Relationship among insulin resistance, growth hormone, and insulin-like growth factor I concentrations in diestrous Swedish Elkhounds.

BACKGROUND: In the dog, the normal estrous cycle includes a prolonged luteal phase. Progesterone stimulates local canine mammary growth hormone (GH) production, which may act systemically and contribute to insulin resistance. Swedish Elkhounds are predisposed to progesterone-related diabetes mellitus, and the relationship among insulin resistance, GH, and insulin-like growth factor I (IGF-I) is of particular interest. OBJECTIVE: To study insulin resistance in relation to GH and IGF-I in nondiabetic Swedish Elkhounds during diestrus. We also assessed whether alterations in these hormones could predict diestrus-linked diseases and all-cause mortality. ANIMALS: Eighty-four privately owned female intact Swedish Elkhounds >4 years of age. METHODS: Blood sampling and clinical examination during luteal phase, with a follow-up questionnaire after 20 months. Insulin resistance was calculated by homeostasis model assessment (HOMA-IR). RESULTS: In multivariable regression analysis, GH was positively associated with HOMA-IR (P = .009). An increase in GH of 1 ng/mL was associated with a 12.7% increase in HOMA-IR. Moreover, C-peptide was positively associated with IGF-I (P = .04), and an increase in C-peptide of 0.1 ng/mL was associated with a 6.9% increase in IGF-I. Structural equation modeling supported these results. Twenty-three animals were found to have previously unrecognized mammary masses and had higher GH (P < .0001) and IGF-I (P = .007) than dogs without mammary masses (n = 61). There was no association between high GH and IGF-I concentrations at sampling and future mammary masses. CONCLUSION: We showed that GH was strongly associated with insulin resistance in older Swedish Elkhounds during diestrus.

Insulin-like growth factor-binding protein-1 in the prediction and development of type 2 diabetes in middle-aged Swedish men.

AIMS/HYPOTHESIS: Insulin-like growth factor-binding protein-1 (IGFBP-1) production in the liver is inhibited by insulin, and low circulating levels are associated with the metabolic syndrome. The aim of this study was to evaluate the predictive role and change in IGFBP-1 concentrations during development of abnormal glucose regulation. METHODS: IGFBP-1 levels were determined at baseline and at 10 years in an incident case-control prospective study of Swedish white men aged 35-56 years. Individuals with normal glucose tolerance at baseline who developed abnormal glucose tolerance during a 10 year period (n = 355) according to WHO criteria were pair-matched to controls for age and family history of diabetes. RESULTS: Fasting IGFBP-1 concentrations were lower in individuals who later developed abnormal glucose regulation and correlated inversely with fasting proinsulin values (r = -0.48; p < 0.0001), and both were significant predictors. Individuals in the highest quartile at baseline for an algorithm incorporating fasting IGFBP-1, blood glucose, proinsulin and waist and height had a 40-fold increased risk of developing type 2 diabetes compared with the lowest quartile (95% CI 7.7-214). IGFBP-1 increased 32% (95% CI 17-49%) during the 10 years in those developing diabetes and was increased in relation to insulin levels, suggesting the emergence of hepatic insulin resistance. Moreover, elevated IGFBP-1 levels at follow-up were associated with higher 2 h glucose values during an OGTT. CONCLUSIONS/INTERPRETATION: Low IGFBP-1 predicts the development of abnormal glucose regulation and, as an inhibitor of the insulin-like actions of insulin-like growth factors, elevated levels of IGFBP-1 after the development of diabetes may also play a pathophysiological role.

Specific cleavage of insulin-like growth factor-binding protein-1 by a novel protease activity.

Insulin-like growth factor-binding protein-1 (IGFBP-1) is secreted in a highly phosphorylated form that binds IGF-I with high affinity and is resistant to proteolysis. We have purified IGFBP-1-specific protease activity from the urine of an individual with multiple myeloma. This protease efficiently cleaves both phosphorylated and non-phosphorylated IGFBP-1 at Ile130-Ser131, generating fragments that together have higher association and dissociation rates for IGFs compared with intact IGFBP-1. The proteolytic fraction contained azurocidin, a protease homologue hitherto considered inactive. After cleavage of IGFBP-1, there was a lower affinity, but higher capacity for IGF-I binding, suggesting both N- and C-terminal fragments may interact with ligand independently. There was decreased inhibition of IGF-II-stimulated cell growth and glucose uptake. Alone, proteolysed IGFBP-1 stimulated glucose uptake in muscle. We conclude that specific cleavage of IGFBP-1 at target tissues is important in cellular growth and metabolism and opens novel strategies for targeting IGFBP-1 in treatment of disease.

Gender difference in the leptin response to feeding in peroxisome-proliferator-activated receptor-alpha knockout mice.

OBJECTIVE: Peroxisome-proliferator-activated receptor-alpha (PPARalpha) has a central role in lipid metabolism. Mice lacking PPARalpha accumulate hepatic fat and are prone to late onset obesity. Leptin, an adipocyte-derived hormone, also plays an important role in regulating energy balance. In order to test the hypothesis that leptin secretion increases in response to PPARalpha knockout, we determined leptin concentrations including the effect of nutritional status in male and female PPARalpha knockout mice compared with wild-type controls. DESIGN: We studied the effect of 16 h fasting and 4 h refeeding on plasma leptin concentrations in male and female wild-type and PPARalpha-knockout mice, aged 14 weeks. In female mice the effect of daily growth hormone (GH) injection on the leptin response to refeeding was determined. RESULTS: Circulating leptin concentrations were higher in female mice compared with males and increased in both sexes after PPARalpha-knockout. There was no change in leptin levels after a 16 h fast, compared with ad libitum feeding. However leptin increased with refeeding, to the greatest extent in female PPARalpha-knockout mice. Intermittent GH administration decreased leptin concentrations in female, wild-type and PPARalpha-knockout animals and abolished the exaggerated leptin response to refeeding. CONCLUSIONS: Leptin concentrations are increased in PPARalpha-knockout mice. There are gender differences in the leptin response to feeding which may be due to differences in insulin sensitivity.

Lithium chloride inhibits the expression and secretion of insulin-like growth factor-binding protein-1.

Insulin-like growth factor-binding protein-1 (IGFBP-1) regulates IGF availability for glucose homeostasis. The IGFBP-1 promoter shares common regulatory response elements with phosphoenol pyruvate carboxykinase (PEPCK), the expression and activity of which is inhibited by lithium chloride, associated with an inhibition of glycogen synthase kinase (GSK)-3 activity, in the rat hepatoma cell line H4-II-E. We therefore determined the effect of lithium chloride on IGFBP-1 expression and secretion in H4-II-E cells. Lithium chloride inhibited IGFBP-1 secretion in a dose response and reversible manner by approx 80% during 5-h and 16-h incubations. An inhibitory effect on IGFBP-1 mRNA expression was observed at 2 h. The inhibitory effect of lithium and insulin were not additive when used alone, but inhibition by lithium occurred when insulin action was blocked by activating AMP-activated protein kinase with 5-aminoimidazole-4-carboxamide-riboside (AICAR). These findings suggest that GSK-3 inhibition, or another pathway activated by lithium, may be involved in a pathway controlling IGFBP-1, inhibiting synthesis when insulin activity is absent or impaired.

Responses of insulin-like growth factor (IGF)-I and IGF-binding proteins to nutritional status in peroxisome proliferator-activated receptor-alpha knockout mice.

Peroxisome proliferator-activated receptor alpha (PPARalpha) plays a central role in glucose and lipid homeostasis. Mice lacking PPARalpha(-/-) have a sexually dimorphic phenotype. We have characterized the IGF system in wild type and PPARalpha-/- mice. In normal mice fasting IGF-I and the IGFBP-3 ternary complex were 2-fold higher in males than in females. PPARalpha influenced the IGF/IGFBP response to feeding, particularly in males. Compared to wild type, male PPARalpha-/- mice had 40% lower total fasting IGF-I concentrations, decreased ALS and less IGFBP-3 ternary complex formation, but within 4 h of refeeding there was an increase in IGF-I and IGFBP-3 ternary complex to values similar to controls. Circulating IGFBP protease activity was induced in male PPARalpha-/- mice during refeeding. IGFBP-1 and insulin concentrations were higher in males than females, and were increased by PPARalpha knockout, suggesting significant hepatic insulin resistance. We speculate that gender differences in the IGF system contribute to the PPARalpha-/- phenotype.

Stimulation of IGF-binding protein-1 secretion by AMP-activated protein kinase.

Insulin-like growth factor-binding protein-1 (IGFBP-1) is stimulated during intensive exercise and in catabolic conditions to very high concentrations, which are not completely explained by known regulators such as insulin and glucocorticoids. The role of AMP-activated protein kinase (AMPK), an important signaling system in lipid and carbohydrate metabolism, in regulating IGFBP-1 was studied in H4-II-E rat hepatoma cells. Arsenic(III) oxide and 5-aminoimidazole-4-carboxamide-riboside (AICAR) were used as activators. AICAR (150 microM) stimulated IGFBP-1 secretion twofold during a 5-h incubation (P = 0.002). Insulin (100 ng/ml) inhibited IGFBP-1 by 80% (P < 0.001), but this was completely abolished in the presence of 150 microM AICAR. The effect of dexamethasone in stimulating IGFBP-1 threefold was additive to the effect of AICAR (P < 0.001) and, in the presence of AICAR, was incompletely inhibited by insulin. In conclusion AMPK is identified as a novel regulatory pathway for IGFBP-1, stimulating secretion and blocking the inhibitory effect of insulin.

Time course changes in IGFBP-1 after treadmill exercise and postexercise food intake in rats.

Prolonged exercise increases circulating insulin-like growth factor binding protein-1 (IGFBP-1) in humans and animals, but its physiological significance is unknown. This study examined 1) time-course changes in plasma IGFBP-1 and hepatic IGFBP-1 mRNA expression after exercise, 2) changes in IGFBP-1 in relation to plasma glucose, insulin, and IGF-I, and 3) the impact of feeding a postexercise meal on the IGFBP-1 response. Food-deprived male rats were vigorously run on a treadmill and compared with nonexercised controls at 15 min and 1, 4, 8, and 12 h after exercise. Circulating insulin concentrations in exercised rats were lower than in controls at 15 min and 1 h, whereas plasma glucose and IGF-I remained unaffected. Circulating and hepatic expression of IGFBP-1 was markedly increased above that of controls at 15 min, 1 h, and 12 h. In a separate experiment, one-half of the exercised animals received a nutritionally complete meal immediately after the experimental run. The meal elevated plasma insulin and glucose concentrations at 15 min and 1 h. Despite this change in nutritional status, serum IGFBP-1 concentrations and hepatic IGFBP-1 abundance remained elevated at 15 min and 1 h. These results demonstrate that the IGFBP-1 response to a single bout of treadmill exercise is short in duration and independent of insulin, glucose, and amino acid availability.

Regulation of insulin-like growth factor-binding protein-3 ternary complex in feline diabetes mellitus.

The 140 kDa ternary complex of insulin-like growth factor-binding protein-3 (IGFBP-3), IGFs and an acid-labile subunit (ALS) has previously been shown to be decreased in diabetes mellitus in humans and rats. We have studied IGF-I levels and ternary complex formation in normal and diabetic cats. Total IGF-I concentrations, measured by RIA using des(1-3)-IGF-I as tracer were (+/-s.e.m.) 54+/-13 nmol/l in eight normal and 227+/-57 nmol/l in eight diabetic cats (P<0.01). The size-distribution of IGFBPs in the cat circulation was determined by incubation with (125)I-IGF-II and Superose 12 chromatography. In normal animals 26+/-2% of the (125)I-IGF-II were in a 140 kDa form compared with 48+/-5% in diabetic cats (P<0.01). When samples from normal and diabetic animals were co-incubated 52+/-3% were at 140 kDa. A similar shift was seen when normal cat and normal human serum were co-incubated. A 2-fold increase in the 140 kDa form in diabetic cats was confirmed first by size-fractionating samples and then performing a ligand-binding assay with (125)I-IGF-I or -II and charcoal separation. SDS-PAGE and Western ligand blotting demonstrated a 45 kDa doublet (presumably IGFBP-3) and 30-35 kDa forms. There were no apparent differences between normal and diabetic profiles on SDS-PAGE, suggesting that a proportion of IGFBP-3 which circulates 'free' in normal cats forms a ternary complex in the diabetic circulation. We conclude that (i) in contrast to humans and rats, ALS is the limiting factor for ternary complex formation in normal cats, (ii) ALS concentrations increase in feline diabetes mellitus and, by promoting ternary complex formation, this leads to an increase in total IGF-I concentrations, and (iii) total IGF-I concentrations may not be reliable in the diagnosis of acromegaly in diabetic cats.

Serum free insulin-like growth factor-I is dose-dependently decreased by methylprednisolone and related to body weight changes in rats.

Glucocorticoids usually inhibit growth despite a paradoxical increase in total IGF-I. To investigate the effect of methylprednisolone on free IGF-I, rats were treated with for 3 days (0, 1, 2, 4, and 6 mg/kg per day). A dose-dependent decrease in ultrafiltrated serum free IGF-I was observed, being lowest after 6 mg/kg (P < 0.001 all groups vs controls). Total IGF-I was increased in the groups receiving 2 mg/kg (P < 0.05). Weight change in the 24 h prior to blood sampling was positively correlated with free IGF-I (R = 0.46, P = 0.0002), but not with total IGF-I. Immunoassayable IGFBP-1 was decreased in rats given 4 mg/kg (P = 0.001), whereas there was no change in IGFBP-3 or acid-labile subunit. We propose that in rats the glucocorticoid-induced weight loss may in part be due to suppression of circulating free IGF-I.

Regulation of insulin-like growth factor-binding protein-3 ternary complex formation in pregnancy.

The IGFs are believed to be important in pregnancy and are implicated in the pathophysiology of pre-eclampsia. In adults the IGFs circulate primarily with IGF-binding protein-3 (IGFBP-3) and an acid-labile glycoprotein (ALS) in a 140 kDa complex which limits IGF bioavailability. Less than 10% of IGFBP-3 is in lower molecular weight forms. We have investigated the developmental regulation of the IGF/IGFBP system in normal and pre-eclamptic pregnancies with particular emphasis on the IGFBP-3 ternary complex. Circulating levels of IGF-I, IGFBP-3 and ALS, and their degree of association in the ternary complex in the fetus increased with gestational age. In neonatal serum from deliveries <35 weeks' gestation IGFBP-3 was predominantly in 30-50 kDa form(s) and ALS was a limiting factor for ternary complex formation. In serum from deliveries >35 weeks both ALS and IGFs were limiting but approximately 25% of IGFBP-3 was unable to form the ternary complex even in the presence of excess ALS and IGF-I. Serum IGFBP-1, -2 and -6 concentrations tended to decrease with increasing gestational age. In pre-eclamptic pregnancies, amniotic fluid IGFBP-2, -3 and -6 levels decreased with gestational age while IGFBP-1 levels did not show the normal decline. We speculate that the endocrine IGF system develops in the fetus during the third trimester of pregnancy when ALS levels increase.

Factors regulating the rat insulin-like growth factor-binding protein-1 response to octreotide.

Insulin-like growth factor-binding protein-1 (IGFBP-1) production is increased by somatostatin and its analogues. In order to determine the time course and identify possible mechanisms of this increase in vivo we administered octreotide to rats and determined IGFBP-1 concentrations by RIA. After 60 min of anaesthesia, the mean baseline IGFBP-1 concentrations were 166 (95% confidence interval 123 to 225) ng/ml and increased in saline-infused animals to 729 (488 to 1086) ng/ml after 180 min. IGFBP-1 was stimulated transiently in response to octreotide, with circulating IGFBP-1 concentrations peaking at 1605 (1220 to 2111) ng/ml at 105 min during a continuous infusion of octreotide (100 micrograms/kg per h). In conscious chronically cannulated rats, baseline IGFBP-1 concentrations were 22 (18 to 28) ng/ml, 8-fold less than in the anaesthetised state, and were stimulated in the short term after administration of an octreotide bolus (100 micrograms/kg s.c.) to reach 88 (62 to 126) ng/ml at 60 min. A similar response was seen after i.v. administration to conscious rats. Intravenous bolus of octreotide (100 micrograms/kg) in rats anaesthetised for 3 h resulted in an increase in IGFBP-1 to peak at 1556 (1268 to 1910) ng/ml at 60 min. The IGFBP-1 response to octreotide was diminished in high-fat fed hyperinsulinaemic rats. The pattern of disappearance of iodinated IGFBP-1 from the circulation was not influenced by octreotide. The changes in GH, insulin and glucose concentrations alone did not sufficiently account for the patterns of response observed. We conclude that, in rats, octreotide stimulates IGFBP-1 acutely and this response is potentiated by factors related to anaesthesia.

Interaction of insulin, glucocorticoids, and protein kinase C in the regulation of insulin-like growth factor-binding protein-1 production by H4IIE rat hepatoma cells.

A sensitive RIA was used to examine regulation of IGFBP-1 in H4IIE rat hepatoma cells. IGFBP-1 was stimulated up to tenfold by dexamethasone and corticosterone, and this stimulation was abolished by RU486. The effect of dexamethasone increased with time in culture. Phorbol 12-myristate 13-acetate (PMA) stimulated IGFBP-1 up to fourfold with a maximal effect in short-term culture. Dexamethasone and PMA were additive in stimulating IGFBP-1. Under basal conditions IGFBP-1 production was linearly related to cell density: however, stimulation by dexamethasone was greatest in confluent cells, and PMA had a greater effect in sparse cultures. Insulin inhibited IGFBP-1 up to 80%, and this effect diminished with time in culture but was unaffected by cell density. Dexamethasone was stimulatory in the presence of a maximal inhibitory concentration of insulin, and insulin was inhibitory in the presence of maximal dexamethasone from 3-48 h in culture, regardless of cell density. PMA abolished the inhibitory action of insulin on IGFBP-1 secretion and mRNA expression during incubation periods of less than 4 h and not during longer incubations. PMA did not influence the stability of IGFBP-1 mRNA. We conclude that, in rat H4IIE cells, dexamethasone and PMA stimulate IGFBP-1 by independent mechanisms and speculate that when protein kinase C is activated the inhibitory action of insulin is blocked.

Growth retardation and hyperglycemia in insulin-like growth factor binding protein-1 transgenic mice.

Transgenic mice that constitutively overexpressed rat insulin-like growth factor binding protein-1 (IGFBP-1) were generated to determine the effects of overexpression of IGFBP-1 on growth and development. In offspring of three of the founders that showed high levels of expression, birth weight was significantly reduced to approximately 83-92% of the weight of their nontransgenic littermates. The transgenic mice gained less weight and were approximately 3.5-8 g lighter than nontransgenic littermates at 40 days of age. The difference in body weight between transgenic and wild-type mice was most apparent around the time of weaning when transgenic mice showed a more marked growth deceleration than wild-type mice. No significant catch-up growth was apparent over the first 3 months of life. In addition, offspring of all three high-expressing founders demonstrated fasting hyperglycemia. The transgene was highly expressed in the brain, uterus, lung, kidney, and heart, but little expression was detected in the liver. The weight of the brain relative to body weight was significantly reduced in transgenic mice compared with wild-type mice, whereas the relative weight of most other organs was similar to wild-type mice. These data demonstrate that IGFBP-1 may function to inhibit IGF action in vivo and that this inhibition selectively impairs development of organs such as the brain.

Developmental regulation of circulating insulin-like growth factor-binding proteins in normal pregnancies and in pre-eclampsia.

The insulin-like growth factors (IGFs) and their binding properties (IGFBPs) are believed to play important roles in the growth and development of the human fetus. They have been implicated in the pathophysiology of pre-eclampsia. In this study we have characterized the developmental regulation, in normal and pre-eclamptic pregnancies, of IGFs and IGFBPs in maternal serum, neonatal serum and amniotic fluid. In neonatal cord serum IGFBP-1, -2 and -6 decreased with increasing gestational age. In contrast, the ternary complex and its components, IGF-I, IGFBP-3 and ALS increased with gestation. We show that while ALS is an important limiting factor for ternary complex formation in the fetal circulation, there is a fraction of IGFBP-3 which is unable to form this complex. IGFs and IGFBPs in the maternal and fetal circulation were similar in normal and pre-eclamptic pregnancies.

Growth hormone (GH) regulation of circulating insulin-like growth factor-I levels during sexual maturation of the GH-deficient dwarf (dw/dw) male rat.

In many mammalian species, circulating levels of insulin-like growth factor-I (IGF-I) rise during puberty. Previous studies manipulating testosterone levels in rats with normal GH secretion suggested that the pubertal IGF-I rise is regulated by an interaction between GH and sex steroids. Therefore, in a reciprocal study, IGF-I levels were examined during sexual maturation of the GH-deficient dwarf (dw/dw) rat which has a selective genetic deficiency of GH but normal sex steroid levels. Male dw/dw rats were treated with daily injections of recombinant human GH (200 micrograms/100 g body weight) or saline vehicle, from 28 to 70 days of age. Sexual maturation was determined to occur primarily between 42 and 63 days of age based on testis and seminal vesicle growth and plasma testosterone levels. GH treatment had no effect on seminal vesicle weights, plasma testosterone or gonadotrophins. GH administration resulted in a 7% increase in absolute testes weight (P < 0.05), but a 50% increase in body weight (P < 0.0001). These results supported previous findings that the reproductive development of dw/dw rats is essentially normal. Untreated dw/dw rats had no rise in IGF-I levels during sexual maturation. In contrast, treatment with GH produced a marked sustained rise in IGF-I levels (P < 0.0001). LIgand blots demonstrated GH induction of IGF-binding protein-3 (IGFBP-3) and an IGFBP cluster at 32 kDa. The initially high immunoreactive IGFBP-1 levels (> 600 ng/ml) decreased by 49 days of age after which untreated dw/dw rats had significantly higher IGFBP-1 levels than GH-treated dw/dw rats (P < 0.01).(ABSTRACT TRUNCATED AT 250 WORDS)

Complex formation by human insulin-like growth factor-binding protein-3 and human acid-labile subunit in growth hormone-deficient rats.

Insulin-like growth factor-binding protein-3 (IGFBP-3), after first associating with IGF-I or IGF-II, is able to associate with the acid-labile subunit (ALS) and form a 140-kDa complex. To investigate the factors regulating ternary complex formation in vivo, human (h) IGFBP-3, hIGF-I, and hALS were administered in various combinations to GH-deficient (dw/dw) rats. hIGFBP-3 had a complex pattern of disappearance from the rat circulation, with an initial phase lasting minutes and a prolonged phase(s) lasting hours. If coinjected with hIGF-I, significantly more hIGFBP-3 was retained over 2 h. The molecular distribution of hIGFBP-3 was determined after size-separation chromatography. After an iv bolus of hIGFBP-3, 36.1 +/- 5.0% was in the 140-kilodalton complex at 5 min; this increased to 55.1 +/- 7.1% if hIGF-I was coinjected (P < 0.05). The 140-kDa complex disappeared slowly over hours, whereas 50- and 30-kDa forms of hIGFBP-3 cleared rapidly, with half-lives of minutes. To determine the importance of ALS in regulating the molecular distribution of hIGFBP-3, hALS was coinjected. Immunoreactive hALS disappeared slowly from the circulation and was shown to retain functional activity after 2 h in vivo. Coadministration of hALS did not influence the pattern of ternary complex formation, consistent with the presence of excess endogenous rat ALS. We conclude that ALS circulates in excess even in GH deficiency, is retained in the circulation for hours, and determines the stability of the 140-kDa complex, whereas IGF-I is a limiting factor in ternary complex formation by hIGFBP-3.

Role of the insulin-like growth factors in the endocrine control of glucose homeostasis.

There is a growing body of evidence that the insulin-like growth factors (IGF-I and IGF-II) are dynamically involved in the regulation of glucose homeostasis, with one of their binding proteins, IGFBP-1, playing a counterregulatory role. The IGFs are structurally and functionally related to insulin and in the circulation they represent a huge hypoglycemic potential which is buffered by their association with the IGFBPs. The predominant IGFBP in serum, IGFBP-3, is able to form a high molecular weight complex with the IGFs and this complex is retained in the circulation and appears to act as a reservoir of IGFs. The IGFs and IGFBP-3 are regulated in the long term by changes in nutritional status. In contrast, IGFBP-1 is acutely regulated in a manner similar to glucose counterregulatory hormones. IGFBP-1 is able to block the insulin-like actions of the circulating IGFs and when administered alone as a bolus infusion causes an increase in blood glucose levels. There is recent evidence that more IGFs are available for an endocrine glucoregulatory role than indicated by estimates of steady-state 'free' IGF levels. The IGF/IGFBP system may thus play a complementary role to insulin and the classical counterregulatory hormones in the control of blood glucose.

Regulation of insulin-like growth factor-binding protein-1 in rat serum.

Previous studies support a role for insulin-like growth factor-binding protein-1 (IGFBP-1) in modulating insulin-like growth factor (IGF) availability for glucose homeostasis. We have developed a radioimmunoassay (RIA) for rat IGFBP-1 (rIGFBP-1) and have examined the regulation of circulating levels by nutritional and hormonal status. Rabbit antisera were raised against pure rIGFBP-1, and an assay was established with a sensitivity of 50 pg. In the rat, serum IGFBP-1 concentrations decrease with increasing developmental age. They were highest in fetal rat serum, exceeding 4 mg/L, and decreased to < 0.1 mg/L in adult animals. Serum rIGFBP-1 levels increased during fasting, 6-fold after 24 h and 18-fold after 48 h, and were suppressed to levels identical to ad libitum-fed control rats within 2 h of refeeding. Fasting levels were > 2-fold higher in female than male animals. IGFBP-1 concentrations were suppressed by > 50% in two rat models of insulin resistance. Levels increased in STZ-induced (streptozotocin) diabetes and were suppressed to normal with insulin treatment. Exercise stimulated rIGFBP-1 concentrations in fasting animals. On immunoblotting after SDS-PAGE (sodium dodecyl sulfate-polyacrylamide gel electrophoresis), rIGFBP-1 in serum appeared as a doublet with molecular masses at 31 and 33 kD. The components of this doublet did not vary across the range of experimental conditions. These observations indicate that the pattern of regulation of rIGFBP-1 is similar to that seen in previous studies of human IGFBP-1, with age, sex, and nutritional status being important regulators.(ABSTRACT TRUNCATED AT 250 WORDS)