Muscle fatty acid oxidative capacity is a determinant of whole body fat oxidation in elderly people

In sedentary elderly people, a reduced muscle fatty acid oxidative capacity (MFOC) may explain a decrease in whole body fat oxidation. Eleven sedentary and seven regularly exercising subjects (65.6 +/- 4. 5 yr) were characterized for their aerobic fitness [maximal O(2) uptake (VO(2 max))/kg fat free mass (FFM)] and their habitual daily physical activity level [free-living daily energy expenditure divided by sleeping metabolic rate (DEE(FLC)/SMR)]. MFOC was determined by incubating homogenates of vastus lateralis muscle with [1-(14)C]palmitate. Whole body fat oxidation was measured by indirect calorimetry over 24 h. MFOC was 40.4 +/- 14.7 and 44.3 +/- 16.3 nmol palmitate. g wet tissue(-1). min(-1) in the sedentary and regularly exercising individuals, respectively (P = nonsignificant). MFOC was positively correlated with DEE(FLC)/SMR (r = 0.58, P < 0. 05) but not with VO(2 max)/kg FFM (r = 0.35, P = nonsignificant). MFOC was the main determinant of fat oxidation during all time periods including physical activity. Indeed, MFOC explained 19.7 and 30.5% of the variance in fat oxidation during walking and during the alert period, respectively (P < 0.05). Furthermore, MFOC explained 23.0% of the variance in fat oxidation over 24 h (P < 0.05). It was concluded that, in elderly people, MFOC may be influenced more by overall daily physical activity than by regular exercising. MFOC is a major determinant of whole body fat oxidation during physical activities and, consequently, over 24 h.

Facilitative glucose transporters in livestock species

The study of facilitative glucose transporters (GLUT) requires carefully done immunological experiments and sensitive molecular biology approaches to identify the various mechanisms which control GLUT expression at the RNA and protein levels. The cloning of species-specific GLUT cDNAs showed that GLUT4 and GLUT1 diverge less among species than other GLUT isoforms. The key role of GLUT in glucose homeostasis has been demonstrated in livestock species. In vitro studies have suggested specific roles of GLUT1 and GLUT3 in avian cells. In vivo studies have demonstrated a regulation of GLUTs (especially of GLUT4) by nutritional and hormonal factors in pigs and cattle, in lactating cows and goats and throughout the foetal life in the placenta and tissues of lambs and calves. All these results suggest that any changes in GLUT expression and activity (such as GLUT4 in muscles) could modify nutrient partitioning and tissue metabolism, and hence, the qualities of animal products (milk, meat).

Age-related changes and location of types I, III, XII and XIV collagen during development of skeletal muscles from genetically different animals

The ontogenesis of total collagen and of different collagen types was studied in four muscle types from genetically different cattle. Hydroxyproline content was 1.2-fold higher in muscles from cross-bred foetuses with normal muscle growth compared to those of the other genetic types (pure bred with different growth rates, double-muscled breed). A similar tendency was observed for type III collagen content. In all muscles of each animal studied, type XII and XIV collagens were colocated in perimysium. Immunolabelling obtained for type XII collagen was higher during foetal life than after birth, while for type XIV collagen, the opposite result was obtained. Whatever the muscle studied, but especially in semitendinosus muscle, during the foetal and the post-natal period until 15 months of age, immunolabelling with antibody anti-type XIV collagen tended to be more intense in muscles of animals from fathers selected for a low muscle growth capacity compared to those from fathers selected for a high muscle growth capacity. In conclusion, this study shows, that during foetal life, selection according to muscle growth capacity has no significant effect on the contents of total hydroxyproline or type III collagen, but minor effects on collagen localization.

Lipoprotein lipase activity and mRNA are up-regulated by refeeding in adipose tissue and cardiac muscle of sheep

Previous studies in rodents have shown that the lipoprotein lipase (LPL) regulation is complex and often opposite in adipose tissue (AT) and muscle in response to the same nutritional treatment. However, neither LPL responses nor the molecular mechanisms involved in the nutritional regulation have been studied in both AT and muscle of ruminant species. To explore this, we measured the LPL activity and mRNA levels in perirenal AT and cardiac muscle (CM) of control, 7-d-underfed or 14-d-refed ewes. Underfeeding decreased (P < 0.01) LPL activity both in AT (-59%) and CM (-31%), and these activities were restored (P < 0.01) by refeeding (AT, +248%; CM, +34%). Variations of LPL mRNA level measured by real-time reverse transcription-polymerase chain reaction or by Northern blot followed variations of LPL activity: underfeeding decreased AT- and CM-LPL mRNA levels (-58 and -53%, respectively), and refeeding restored (P < 0.01) them in CM (+117%) and increased them over the baseline in AT (+640%). Quantification of either 3.4- or 3.8-kb LPL mRNA levels revealed a predominant (P < 0.001) expression of the 3.4-kb mRNA in AT (60%) and of the 3.8-kb mRNA in CM (56%), without any preferential regulation of one of these mRNA species by the nutritional status. This work reveals a tissue-specific expression pattern of the ovine LPL gene and a pretranslational nutritional regulation of its expression, which is achieved in the same direction in perirenal AT and CM. The different regulation of CM-LPL between ewes and rats probably arises from peculiarities of ruminant species for nutrient digestion and absorption and liver lipogenesis.

Dietary coconut oil affects more lipoprotein lipase activity than the mitochondria oxidative capacities in muscles of preruminant calves

The presence of coconut oil in a milk replacer stimulates the growth rate of calves, suggesting a better oxidation of fatty acid in muscles. Because dietary fatty acid composition influences carnitine palmitoyltransferase I (CPT I) activity in rat muscles, this study was designed to examine the effects of a milk replacer containing either tallow (TA) or coconut oil (CO) on fatty acid utilization and oxidation and on the characteristics of intermyofibrillar (IM) and subsarcolemmal (SS) mitochondria in the heart and skeletal muscles of preruminant calves. Feeding CO did not affect palmitate oxidation rate by whole homogenates, but induced higher palmitate oxidation by IM mitochondria (+37%, P < 0.05). CPT I activity did not significantly differ between the two groups of calves. Heart and longissimus thoracis muscle of calves fed CO had higher lipoprotein lipase activity (+27% and 58%, respectively; P < 0.05) but showed no differences in fatty acid binding protein content or activity of oxidative enzymes. Whatever the muscle and the diet, IM mitochondria had higher respiration rates and enzyme activities than those of SS mitochondria (P < 0.05). Furthermore, CPT I activity of the heart was 28-fold less sensitive to malonyl-coenzyme A inhibition in IM mitochondria than in SS mitochondria. In conclusion, dietary CO marginally affected the activity of the two mitochondrial populations and the oxidative activity of muscles in the preruminant calf. In addition, this study showed that differences between IM and SS mitochondria in the heart and muscles were higher in calves than in other species studied so far.

Effects of muscle type, castration, age, and compensatory growth rate on androgen receptor mRNA expression in bovine skeletal muscle

The effect of testosterone on sexual dimorphism is evident by differential growth of forelimb and neck muscles in bulls and steers. Divergent hormone sensitivites may account for the differential growth rates of individual muscles. Therefore, the objective of this study was to compare androgen receptor (AR) expression in three different muscles of bulls and steers at various ages and growth rates. Thirty Montbéliard bulls and 30 steers were assigned to four slaughter age groups. Four or five animals of each sex were slaughtered at 4 and 8 mo of age. Animals in the remaining two slaughter groups (12 and 16 mo) were divided into groups of either restricted (R) or ad libitum (AL) access to feed. Five animals of each sex and diet were slaughtered at the end of the restricted intake period at 12 mo of age. To simulate compensatory growth, the remaining animals (R and AL) were allowed ad libitum access to feed until slaughter at 16 mo of age. Total RNA was extracted from samples of semitendinosus (ST), triceps brachii (TB), and splenius (SP) muscles. Androgen receptor mRNA was quantified in 200-ng total RNA preparations using an internally standardized reverse transcription (RT) PCR assay. Data were analyzed using 18S ribosomal RNA concentrations as a covariable. Steers had higher AR mRNA levels per RNA unit than bulls (P < .01). Androgen receptor mRNA levels differed between muscles (P < .05), with lowest expression in the SP. The pattern of AR expression differed (P < .05) for each muscle with increasing age. Between 4 and 12 mo of age, AR mRNA levels increased (P < .05) in SP but remained unchanged in the ST and TB. Feeding regimen had no effect on muscle AR expression, but steers exhibiting compensatory growth had higher AR mRNA levels than AL steers (P < .01) or bulls (P < .01). Our results show that AR expression is muscle-specific and may be modulated by circulating testicular hormones. These data suggest that the regulation of AR expression may be linked to allometric muscle growth patterns in cattle and compensatory gain in steers.

Effects of dietary coconut oil on fatty acid oxidation capacity of the liver, the heart and skeletal muscles in the preruminant calf

The oxidative capacity of the liver, the heart and skeletal muscles for fatty acids were investigated in preruminant calves fed for 19 d on a milk-replacer containing either coconut oil (CO, rich in 12:0) or tallow (rich in 16:0 and 18:1). Weights of the total body and tissues did not differ significantly between the two groups of animals but plasma glucose and insulin concentrations were lower in the CO group. Feeding on the CO diet induced an 18-fold increase in the hepatic concentration of triacylglycerols. Rates of total and peroxisomal oxidation of [1-14C]laurate, [1-14C]palmitate and [1-14C]oleate were measured in fresh tissue homogenates. Higher rates of total oxidation in liver homogenate and of peroxisomal oxidation in liver, heart and rectus abdominis muscle homogenates were observed with laurate used as substrate. Furthermore, the relative contribution of peroxisomes to total oxidation was 1.9-fold higher in the liver and in the heart with laurate than with oleate or palmitate. Finally, the peroxisomal oxidation rate of oleate was 1.5-fold higher in the hearts of calves fed on the CO diet. Whatever the tissue, citrate synthase (CS, EC 4.1.3.7) and cytochrome c oxidase (COX, EC 1.9.3.1) activities were similar between the two groups of calves but the COX: CS activity ratio was lower in the liver of the CO group. In conclusion, laurate is better catabolized by peroxisomes than long-chain fatty acids, especially in the liver. Elongation of lauric acid after partial oxidation might explain the hepatic triacylglycerol accumulation in calves fed on the CO diet.

Fat partitioning and biochemical characteristics of fatty tissues in relation to plasma metabolites and hormones in normal and double-muscled young growing bulls

Plasma metabolites and hormones, and the biochemical characteristics of four fatty tissues (FT) were studied in two groups of six normal (N) or six double-muscled (DM) Belgian Blue young growing bulls fed the same net energy amount at the same live weight and slaughtered at 10 months of age. Average daily gain and feed efficiency did not significantly differ between the two groups. However, the DM bulls exhibited a higher proportion of muscles (+22%, P < 0.01) and a reduced proportion of fat (-49%, P < 0.01) mainly in the subcutaneous FT (-80%, P < 0.05). Triiodothyronine, insulin and glucose plasma concentrations tended to be lower in DM bulls (-24%, P < 0.02; -27%, P = 0.14; -7%, P = 0.06, respectively) and were positively related to the higher fat development in N bulls. From the results of total protein. DNA, lipid and TG contents of FT, it appeared that a reduction in fat storage per fat cell (hypotrophy) or a reduction in total fat cell number (hypoplasia) could explain, in DM bulls, two-thirds and one-third of the reduction of perirenal and subcutaneous FT weights, respectively, as compared to N bulls. In contrast, either hypotrophy or hypoplasia was the main cause of omental or intermuscular FT weight reduction in DM animals.

Intestinal absorption, blood transport and hepatic and muscle metabolism of fatty acids in preruminant and ruminant animals

Current research on lipid metabolism in ruminants aims to improve the growth and health of the animals and the muscle characteristics associated with meat quality. This review, therefore, focuses on fatty acid (FA) metabolism from absorption to partitioning between tissues and metabolic pathways. In young calves, which were given high-fat milk diets, lipid absorption is delayed because the coagulation of milk caseins results in the retention of dietary fat as an insoluble clot in the abomasum. After weaning, the calves were fed forage- and cereal-based diets containing low levels of long-chain fatty acids (LCFA) but leading to high levels of volatile fatty acid (VFA) production by the rumen microflora. Such differences in dietary FA affect: i) the lipid transport system via the production of lipoproteins by the intestine and the liver, and (ii) the subsequent metabolism of lipids and FA by tissues. In preruminant calves, high-fat feed stimulates the secretion of triacylglycerols (TG)-rich lipoproteins (chylomicrons, very-low density lipoproteins (VLDL)). Diets rich in polyunsaturated FA (PUFA) stimulate the production of chylomicrons by the intestine (at peak lipid absorption) and of high density lipoproteins by the liver, leading to high blood concentrations of cholesterol. High levels of non-esterified FA (NEFA) uptake by the liver in high-yielding dairy cows in early lactation leads to TG infiltration of the hepatocytes (fatty liver). This is due to the low chronic capacity of the liver to synthesise and secrete VLDL particles. This abnormality in hepatic FA metabolism involves defects in apolipoprotein B synthesis and low availability of apolipoproteins and lipids for VLDL packaging. Fatty liver in calves is also caused by milk containing either soybean oil (rich in n-6 PUFA), or coconut oil (rich in C12:0 and C14:0). The ability of muscle tissue to use FA as an energy source depends on its mitochondrial content and, hence, on many physiological factors. The uptake and partitioning of LCFA between oxidation and storage in muscle is regulated by the activity of key intracellular enzymes and binding proteins. One such protein, carnitine palmitoyltransferase I (CPT I) controls the transport of LCFA into mitochondria. Metabolites derived from LCFA inhibit glucose oxidation, decrease the activity of CPT I and decrease the efficiency of ATP production by mitochondria. Most research on tissue lipid metabolism in ruminants is focused on: i) the partitioning of FA oxidation between intracellular peroxisomes and mitochondria in the liver and in muscles; (ii) the regulation of lipid metabolism by leptin, a recently discovered hormone secreted by mature adipocytes; and iii) the effects of activation of the nuclear receptors (PPARs and RXR) by LCFA or by phytol metabolites derived from chlorophyll.

Lipoprotein lipase activity and mRNA levels in bovine tissues

Lipoprotein lipase (LPL) in cattle has been extensively studied in adipose tissue, milk and mammary gland, but only to a limited extent in muscles. Therefore, we have adapted our in vitro LPL assay method for the measurement of LPL activity and describe, for the first time, sensitive procedures to quantify LPL activity and mRNA levels in bovine muscles. In vitro activation of bovine LPL activity is approximately 5-fold greater with rat than with bovine sera for heart and muscles, but not for adipose tissues. Values of LPL activity are in the upper range of those previously reported for rat or bovine tissues. With rat serum as activator, LPL activity in the heart of seven calves (662-832 mU g-1) is at least 3-fold lower than in the rat heart (2150-2950 mU g-1, P < 0.05). LPL activity is higher in bovine heart and oxidative muscles (412-972 mU g-1), except the diaphragm, than in mixed or glycolytic muscles (33-154 mU g-1, P < 0.05). The levels of LPL transcripts are positively related to LPL activity in bovine tissues, including muscles and adipose tissues.

Contribution of mitochondria and peroxisomes to palmitate oxidation in rat and bovine tissues

Total and peroxisomal palmitate oxidation capacities and mitochondrial enzyme activities were compared in tissues from growing rats, preruminant calves and 15-month-old bulls. Total palmitate oxidation rates were 1.9-5.2-fold higher in rat than in bovine tissues and 1.7-fold higher in the heart and muscles from calves than from growing bulls. The peroxisomal contribution to palmitate oxidation was similar between rats and bovines (i.e. calves and bulls) in liver (35-51%), heart (26%) but not in muscles (14 +/- 3% in rats vs 33 +/- 4.5% in bovines, P < 0.05). Mitochondrial enzyme activities were 1.8-4.8-fold higher in rat than in bovine tissues but the citrate synthase to cytochrome-c oxidase ratio was the highest in the liver (17-38), intermediate in the heart and muscles from calves and rats (6-10) and the lowest in heart and muscles from bulls (2-3, P < 0.05). In all tissues and animal groups, palmitate oxidation rates were similar per unit cytochrome-c oxidase activity, but not always per unit citrate synthase activity. Therefore, differences in mitochondrial contents (as between rats and bovines) or in mitochondrial characteristics (as between liver and muscles) relate to the differences in palmitate oxidation capacity.

Acute hyperinsulinemia fails to change GLUT-4 content in crude membranes from goat skeletal muscles and adipose tissue

The effect of insulin on GLUT-4 protein level in samples of adipose tissue and skeletal muscles from goats was studied in vivo using an euglycemic hyperinsulinemic clamp. The clamp was maintained in conscious goats for 6 h in the presence of amino acids to prevent insulin-induced hypoaminoacidemia. GLUT-4 protein was assessed in crude membrane preparations from adipose tissue and four skeletal muscles (longissimus dorsi, tensor fasciae latae, anconeus and diaphragm) by Western blot analysis. No changes of GLUT-4 protein content were detected after 6 h of hyperinsulinemia in either adipose tissue or skeletal muscles from goats. These results suggest that insulin is not the prime factor involved in the short-term regulation of GLUT-4 protein transporter content in insulin-sensitive tissues from goats.

Messenger RNAs encoding lipoprotein lipase, fatty acid synthase and hormone-sensitive lipase in the adipose tissue of underfed-refed ewes and cows

The mechanisms involved in the nutritional regulation of genes encoding lipogenic (lipoprotein lipase (LPL) and fatty acid synthase (FAS)) and lipolytic (hormone-sensitive lipase (HSL)) enzymes were investigated by comparing the levels of the corresponding mRNAs in the adipose tissue (AT) of underfed or underfed-refed ewes and cows. Refeeding sharply increased LPL and FAS activities (19-25- and 6-8-fold, respectively) and moderately increased (2-4 fold) the activities of glucose-6-phosphate dehydrogenase (G6PDH), malic enzyme (ME) and glycerol-3-phosphate dehydrogenase (G3PDH). Northern blot analysis revealed three LPL transcripts and a single FAS transcript in cow and ewe AT. A single HSL mRNA was detected in cow AT and two transcripts in ewe AT. Refeeding sharply increased LPL and FAS mRNA levels, while restriction slightly increased (cows) or had no effect (ewes) on the HSL mRNA levels. This suggests that nutritional factors regulate sharply the expression of LPL and FAS genes by pretranslational mechanisms, but less clearly that of HSL gene.

Weaning marginally affects glucose transporter (GLUT4) expression in calf muscles and adipose tissues

The nutritional regulation of glucose transporter GLUT4 was studied in eight muscles and four adipose tissues from two groups of preruminant (PR) or ruminant (R) calves of similar age (170 d), empty body weight (194 kg) at slaughter, and level of net energy intake from birth onwards. Isocitrate dehydrogenase (EC 1.1.1.41) activity in muscles was not different between PR and R except in masseter muscle from the cheek (+71% in R; P < 0.003), which becomes almost constantly active at weaning for food chewing. Basal and maximally-insulin-stimulated glucose transport rate (GTR) per g tissue wet weight in rectus abdominis muscle were significantly higher in R calves (+31 and 41% respectively; P < 0.05). GLUT4 protein contents did not differ in muscles from PR and R except in masseter (+74% in R; P < 0.05) indicating that the increased GTR in rectus abdominis cannot be accounted for by an enhanced GLUT4 expression. GLUT4 mRNA levels did not differ between the two groups of animals in all muscles suggesting a regulation of GLUT4 at the protein level in masseter. GLUT4 number expressed on a per cell basis was lower in adipose tissue from R calves (-39%; P < 0.05) and higher in internal than in peripheral adipose tissues. In summary, the regulation of GLUT4 in calves at weaning differs markedly from that previously described in rodents (for review, see Girard et al. 1992). Furthermore, significant inter-individual variations were shown for metabolic activities in muscle and for biochemical variables in adipose tissue.

Insulin-sensitive glucose transporter transcript levels in calf muscles assessed with a bovine GLUT4 cDNA fragment

Previous studies have shown that the expression of the insulin-sensitive glucose transporter (GLUT4) is lower in oxidative muscles than in glycolytic muscles in bovines and goats in contrast to observations in rats. Additional experiments in this work provide very strong arguments that the immunoreactive band detected does represent GLUT4 protein, which further validates our previous results. Therefore, to determine the level of regulation, the main objective of the present study was to measure GLUT4 mRNA amounts in various bovine muscles. A 241-bp fragment of the bovine GLUT4 cDNA was cloned by polymerase chain reaction (PCR). It shares 80-90% sequence identity with related sequences in other species. This PCR-amplified bovine GLUT4 probe was used to determine the distribution of GLUT4 mRNA in bovine tissues in comparison with that of GLUT1 mRNA. Moreover, GLUT4 mRNA amounts were quantified by Northern-blot analysis in heart and seven skeletal muscles with various oxidative and glycolytic activities from seven ruminant calves. GLUT4 mRNA was detected by Northern-blot analysis only in calf insulin-sensitive tissues. In contrast, GLUT1 mRNA was detected in all tissues studied except liver. GLUT4 mRNA amount was the highest in masseter and heart, which are oxidative muscles (1.67 +/- 0.16 and 1.53 +/- 0.19 units/g wet tissue weight, respectively) and the lowest in glycolytic or oxido-glycolytic muscles (0.31 +/- 0.04 to 1.00 +/- 0.09 units/g wet tissue weight; SEM, n = 7). These data and our previous results provide evidence for translational and/or post-translational control mechanisms of bovine GLUT4 protein expression in a muscle type-specific manner.

Glucose-transporter (GLUT4) protein content in oxidative and glycolytic skeletal muscles from calf and goat

It is well accepted that skeletal muscle is a major glucose-utilizing tissue and that insulin is able to stimulate in vivo glucose utilization in ruminants as in monogastrics. In order to determine precisely how glucose uptake is controlled in various ruminant muscles, particularly by insulin, this study was designed to investigate in vitro glucose transport and insulin-regulatable glucose-transporter protein (GLUT4) in muscle from calf and goat. Our data demonstrate that glucose transport is the rate-limiting step for glucose uptake in bovine fibre strips, as in rat muscle. Insulin increases the rate of in vitro glucose transport in bovine muscle, but to a lower extent than in rat muscle. A GLUT4-like protein was detected by immunoblot assay in all insulin-responsive tissues from calf and goat (heart, skeletal muscle, adipose tissue) but not in liver, brain, erythrocytes and intestine. Unlike the rat, bovine and goat GLUT4 content is higher in glycolytic and oxido-glycolytic muscles than in oxidative muscles. In conclusion, using both a functional test (insulin stimulation of glucose transport) and an immunological approach, this study demonstrates that ruminant muscles express GLUT4 protein. Our data also suggest that, in ruminants, glucose is the main energy-yielding substrate for glycolytic but not for oxidative muscles, and that insulin responsiveness may be lower in oxidative than in other skeletal muscles.

Study of the influence of age and weaning on the contractile and metabolic characteristics of bovine muscle

Weaning is an interesting period for the study of the nutritional regulation of muscle energy metabolism, since during this stage the nature of the substrates supplied to the muscle and their energy balance are profoundly changed. The aim of this study was to determine the effect of these modifications on the contractile and metabolic characteristics of bovine muscle. Two similar groups of 7 male Montbéliard calves were used with the same age and weight, and with the same energy intake. One group consisted of milk-fed calves, the other of weaned animals. The latter were progressively weaned over a period between 107 and 128 d. The average age at slaughter in the 2 groups was 170 d. Biopsy specimens of semitendinosus (ST) muscle were taken at the ages of 66 d, 94 d (before the beginning of weaning) and 136 d (at the end of weaning) to follow the evolution of muscle characteristics. Samples of longissimus thoracis (LT) muscle were taken 24 h after slaughter and used to study the changes in protein and DNA content. The proportion and area of the different types of fiber, I (slow, oxidative), IIA (fast, oxido-glycolytic), IIB (fast, glycolytic) and IIC (fast/slow, oxidoglycolytic) were measured by immunohistochemistry and image analysis. The metabolism of the muscles was determined by studying isocitrate dehydrogenase (ICDH, oxidative) and lactate dehydrogenase (LDH, glycolytic) activity. The results obtained between 2 and 6 months of life showed an overall increase in the area of the fibers (I, IIA, IIB and IIC) and a conversion of type IIA fibers into type IIB accompanied by a shift in the energy metabolism towards a glycolytic type. Weaning caused temporary stress, whose main consequences were to decrease overall muscle fiber area and the percentage of type IIB fibers, and increase the proportion of type IIC fibers in weaned animals. These effects may have been due to the nutritional and behavioral disturbances that accompany weaning, because 42 d after the end of weaning there was no difference in the size of ST and LT fibers between the 2 groups whereas the proportion of type IIA fibers was still higher in weaned animals.

[Analysis of human growth hormone-binding protein in plasma by high pressure liquid chromatography]

A technique using high pressure liquid chromatography gel filtration was used to evaluate GH-binding proteins (GH-BPs) in human plasma; eluate was monitored for radioactivity in a gamma-detection system connected to a computer. Plasma (200 microliters) was incubated with 125I-human (h) GH (200,000 cpm) at 4 degrees C for 20 hours. Our GH binding assay offers important gains in terms of rapidity and resolution; it has permitted a clear separation and characterization of the two GH-binding components present in human plasma.

Human plasma growth hormone (GH)-binding proteins are regulated by GH and testosterone

Possible regulation of GH-binding proteins (GH-BPs) in human plasma was examined. Eight children with isolated GH deficiency had a very low level of plasma GH-binding activity (10.2 +/- 1.1% of radioactivity). Under GH treatment the hormone binding to the high affinity BP (peak II-BP) increased in every patient to reach the mean value of 18.5 +/- 1.4%. In one patient, Scatchard plot analysis indicated that this increase was related to a higher binding capacity without any significant change in the binding affinity. A positive correlation existed between the GH-binding activity and insulin-like growth factor-I plasma levels. In nine boys with pubertal delay, the GH-specific binding to peak II-BP was normal (30.6 +/- 3.7% of radioactivity); it decreased significantly after testosterone treatment. In four boys with precocious puberty, the specific GH binding to peak II-BP was low (16.6 +/- 1.1%). It increased significantly to 21.6 +/- 1.1% of radioactivity after treatment with a LHRH analog. A negative correlation existed between plasma testosterone levels and GH binding to peak II-BP in boys presenting pubertal delay or precocious puberty. The high affinity GH-BP is regulated, and among the factors that play a role in this regulation, GH and testosterone have opposite effects.

Evaluation of the growth hormone-binding proteins in human plasma using high pressure liquid chromatography gel filtration

A technique using high pressure liquid chromatography gel filtration was used to evaluate GH-binding proteins (BP) in human plasma; eluate was monitored for radioactivity in a gamma-detection system connected to a computer. Plasma (200 microL) was incubated with [125I]human (h) GH (200,000 cpm) at 4 C for 20 h. The main GH-BP (peak II) was well separated from free [125I]hGH (peak III) and from a higher mol wt complex (peak I), which was minor. In our control plasma, the specific binding of [125I]hGH to peak II BP (II-BP) was 32.2 +/- 0.6% of the radioactivity. Scatchard analyses indicate an association constant of 3.6-7.4 X 10(8) M-1 and a binding capacity ranging from 24-86 ng/mL for peak II-BP in five normal adult plasma samples. Peak I material, separated from plasma of boys with pubertal delay, bound hGH with a low affinity (3 x 10(6) M-1) and a very high capacity (2 micrograms/mL). In cross-linking experiments, peak I appeared as two proteins of 165 and 174 kD; these mol wt were much higher than that of peak II-BP, previously estimated at 53,000. hGH complexed to peak II-BP remained fully immunoreactive with use of the anti-hGH antibodies of our assay. In plasma containing 10-20 micrograms/L hGH, the proportion of bound hormone (peak II) was 44.5 +/- 2.3%, whereas the amount of hGH in peak I was very low or undetectable. Specific binding of hGH to II-BP was lowest during the first year of life and highest in adulthood. No sex difference was found. I-BP is differentially regulated, since its binding activity was significantly lower in adults than in prepubertal children. Normal values for age should be taken into account to interpret GH-binding activity, particularly in children 2 yr of age or younger. Our GH binding assay offers important gains in terms of rapidity and resolution; it has permitted a clear separation and characterization of the two GH-binding components present in human plasma.

Human liver growth hormone receptor and plasma binding protein: characterization and partial purification

The human liver GH receptor has been further characterized using several biochemical approaches. Crosslinking of [125 I]human GH (hGH) to microsomal receptors and to particulate and solubilized plasma membrane receptors, followed by gel electrophoresis and autoradiography, revealed two predominant receptor-hormone complexes with a mol wt of 124,000 and 75,000, respectively. As previously shown, the 70-80 k band appears to be generated from the 124 k band in the presence of beta-mercaptoethanol, suggesting intersubunit disulfide linkages. A minor complex of mol wt 150,000 was sometimes found. By immunoblot, using a polyclonal antibody raised against a synthetic peptide (residues 391-405 of the mature human GH receptor), a single band of 100 k was detected. Human liver GH receptor and plasma GH-binding protein (BP) were purified 1,000- and 4,000-fold, respectively. The partially purified membrane receptor, analyzed by ligand-blot, showed two major bands of 55 and 32 k and minor bands of 68 and 47 k. Crosslinking of the purified GH-BP or purified receptor with [125I]hGH revealed a 75 k receptor-hormone complex. Polyclonal antibodies raised against the hepatic GH receptor inhibited the binding of hGH to the human receptor and were able to immunoprecipitate the GH receptor and also the GH-BP complex. Our findings demonstrate the existence of multiple forms of the GH receptor in human liver (major components of 100 and 50-55 k, minor component of 130 k); they lend more support to the possible subunit structure of the GH receptor; and finally, they also suggest a close relationship, with common antigenic properties, of the membrane receptor and the plasma GH-BP.

The human liver growth hormone receptor

Human livers, obtained from donors at the time of transplant, were homogenized in 0.25 M sucrose and fractionated by differential centrifugation. The specific binding of [125I] human (h) GH to total particulate fractions from 18 livers varied from 0.4-5.1% of the total radioactivity/100 micrograms protein. Binding affinity was 2.0 +/- 0.3 X 10(9) M-1, and binding capacity ranged from 14-53 fmol/mg protein. A different proportion of receptors occupied by endogenous hGH did not explain the large variation in binding. Binding sites were specific for hGH. Dissociation of the hormone-receptor complex was extremely slow. No specific binding of [125I]hPRL was observed. Specific binding of insulin was found in fractions from all livers and varied less than hGH binding. Cross-linking of [125I]hGH to plasma membrane and microsome receptors yielded two major autoradiographic bands corresponding to an estimated mol wt of 103,000 for the receptor, with a possible subunit of 54,000. Human liver primary fractions were characterized. The binding of hGH and insulin displayed a nucleo-microsomal distribution pattern in the primary fractions; 54.2% and 27.9% of the hGH-binding activity were found in the microsomes and the nuclear fraction, respectively, whereas insulin binds equally to nuclear and microsomal elements. Our findings suggest that hGH-binding sites are present in the plasma membrane and also in one or more intracellular compartments, whereas a high proportion of insulin receptors is associated with the plasma membrane.