Insulin-like growth factor characteristics of an acidic non-suppressible insulin-like activity

Modeling of neutral solute transport in a dynamically loaded porous permeable gel: implications for articular cartilage biosynthesis and tissue engineering

A primary mechanism of solute transport in articular cartilage is believed to occur through passive diffusion across the articular surface, but cyclical loading has been shown experimentally to enhance the transport of large solutes. The objective of this study is to examine the effect of dynamic loading within a theoretical context, and to investigate the circumstances under which convective transport induced by dynamic loading might supplement diffusive transport. The theory of incompressible mixtures was used to model the tissue (gel) as a mixture of a gel solid matrix (extracellular matrix/scaffold), and two fluid phases (interstitial fluid solvent and neutral solute), to solve the problem of solute transport through the lateral surface of a cylindrical sample loaded dynamically in unconfined compression with frictionless impermeable platens in a bathing solution containing an excess of solute. The resulting equations are governed by nondimensional parameters, the most significant of which are the ratio of the diffusive velocity of the interstitial fluid in the gel to the solute diffusivity in the gel (Rg), the ratio of actual to ideal solute diffusive velocities inside the gel (Rd), the ratio of loading frequency to the characteristic frequency of the gel (f), and the compressive strain amplitude (epsilon 0). Results show that when Rg > 1, Rd < 1, and f > 1, dynamic loading can significantly enhance solute transport into the gel, and that this effect is enhanced as epsilon 0 increases. Based on representative material properties of cartilage and agarose gels, and diffusivities of various solutes in these gels, it is found that the ranges Rg > 1, Rd < 1, correspond to large solutes, whereas f > 1 is in the range of physiological loading frequencies. These theoretical predictions are thus in agreement with the limited experimental data available in the literature. The results of this study apply to any porous hydrated tissue or material, and it is therefore plausible to hypothesize that dynamic loading may serve to enhance solute transport in a variety of physiological processes.

The growth hormone independent insulin-like growth factor-I binding protein BP-28 is associated with serum insulin-like growth factor-I inhibitory bioactivity in adolescent insulin-dependent diabetics

The relationship between the growth hormone independent insulin-like growth factor binding protein (BP-28) and serum insulin-like growth factor-I (IGF-I) inhibitory bioactivity observed in diabetic serum was investigated in five poorly controlled adolescent type I diabetics. We have measured the in-vitro effects of purified BP-28 from amniotic fluid on serum IGF-I stimulated and basal cartilage sulphation and compared serum IGF-I bioactivity obtained from 24-h serum profiles from each diabetic subject with serum concentrations of BP-28 and IGF-I measured by specific radioimmunoassays. Purified BP-28 inhibited serum IGF-I stimulated and basal cartilage sulphation in vitro, in a dose-dependent manner. Serum IGF-I bioactivity of diabetic sera showed a change in activity over the 24-h period, with peak inhibitory bioactivity observed in each subject between 0800 and 1000 h. BP-28 concentrations in each individual showed a marked circadian rhythm with maximum peak levels occurring at 0800 h. Long-acting insulin administered in the evening in two of the diabetic subjects blunted the maximum peak level attained compared to the three diabetics who had long-acting insulin administered in the morning. IGF-I concentrations did not change over the 24-h period in each individual. The data shows that BP-28 inhibits serum IGF-bioactivity on cartilage in vitro. The changes in inhibitory bioactivity observed in diabetic serum are associated with similar changes in serum concentrations of BP-28. We propose that BP-28 is one of the IGF-I inhibitors observed in diabetic serum and that it may play a role in retarded growth and delayed puberty often seen in the adolescent diabetic.

Nutrition and somatomedin. XVI: Somatomedins and somatomedin inhibitors in fasted and refed rats

Nutritional deprivation is associated with poor growth and decreased levels of net circulating somatomedin activity, as measured by bioassay. Since somatomedin activity reflects the contributions both of somatomedins (which stimulate cartilage) and of somatomedin inhibitors (which antagonize the ability of the somatomedins to stimulate cartilage), we asked if changes in net somatomedin activity could involve progressive underlying alterations in levels of both somatomedins and somatomedin inhibitors. Groups of rats were killed during three days of fasting and 24 hours of refeeding. Fasting was associated with a rise in serum beta-hydroxybutyrate from 1.6 to 12.6 mmol/L after one day, followed by a decline to 4 mmol/L at three days. Somatomedins (low-MW) were separated from somatomedin inhibitors (high-MW) by gel permeation chromatography at acid pH on Sephadex G-50 and TSK-2000 HPLC. Somatomedins fell 35% after one day of fasting, and decreased to 59% below control levels after three days (P less than .05). Somatomedins did not change with six hours of refeeding, but then rose rapidly, reaching control levels after 24 hours. Somatomedins were correlated with change in weight (r = .41, P less than .05), but not with glucose or beta-hydroxybutyrate. Inhibitors rose to 195% above control-levels after one day of fasting, and continued to rise to 375% above control after three days (P less than .01). In contrast to the delayed change in somatomedins with refeeding, there was an abrupt fall in inhibitors (41% below three-day fasted levels after six hours), returning to control levels after 24 hours.(ABSTRACT TRUNCATED AT 250 WORDS)

Preferential association of the insulin-like growth factors I and II with metabolically inactive and active carrier-bound complexes in serum

Ion-exchange chromatography of serum on DEAE-Sephadex A-50 using a stepwise NaCl gradient showed that complexes enriched with insulin-like growth factors I and II (IGF-I and IGF-II) could be preferentially eluted. A fraction eluted with 0.075 M-NaCl preferentially contained immunoreactive IGF-I with peak levels appearing in fractions of Mr approx. 110,000. The IGF-I-binding protein complex itself had low bioactivity as measured in a non-suppressible insulin-like (NSILA) bioassay. On conversion to free IGF-I by gel-permeation chromatography on Sephadex G-75 in 1% formic acid, however, the IGF-I did express its intrinsic NSILA bioactivity. In contrast, an IGF-II-enriched complex was eluted from the DEAE-Sephadex with 0.15 M-NaCl. Practically all of the recovered NSILA of the original serum was present in this fraction, in the Mr range 70,000-300,000 with a peak of 150,000. Chromatography on Sephadex G-75 in 1% formic acid separated this high-Mr NSILA into low-Mr (less than 15000) IGF-II and high-Mr acid-stable NSILA-P. The high-Mr IGF-II complex bound to concanavalin A-Sepharose, suggesting that it was a glycoprotein. The results confirm previous reports that a large portion of the NSILA of whole serum can be accounted for by a biologically active acid-dissociable complex. These data show for the first time that this active complex consists of an IGF-II-preferring binding protein. In direct contrast, the IGF-I-preferring complex does not express NSILA bioactivity until the IGF-I is liberated through acidification. The presence of a metabolically active IGF-II complex in serum raises questions as to its possible biological role in the adult.

Specific non-saturable binding of insulin by human platelets

Studies on the nature and characterization of the specific binding of 125I-insulin to intact human platelets have been undertaken. Although under conditions of physiological buffer osmolality, a binding equilibrium was attained in 4-6 h at 4 degrees C, at higher temperatures (17 degrees C or 24 degrees C) equilibrium was reached only in the presence of very high buffer osmolality or 25 mM NaF. Under conditions of normal osmolality and in the absence of NaF, binding at 17 degrees C was not saturable. This phenomenon was specific for insulin or insulin-like hormones (the insulin-like growth factors) and did not occur with 125I-labelled growth hormone, ACTH or beta-endorphin. The non-saturable uptake of insulin appeared due to an energy-dependent specific sequestration or internalization of insulin by mechanisms probably involving the microtubule system. This study emphasizes the need to restrict this non-saturable uptake of insulin if one wishes to adequately study the platelet plasma membrane receptors for insulin. These data also indicate that there is a major interaction of insulin and insulin-like hormones with normal human platelets and support previous demonstrations of major insulin effects on platelet physiology in both normal and diabetic states.

Proteolytic conversion of insulin-like growth factors to an acidic form(s)

The relative amounts of the various forms of bioassayable insulin-like growth factors (IGF) isolated from human serum or serum fraction Cohn IV-1 depend on the purification procedure. With acid gel filtration or acid/ethanol extraction as the initial step, IGF-II (pI approximately 6.5) was the most abundant (40-70%) followed by somatomedin A (pI approximately 7.4; 15-23%), an acidic form of insulin-like activity (ILA pI 4.8) (13-21%) and IGF-I (pI approximately 8.5; 5-27%). If, however, pH 5.5 ion-exchange chromatography on SP-Sephadex was used prior to acid gel filtration, the acidic pI 4.8 form was the major (greater than 90%) species recovered and was accompanied by a quantitative loss of the other IGF species. This suggested a possible conversion of IGF-I, somatomedin A and/or IGF-II to the acidic ILA pI 4.8 form(s) during the SP-Sephadex procedure. Further experiments indicated that differences in the yields of ILA pI 4.8 were not due simply to differences in the initial pH conditions of the various methods (i.e. acid versus neutral), although exposure to pH 9.7 (a pH experienced during elution of IGF activity from the SP-Sephadex) did appear to play a role. The involvement of the carrier protein in the conversion process was tested by subjecting carrier-free IGF-I and IGF-II to the SP-Sephadex procedure. No conversion of the free forms to ILA pI 4.8 occurred. To examine the possible role of proteinase in the conversion of IGFs to ILA pI 4.8, SP-Sephadex chromatography was performed in the presence of a broad spectrum proteinase inhibitor. The IGF distribution pattern obtained closely resembled the 'normal' pattern seen with acid gel filtration, indicating that proteinase inactivation had prevented conversion to ILA pI 4.8. These data suggest that proteolytic conversion of IGF-I, somatomedin A and IGF-II to more acidic ILA pI 4.8 form(s) (i) occurs during SP-Sephadex chromatography, (ii) is not prevented simply by prior acid exposure, and (iii) takes place only when IGF-I and -II are in their high-Mr carrier-bound forms. Since IGF-I and IGF-II, although homologous, have unique amino acid sequences, the conversion of both IGFs implies that at least two acidic ILA forms exist. Nevertheless, because ILA pI 4.8 retains the full spectrum of IGF bioactivities in vitro, and significant quantities are present in normal human serum (21%), it would suggest that proteolytic conversion of IGF-I, somatomedin A and IGF-II to ILA pI 4.8 in vivo may be a physiologically significant event.