I could have a role in the regulation of muscle growth.

Effects of streptozotocin-induced diabetes mellitus on growth and hepatic insulin-like growth factor I gene expression in the rat

Poorly controlled diabetes mellitus in humans and animals is often accompanied by impaired growth. We undertook this study in young rats to determine whether the reduction in growth rate associated with streptozotocin (STZ)-induced diabetes might be related to changes in both serum insulin-like growth factor I (IGF-I) and IGF-II levels, and, if so, whether these changes reflect alterations in serum growth hormone (GH) and in hepatic IGF-I and IGF-II gene expression. Serum rat GH (rGH) levels were variable during the first 4 days after STZ administration, but during the subsequent 5- to 11-day period the mean (+/- SEM) levels in insulin-treated (DI) (21.4 +/- 4.9 ng/mL) and untreated (D) (8.5 +/- 1.5 ng/ml) diabetic rats were significantly (P less than .001) lower than in controls (C) (117.8 +/- 22.9 ng/mL). Multiple transcripts of IGF-I (7.0, 4.0, 1.9, 1.0 kb), but barely detectable amounts of IGF-II mRNA, were found in the livers of normal and diabetic rats by Northern blot analysis. Using dot blot analysis, we have shown that the abundance of total hepatic IGF-I mRNA in untreated, growth-retarded diabetic animals decreases rapidly over a period of 3 days after STZ administration. Both serum IGF-I and IGF-II levels are also diminished during this interval in these markedly hyperglycemic rats. Insulin treatment for 3 to 4 days, started either immediately (6 hours) or within 3 days after administering STZ, blunts diabetes-induced impairment of growth and restores mean hepatic IGF-I mRNA abundance to control levels, but does not normalize serum IGF-I and IGF-II concentrations.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of Fasting on Insulin-Like Growth Factor-I (IGF-I) and Growth Hormone Receptor mRNA Levels and IGF-I Gene Transcription in Rat Liver

Previous studies have indicated that the concentration of circulating insulin-like growth factor-I (IGF-I) declines in young growing rats that have been fasted or maintained on a protein-deficient diet. To investigate the molecular mechanism(s) by which IGF-I levels are regulated by nutrition, we measured the levels of IGF-I mRNA in 6-week-old male control rats fed ad libitum, rats fasted for 24, 48, or 72 h, and rats fasted for 48 or 72 h and then refed for 24 h. The abundance of several IGF-I mRNA species (8.0, 4.0, 1.7, and 1.0 kilobases) decreased in the fasting animals and rebounded after 24 h of refeeding, although not to the initial control levels. The 1 kilobase IGF-I mRNA species exhibited a 43% decrease after 24 h of fasting, a 76% decrease after 48 h of fasting, and an 82% decrease after 72 h of fasting. Hepatic GH receptor mRNA also decreased in fasting rats. This indicates that the GH receptor down-regulation that occurs in fasting is accompanied by and probably at least partly caused by a decline in GH receptor mRNA. The magnitude and kinetics of the decline in GH receptor mRNA were similar to the magnitude and kinetics of the decline in IGF-I mRNA, suggesting that the two mRNAs may be regulated by a similar mechanism. There was no significant change in the levels of liver beta-actin or serum albumin mRNA under the same conditions, indicating that the regulation of IGF-I and GH receptor mRNA was specific. In addition, the levels of brain IGF-II, beta-actin, and alpha-tubulin mRNAs were not significantly changed by fasting. To further elucidate the molecular mechanism for regulation of hepatic IGF-I mRNA, nuclear transcription elongation assays were performed using nuclei isolated from the liver of control rats, rats fasted for 72 h, and fasted-refed rats. There was considerable animal-to-animal variability in IGF-I gene transcription within each group. The mean level of IGF-I gene transcription was lower in the fasting animals than in the fed controls. However, this decrease was not statistically significant, and the magnitude of the decrease did not account for the 79% decrease in total IGF-I mRNA. These results suggest that IGF-I mRNA is regulated at least partly at the posttranscriptional level.

Pituitary insulin-like growth factor-I gene expression: regulation by triiodothyronine and growth hormone.

Most of the growth-promoting effects of GH are mediated through insulin-like growth factor-I (IGF-I). Pituitary GH gene expression is, in turn, inhibited by IGF-I. Since rat pituitary tissue and GH3 pituitary tumor cells express both the GH and the IGF-I genes, we have attempted to clarify their potential interactions in the somatotroph by examining hormonal factors involved in the regulation of pituitary IGF-I gene expression. IGF-I mRNA was measured in GH3 cells by a solution hybridization/RNase protection assay, using riboprobes to differentially protect the IGF-I variant mRNAs arising by alternative splicing at both the 5' untranslated (UT) and 3' ends of the primary transcript. GH3 cells contained both class A and class C 5' UT variant mRNAs, with a relative abundance similar to that found in the liver. Sixty-five percent of the total IGF-I mRNA in GH3 cells was processed at the 3' end to IGF-Ia, and 35% to IGF-Ib mRNAs, whereas in the liver the proportions were 85% and 15%, respectively. GH3 cells grown in thyroid hormone-depleted medium for 4 days contained low levels of IGF-I mRNA. T3 and human (h) GH induced total IGF-I mRNA content in thyroid hormone-depleted cells, with both 5' and 3' alternative transcripts regulated coordinately, an effect that was maximal at 48-72 h. T3 stimulation of GH3 IGF-I mRNA over 48 h was dose dependent (0.01-5 nM). Similarly, hGH (0.5-10 micrograms/ml) evoked a dose-dependent induction of IGF-I mRNA in the thyroid hormone-deficient GH3 cells. The effects of T3 (5 nM) and hGH (10 micrograms/ml) on IGF-I mRNA were not additive. Furthermore, the effects of both T3 and hGH were selective for IGF-I mRNA, as neither of these treatments stimulated PRL mRNA, and treatment with hGH decreased GH3 cell GH mRNA content. This model does not discriminate whether T3 has an independent effect on IGF-I gene expression or if its action is mediated solely through induction of GH. In conclusion, IGF-I mRNA transcripts are present in GH3 cells and are modulated by T3 and GH. Local paracrine or autocrine interactions may, therefore, be involved in the feedback control of GH secretion.

Regulation by fasting of rat insulin-like growth factor I and its receptor. Effects on gene expression and binding.

We have examined, in liver and extrahepatic tissues, the effects of fasting on total insulin-like growth factor I (IGF-I) mRNA levels, on levels of different IGF-I mRNAs generated by alternative splicing of the primary IGF-I transcript, and on IGF-I receptor binding and mRNA levels. A 48-h fast decreased total IGF-I mRNA levels by approximately 80% in lung and liver, approximately 60% in kidney and muscle, and only approximately 30-40% in stomach, brain, and testes. In heart, IGF-I mRNA levels did not change. The levels of the different splicing variants, however, were essentially coordinately regulated within a given tissue. Specific 125I-IGF-I binding in lung, testes, stomach, kidney, and heart was increased by fasting by approximately 30-100%, whereas in brain 125I-IGF-I binding did not change in response to fasting. In tissues in which fasting increased IGF-I receptor number, receptor mRNA levels increased approximately 1.6- to 2.5-fold, whereas when IGF-I receptor number was unchanged in response to fasting, receptor mRNA levels did not change. These data demonstrate that the change in IGF-I and IGF-I receptor mRNA levels during fasting is quantitatively different in different tissues and suggest that regulation of IGF-I and IGF-I receptor gene expression by fasting is discoordinate.

Insulin-like growth factor I messenger ribonucleic acids with alternative 5'-untranslated regions are differentially expressed during development of the rat.

Solution hybridization/RNase protection assays were used to study the developmental expression of insulin-like growth factor I (IGF-I) mRNA levels in rats. In liver, heart, and kidney, total IGF-I mRNA levels were low at birth and increased during the 50-day postnatal period, with liver levels increasing by over 100-fold. In contrast, stomach, muscle, and testicular IGF-I mRNA levels were highest at the earliest stages examined (late fetal or early neonatal) and declined thereafter to the levels observed in 50-day-old rats. In brain, IGF-I mRNA levels rose 2-fold during the first week of life and declined over the next 6-7 weeks. Lung IGF-I mRNA levels were highest in 20-day-old fetuses and exhibited some fluctuation during the postnatal period. Alternative splicing in the 5'-untranslated region of the primary rat IGF-I transcript gives rise to three transcripts, classes A, B, and C, which have divergent 5'-untranslated region sequences associated with a common region that encodes the mature IGF-I peptide. These sequences contain upstream in-frame translation initiation codons and may, therefore, encode alternate IGF-I prepropeptides. The class C variant was the predominant mRNA species at all stages of development studied and was the only IGF-I transcript in brain, heart, and muscle. In tissues where multiple 5'-untranslated region splicing variants occurred, therefore, changes in total IGF-I mRNA primarily reflected changes in this splicing variant. However, the class C and class A (as well as class B in liver) transcripts exhibited temporally divergent changes over some developmental intervals. Class A transcripts in the liver, stomach, testes, and lung as well as class B transcripts in liver, exhibited sustained increases from 15 or 22 postnatal days to maximal levels at 50 postnatal days. In kidney, class A transcripts also increased steadily, but beginning at an earlier stage, i.e. at 8-15 days of postnatal life. These results demonstrate that the temporal expression of total IGF-I mRNA in the developing rat occurs in a tissue-specific manner, and additionally, that IGF-I mRNA variants are differentially expressed during development.