Prevalence of stimulant use for attentional dysfunction in children with phenylketonuria.

Recent data suggest that children with phenylketonuria (PKU) and poor metabolic control may have an increased prevalence of attentional dysfunction. However, few formal studies have addressed this topic in detail. We reviewed the medical records of 38 school-aged children with early and continuously treated PKU to determine the prevalence of stimulant use for attentional dysfunction, and to determine the relationship between metabolic control and attentional symptoms. Twenty-six per cent of the PKU children used a stimulant medication for attentional dysfunction. This is significantly higher than in an age- and sex-matched control group consisting of children with type I diabetes mellitus (6.5%, p <0.006), and also considerably higher than population norms for attention deficit hyperactivity disorder (ADHD) (5%). We also found a significant relationship between phenylalanine levels and stimulant use or attentional symptoms. Mean plasma phenylalanine concentration was 486 micromol/L in the non-stimulant-using group and 792 micromol/L in the stimulant-using group (p <0.02). Mean phenylalanine concentration was 462 micromol/L in the group not reporting attentional symptoms, and was 702 micromol/L in the symptomatic group (p <0.05). Parents of the stimulant-using children felt that the stimulants were efficacious in treating their child's attentional symptoms. Stimulant use and parent reports of attentional dysfunction are quite common in our PKU patients and appear to be strongly related to higher phenylalanine concentrations.

Hyperoxia increases keratinocyte growth factor mRNA expression in neonatal rabbit lung.

Acute hyperoxic lung injury remains a major factor in the development of chronic lung disease in neonates. A critical step in the repair of acute lung injury is the proliferation of type II alveolar epithelial cells. Type II cell proliferation is stimulated by keratinocyte growth factor (KGF), an epithelial cell-specific mitogen. We sought to investigate KGF mRNA expression in relation to type II cell proliferation during hyperoxic lung injury. We studied a previously described newborn (NB) rabbit model of acute and chronic hyperoxic injury [C. T. D'Angio, J. N. Finkelstein, M. B. LoMonaco, A. Paxhia, S. A. Wright, R. B. Baggs, R. H. Notter, and R. M. Ryan. Am. J. Physiol. 272 (Lung Cell. Mol. Physiol. 16): L720-L730, 1997]. NB rabbits were placed in 100% O2 for 9 days and then recovered in 60% O2. RT-PCR was used to synthesize and amplify a 267-bp fragment of rabbit KGF cDNA from whole lung RNA. KGF mRNA expression was analyzed by ribonuclease protection assay, and mRNA abundance was quantified by phosphorimaging. Proliferating cell nuclear antigen immunohistochemistry was used on lung sections to identify proliferating cells. The rabbit partial cDNA sequenced was >95% homologous to human cDNA, and all amino acids were conserved. Whole lung KGF mRNA expression was increased 12-fold after 6 days of hyperoxia compared with control lungs, and remained increased throughout the 100% O2 exposure period. Proliferating cell nuclear antigen immunohistochemistry showed an increase in type II cell proliferation after 8-12 days of hyperoxia. NB rabbits exposed to hyperoxic injury exhibit increased whole lung KGF mRNA expression preceding type II cell proliferation. KGF may be an important mitogen in the regulation of alveolar epithelial repair after hyperoxic lung injury.

The mammalian cell cycle in normal and abnormal growth.

Cell division, a complex array of intracellular events, occurs in a highly ordered and carefully coordinated manner. This regulation is achieved by the sequential activation and deactivation of the members of a family of serine-threonine-specific protein kinases that consist of regulatory and enzymatic subunits, the cyclins and cyclin-dependent kinases. These enzymes, in turn, regulate the activity of other proteins involved in the mitogenic pathway. Mutations in the components of the regulatory pathways can lead to aberrant growth, including malignancies.

Comparison of transdermal and oral estrogen therapy in girls with Turner's syndrome.

Eight girls with Turner's syndrome were given low dose oral ethinyl estradiol or transdermal 17 beta-estradiol in order to compare the effect of the route of administration on selected markers of hepatic metabolism, and various hormonal concentrations. Oral estrogen was given at a dose of 100 ng/kg/day and transdermal estrogen via adhesive skin patch at 0.0125 mg/kg/day. The subjects received one form of estradiol for one month, and after a one month washout period, received the other form. Both oral and transdermal estradiol caused a significant decrease in FSH while only transdermal resulted in a significant decrease in LH. Oral estradiol, though not transdermal estradiol, increased serum high density lipoprotein, thyroxine binding protein and growth hormone binding protein. Urinary growth hormone excretion increased after both forms of therapy, while insulin-like growth factor-I and insulin-like growth factor binding protein-3 remained unchanged. Thus, in girls with Turner's syndrome, estrogen replacement by the transdermal route may have less deleterious effect on hepatic metabolism than oral estrogen.

Insulin rapidly inhibits insulin-like growth factor-binding protein-1 gene expression in H4-II-E rat hepatoma cells.

The insulin-like growth factor-binding proteins (IGFBPs) are thought to determine the distribution of IGF-I and IGF-II between the blood and tissue compartments and to modulate their biological activities. A dynamic metabolic role for one of the IGFBPs, IGFBP-1, is suggested by the fact that plasma IGFBP-1 was increased after fasting and diabetes and rapidly decreased by refeeding or insulin treatment, respectively. IGFBP-1 mRNA also is increased in the livers of diabetic rats and decreased by insulin treatment. To understand the molecular basis for this regulation, we have examined the effects of insulin on IGFBP-1 and IGFBP-1 mRNA in the H4-II-E cell line derived from the well differentiated H35 rat hepatoma. IGFBP-1, identified by ligand blotting and immunoblotting, is the major IGFBP in H4-II-E cells. Incubation of H4-II-E cells with insulin for 24 h decreased IGFBP-1 in the culture medium by approximately 50%. Inhibition was observed at physiological concentrations of insulin (ED50, less than 0.5 nM), but not at higher concentrations of IGF-II. These results, together with the fact that H4-II-E cells do not possess IGF-I receptors with which insulin might cross-react, suggest that insulin acts via the insulin receptor. Insulin inhibited IGFBP-1 in the medium by 80% in the absence of glucose, suggesting that the inhibition is a direct effect of insulin; glucose exerted a smaller independent effect in the absence of insulin. Insulin decreased IGFBP-1 mRNA in H4-II-E cells by 50% within 1 h and by 90% after 2-12 h of incubation. Nuclear run-on transcription assays indicated a corresponding decrease in the rate of IGFBP-1 gene transcription. Pretreatment of H4-II-E cells with dexamethasone stimulated IGFBP-1 transcription and increased steady state IGFBP-1 mRNA; stimulation was abolished by insulin treatment, indicating that inhibition by insulin was dominant over induction by dexamethasone. Thus, insulin, acting through the insulin receptor, rapidly decreases the abundance of IGFBP-1 mRNA in H4-II-E cells. Regulation occurs at least in part at the level of gene transcription. We propose that regulation of IGFBP-1 synthesis is an important component of the regulation of IGFBP-1 by insulin in vivo.

Expression of the genes for insulin-like growth factor-I (IGF-I), IGF-II, and IGF-binding proteins-1 and -2 in fetal rat under conditions of intrauterine growth retardation caused by maternal fasting.

Evidence suggests that insulin-like growth factors-I and -II (IGF-I and II) play a role in regulating fetal growth and development. In the fetus, IGF-I and -II are complexed with two specific binding proteins (IGFBP-1 and -2), which are thought to modulate the actions of the IGFs in target tissues. We examined regulation of the genes for IGF-I, IGF-II, IGFBP-1, and IGFBP-2 in fetal rat liver in an experimental model for intrauterine growth retardation caused by maternal fasting on days 17-21 of gestation. The mean weight of fetuses from the fasted dams was 27-32% lower than the mean weight of fetuses from the fed dams. The concentration of immunoreactive IGF-I was decreased by 71% in serum of fetuses from the fasting dams. The concentration of immunoreactive IGF-II was slightly decreased (by 12%) in serum of fetuses from the fasting dams, whereas the concentration of immunoreactive pro-IGF-II E-domain peptide was decreased by 31%. The abundance of hepatic IGF-I mRNA was decreased by 55% in fetuses from the fasting dams. In contrast, the abundance of IGF-II mRNA in fetal liver was not significantly decreased by maternal fasting. Maternal fasting caused a 2-fold increase in the abundance of IGFBP-1 mRNA in fetal liver, whereas it did not change the abundance of IGFBP-2 mRNA. The induction of IGFBP-1 mRNA in liver of the growth-retarded fetuses is similar to the induction that occurs in liver of fasting adults, while the lack of regulation of IGFBP-2 mRNA differs from the strong induction of IGFBP-2 mRNA that occurs in liver of fasting adults. In summary, these results indicate that maternal fasting causes a decrease in fetal IGF-I gene expression, a decrease in fetal serum IGF-I, and a slight decrease in fetal serum IGF-II and pro-IGF-II E-domain peptide concentrations. Maternal fasting also causes an increase in fetal IGFBP-1 gene expression. Changes in fetal insulin and glucose may be related to changes in expression of the IGF-I and IGFBP-1 genes in the growth-retarded fetuses. The decreased expression of IGF-I and -II and increased expression of the IGFBP-1 gene may contribute to the fetal growth retardation observed in this model system.

Dexamethasone stimulates transcription of the insulin-like growth factor-binding protein-1 gene in H4-II-E rat hepatoma cells.

Binding proteins for the insulin-like growth factors (IGFBP) are important modulators of the biological actions of IGF-I and IGF-II. Concentrations of one of these proteins, IGFBP-1, in human plasma and IGFBP-1 mRNA in rat liver are markedly altered in diabetes and fasting. We now examine the regulation of IGFBP-1 and IGFBP-I mRNA in H4-II-E cells, a rat cell line derived from the minimal deviation H35 Reuber hepatoma previously reported to synthesize IGFBP-1 as its predominant IGF-binding protein. Confluent H4-II-E cells in serum-free medium were incubated with different hormones for 48 h, and the conditioned medium was analyzed by ligand blotting. Dexamethasone (10(-6) M) increased levels of 30-kDa IGFBP-1 approximately 10-fold; stimulation was half-maximal at 6 x 10(-9) M dexamethasone. No stimulation was seen with progesterone, testosterone, IGF-I, or rat GH, whereas insulin gave a small inhibition. Immunoblot analysis using a monoclonal antibody to human IGFBP-1 confirmed that the 30-kDa IGFBP induced by dexamethasone was IGFBP-1. IGFBP-1 mRNA was increased to a similar extent (7-fold), as determined by Northern blot hybridization using human or rat IGFBP-1 cDNA probes. The stimulation of IGFBP-1 mRNA was observed within 3 h after the addition of dexamethasone; IGFBP-1 in the medium increased more slowly. After withdrawal of dexamethasone from stimulated cells, IGFBP-1 mRNA decreased by 80% after 48 h; IGFBP-1 decreased more slowly. The increased abundance of IGFBP-1 mRNA in dexamethasone-treated cells primarily reflected increased transcription rather than increased mRNA stability.(ABSTRACT TRUNCATED AT 250 WORDS)

Different tissue distribution and hormonal regulation of messenger RNAs encoding rat insulin-like growth factor-binding proteins-1 and -2.

The insulin-like growth factor-binding proteins IGFBP-1 and IGFBP-2 are low mol wt IGFBPs that are similar in structure. They are not glycosylated and have a homologous amino acid sequence, including the number and position of 18 cysteine residues and a carboxyl-terminal Arg-Gly-Asp sequence that can be recognized by cell adhesion receptors. The present study demonstrates that expression of mRNAs encoding the two BPs differs in some fetal rat tissues and in the livers of adult rats after hypophysectomy, fasting, or streptozotocin-induced diabetes. As determined by Northern blot hybridization using cDNA probes for rat IGFBP-2 or human IGFBP-1, both mRNAs are expressed at high levels in liver of 21-day gestation and 1-day-old rats and at lower levels in 21- and 65-day-old rat liver. Levels of both mRNAs are higher in liver than in other fetal rat tissues. The relative abundance of the two mRNAs in most fetal tissues is similar to that in liver, except that kidney and brain have 8-fold and more than 25-fold higher relative levels of IGFBP-2 mRNA, respectively. IGFBP-2 mRNA is about 10- to 20-fold increased after hypophysectomy or fasting, whereas IGFBP-1 mRNA is relatively unchanged. IGFBP-2 mRNA levels are decreased completely by refeeding fasted rats for 3 days, but only partially decreased by treatment of hypophysectomized rats with GH, cortisone acetate, T4, and testosterone for 4 days.(ABSTRACT TRUNCATED AT 250 WORDS)

Tissue, developmental, and metabolic regulation of messenger ribonucleic acid encoding a rat insulin-like growth factor-binding protein.

Insulin-like growth factor-II (IGF-II) is the predominant insulin-like growth factor in fetal and neonatal rat serum and tissues. In serum, it occurs complexed to a 30-kDa nonglycosylated IGF-binding protein (IGFBP) that is immunologically related to the IGFBP in BRL-3A rat liver cells (rIGFBP-2). Levels of rIGFBP-2 and IGF-II decrease in rat serum after birth. Using a recently isolated cDNA clone for rIGFBP-2 as hybridization probe, we now compare the expression of rIGFBP-2 and IGF-II in fetal tissues and the effects of hypophysectomy and fasting on the abundance of these mRNAs in adult rat liver. rIGFBP-2 mRNA is expressed at high levels in term gestation liver and at lower levels in other tissues. The ratio of rIGFBP-2 to IGF-II mRNAs in stomach, kidney, and lung is similar to that seen in liver, whereas IGF-II mRNA is more abundant than rIGFBP-2 mRNA in muscle, intestine, heart, and skin. Both mRNAs are more abundant in fetal tissues than in the corresponding tissues from adult rats. Dexamethasone treatment of 4-day-old rats for 4 days caused a greater (90%) decrease in hepatic IGF-II mRNA than in rIGFBP-2 mRNA (50%), suggesting subtle differences in the developmental regulation of the two mRNAs. Even more striking differences were observed in the regulation of the two mRNAs in adult rats after hypophysectomy or fasting. Hepatic rIGFBP-2 mRNA was increased 10- to 20-fold compared to age-matched control rats, whereas IGF-II mRNA was not increased. A parallel increase in serum rIGFBP-2 was observed, suggesting that this regulation may result at least in part from the increased abundance of rIGFBP-2 mRNA. Thus, in addition to modulating the stimulation of growth and differentiation by IGF-II in fetal tissues, rIGFBP-2 may play a homeostatic role during catabolic states in the adult rat.

Identification of rat cell lines that preferentially express insulin-like growth factor binding proteins rlGFBP-1, 2, or 3.

The bioavailability and action of the insulin-like growth factors (IGFs) are determined by specific IGF-binding proteins (IGFBP) to which they are complexed. Complementary DNA clones have been isolated that encode three related IGFBPs: human IGFBP-1 (hIGFBP-1), human IGFBP-3 (hIGFBP-3), and rat IGFBP-2 (rIGFBP-2). IGFBP-1 and IGFBP-3 are regulated differently in human plasma, suggesting that they have different functions. In order to study the molecular basis of the regulation of the different IGFBPs, we have identified a panel of rat cell lines that express a single predominant binding protein and developed an assay strategy to distinguish the different binding proteins. Proteins in conditioned medium were examined by ligand blotting, and by immunoprecipitation and immunoblotting using antibodies to rIGFBP-2 and hIGFBP-1; RNAs were hybridized to cDNA probes for rIGFBP-2 and hIGFBP-1. 1) C6 glial cells and B104 neuroblastoma cells express an approximately 40 kilodalton (kDa) glycosylated binding protein that most likely represents rIGFBP-3, the binding subunit of the 150 kDa IGF: binding protein complex in adult rat serum. The C6 and B104 binding proteins do not react with antibodies to rIGFBP-2, and RNAs from C6 and B104 cells do not hybridize to cDNA probes for rIGFBP-2 or hIGFBP-1. 2) BRL-3A, Clone 9, and TRL 12-15 cell lines derived from normal rat liver express rIGFBP-2, a 30 kDa nonglycosylated IGF-binding protein that is recognized by antibodies to rIGFBP-2 but not by antibodies to hIGFBP-1. RNAs from these cells hybridize to a rIGFBP-2 cDNA probe, but not to a hIGFBP-1 probe. 3) H35 rat hepatoma cells express a 30 kDa nonglycosylated IGFBP that is presumptively identified as rIGFBP-1. It does not react with antibodies to rIGFBP-2, but is recognized by polyclonal and monoclonal antibodies to hIGFBP-1. RNA from H35 cells hybridizes to a hIGFBP-1 cDNA probe, but not to a rIGFBP-2 probe. Expression of rIGFBP-1 by the H35 cell line has enabled us to establish and validate specific assays for this protein that allow us to study its regulation in intact rats. Identification of a panel of rat cell lines expressing specific IGFBPs should be useful in elucidating the molecular mechanisms of IGFBP regulation.

The fetal rat binding protein for insulin-like growth factors is expressed in the choroid plexus and cerebrospinal fluid of adult rats.

The biological effects of the insulin-like growth factors, IGF-I and IGF-II, on their receptors are modulated by IGF-binding proteins. Recently, we isolated a cDNA clone for one member of the family of IGF-binding proteins, BP-3A, a 30 kilodalton (kDa) protein synthesized by the BRL-3A rat liver cell line. BP-3A is related to but distinct from two other cloned IGF-binding proteins, the human amniotic fluid binding protein and the glycosylated binding subunit of the 150 kDa IGF-binding protein complex in serum. It is expressed in multiple nonneural tissues and in serum in the fetal rat and decreases after birth, similar to the developmental pattern of IGF-II expression. IGF-I, IGF-II, and their receptors are expressed in brain. The present study examines the expression of BP-3A in the rat central nervous system. By Northern blot analysis, BP-3A mRNA is present at high levels in brain stem, cerebral cortex, and hypothalamus from 21-day gestation rats and, like IGF-II mRNA, persists in adult rat brain. The site of BP-3A mRNA synthesis was localized by in situ hybridization to coronal sections of adult rat brain using 35S-labeled oligonucleotides, 48 bases in length, complementary and anticomplementary to the coding region of BP-3A. Specific hybridization of the BP-3A probe was observed exclusively to the choroid plexus extending from the level of the medial preoptic nucleus to the arcuate nucleus of the hypothalamus, similar to the previously reported preferential localization of IGF-II mRNA to the choroid plexus. Synthesis of BP-3A mRNA by choroid plexus suggested that BP-3A might be secreted into the cerebrospinal fluid. A 30 kDa IGF-binding protein was demonstrated in rat cerebrospinal fluid that is recognized by antibodies to BP-3A and, like purified BP-3A, has equal affinity for IGF-I and IGF-II. By analogy with other transport proteins synthesized by the choroid plexus, BP-3A may facilitate the secretion of IGF-II to the cerebrospinal fluid and modulate its biological actions at distant sites within the brain.

Structure, specificity, and regulation of the insulin-like growth factor-binding proteins in adult rat serum.

Insulin-like growth factor-I (IGF-I), the principal IGF in adult rat serum, occurs complexed to specific binding proteins. After fractionation of serum on Sephadex G-200 at neutral pH, 62% of the immunoreactive IGF-I is recovered in the 150K region, 38% in the 40K region, and none is present as free 7.5K IGF-I. Adult rat serum also contains unoccupied binding sites for IGFs that also are predominantly (77%) located in the 150K region and have preferential binding affinity for IGF-II. IGF-binding protein components in the 150K and 40K regions were evaluated by affinity cross-linking to 125I-labeled IGFs and by ligand blotting (i.e. incubation of nitrocellulose blots of sodium dodecyl sulfate (SDS)-gels with [125I]IGFs). Affinity cross-linking of the 150K region revealed a major 43K binding protein complex and several minor covalent complexes of 97-210K that are formed during the cross-linking reaction. The 40K region of the gel filtration column contains a predominant 32K complex and smaller amounts of the 43K complex. Ligand blotting of the 150K region identifies a predominant cluster of binding components of about 40K and a smaller 29K protein. The apparent molecular masses of the 40K and 29K proteins are decreased by incubation with N-glycanase, indicating that they contain N-linked oligosaccharides. These glycoprotein components, designated gp40 and gp29, presumably combine with an acid-labile nonbinding subunit of about 100K to generate the 150K complex. The gp40 cluster represents glycosylation variants of a 34K protein; gp29 has been shown to correspond to an amino-terminal fragment of gp40. Ligand blotting of the 40K region indicates that it contains smaller amounts of gp40 and gp29, possibly representing free subunits not combined with the nonbinding subunit, as well as two proteins of apparent molecular mass 24K and 30K (p24 and p30) that are not glycosylated. Although p30 is similar in size to the binding protein from BRL-3A cells (BP-3A) that is present in fetal rat serum, immunoprecipitation and immunoblotting of whole and fractionated adult serum with an antiserum to BP-3A indicate that p30 in adult rat serum is an antigenically distinct protein. Serum levels of gp40 and gp29 are decreased by hypophysectomy and are restored by GH treatment; p24 and p30 show similar but smaller changes.

Actions of insulin-like growth factor-I on the B104 neuronal cell line: effects on cell replication, receptor characteristics, and influence of secreted binding protein on ligand binding.

Several peptide growth factors influence the growth and differentiation of neural cells. To investigate further the growth-promoting effects of the somatomedins on cells of neural origin, the authors characterized the binding and mitogenic effects of insulin-like growth factor-I (IGF-I) on a functionally differentiated rat neuronal cell line (B104). Specific, high-affinity (Kd approximately equal to 10(-9) M) receptors for IGF-I were abundant (approximately 124,000 binding sites/B104 cell). These IGF-I receptors were similar to those of non-neural tissue in that they contained 135,000 dalton binding subunits (demonstrated by affinity labeling and autoradiography) and recognized insulin at high concentrations. IGF-I was more potent than insulin at stimulating B104 cell replication in serum-free medium and, at an initial concentration of 100 ng/ml, was the only exogenous growth factor needed to maintain growth through several cell divisions. Furthermore, cells of later passage were found to secrete specific IGF binding proteins that produced an unusual, biphasic binding curve in radioligand displacement studies. These binding proteins apparently sequester IGF-I, limiting its access to the cell. Experiments with B104 cells may provide useful information about the role of IGFs and their binding proteins as potential regulators of growth and differentiation of the primitive neuroblast.

Nucleotide sequence and expression of a cDNA clone encoding a fetal rat binding protein for insulin-like growth factors.

The insulin-like growth factors (IGFs), IGF-I and IGF-II, occur in plasma and tissue fluids complexed to specific binding proteins. Although the role of the binding proteins is not completely defined, they are capable of modulating the biological activity of the IGFs. In order to better understand the function of these proteins, we have isolated a clone from the BRL-3A rat liver cell line that encodes a protein corresponding to the IGF binding protein in fetal rat serum. The cDNA clone encodes a precursor protein of 304 amino acids (32,886 daltons), comprised of a 34-residue hydrophobic prepeptide and a 270-residue mature protein (29,564 daltons). The deduced amino acid sequence agrees with the sequence of 173 amino acid residues determined by Edman degradation. The mature protein contains 18 cysteines and no N-glycosylation sites. It contains an Arg-Gly-Asp (RGD) sequence near the carboxyl terminus. A similar sequence is present on many extracellular matrix proteins and contributes to their recognition by cellular adhesion receptors. The cloned cDNA has been transcribed in vitro and the resulting RNA expressed in Xenopus oocytes. Injected oocytes secrete a 33-kDa protein that is immunoprecipitated by polyclonal antibodies to the BRL-3A binding protein and binds IGF-I and IGF-II with the same affinity and specificity as does purified BRL-3A binding protein. The binding protein cDNA probe hybridizes to an approximately 2-kilobase mRNA in BRL-3A cells and in multiple fetal rat tissues including liver, kidney, intestine, and lung. Levels of this mRNA are greatly reduced in the corresponding adult tissues. The rat IGF binding protein is closely related to the partial amino acid sequences reported for a bovine IGF binding protein and more distantly related to a human IGF binding protein that recently has been cloned. No significant homologies were identified to other proteins. Thus, the rat IGF binding protein that we have cloned appears to be a distinct member of a family of related IGF binding proteins. We postulate that the structurally distinct IGF binding proteins may have different biological functions.

Discordance of serum and tissue somatomedin levels in growth hormone-stimulated growth in the rat.

Although the somatomedins are believed to mediate GH-induced somatic growth, circulating levels of insulin-like growth factor I (IGF-I) do not always correlate with growth rate. To evaluate a paracrine or autocrine effect that might explain this discordance, the concentrations of plasma and tissue IGF-I were compared with indices of growth in hypophysectomized rats treated with rat GH (rGH). The rats received a total of 250 micrograms rGH by either continuous pump infusion (P) or twice daily injections (I). After 4 days of treatment, the amounts of IGF-I present in acetic acid extracts of liver and kidney and in native serum were determined by RIA and related to proximal tibial epiphyseal plate width. Tibial epiphyseal plate width increased from 198 +/- 6 microns (mean +/- SE) in untreated controls to 339 +/- 13 microns in group P and 347 +/- 9 micron in group I; weight gain was 11.6 +/- 1.0 g in group P and 9.6 +/- 0.8 g in group I, while control animals lost 1.0 g. Serum concentrations of IGF-I were no different between control animals and those in the injection group (0.91 +/- 0.06 vs. 0.90 +/- 0.06 U/ml), whereas levels in the infusion group increased slightly (1.1 +/- 0.05 U/ml). In contrast, rGH administration caused tissue IGF-I to double in liver (control, 0.24 +/- 0.04 U/g; P, 0.51 +/- 0.03 U/g; I, 0.46 +/- 0.05 U/g) and nearly triple in kidney (control, 0.52 +/- 0.05 U/g; P, 1.51 +/- 0.09 U/g; I, 1.36 +/- 0.08 U/g). There was no detectable change in somatomedin-binding protein by gel exclusion chromatography. Since the rGH administered was sufficient to stimulate growth and increase tissue somatomedin levels without corresponding increases in circulating IGF-I, an autocrine or paracrine action of IGF-I appears to mediate GH's initial somatogenic actions in the young rat.