Factors controlling blood concentration of somatomedin C

Somatomedin-C levels in treated and untreated patients with acromegaly

With the aid of a recently developed commercially available radioimmunoassay for Somatomedin-C (Sm-C) we measured the Sm-C levels in 38 controls, 24 untreated acromegalics, 45 inactive acromegalics and five pituitary dwarfs. With the exception of the dwarfs, the age and sex distributions for the various groups were similar. In inactive acromegalics the basal GH level (calculated as the mean of four blood samples taken during the day) was less than 5 mU/l; it was depressed to, or less than, 2.5 mU/l during the 100 g oral glucose tolerance test. The mean Sm-C level found for control subjects was 20.4 +/- 5.1 nmol/l and for untreated patients 85.6 +/- 25.7 nmol/l (mean +/- SD, P less than 0.001). The mean Sm-C level for inactive patients who had undergone surgery and invariably showed a normalized paradoxical reaction to TRH was 18.2 +/- 7.1 nmol/l (NS) vs 16.1 +/- 8.2 nmol/l (NS) for those who underwent surgery plus postoperative pituitary radiation therapy. Only one out of 45 inactive acromegalics exhibited an increased Sm-C level. The Sm-C levels correlated significantly not only with the log GH levels (r = 0.82) but also with the fasting insulin/glucose ratio the integrated incremental insulin levels, the maximal insulin increase and the integrated glucose levels during the oral glucose tolerance test as well as the 24-h urinary excretion of calcium and hydroxyproline.(ABSTRACT TRUNCATED AT 250 WORDS)

Physiology of the somatotropic axis with particular reference to the ruminant

The physiology of growth hormone and the insulin-like growth factors are reviewed with particular reference to the dairy industry. Growth hormone secretion in the ruminant is pulsatile in nature and nutritional factors have a major impact on its secretion. Isolation of growth hormone-releasing factor has allowed further progress in understanding the mechanisms underlying growth hormone release. The receptors appear to be under active endocrine and metabolic control, and nutritional influences on the somatotropic axis are in large part mediated through changes in somatotropic receptors. The mode of action of growth hormone to induce acute metabolic affects and lipolysis remains to be resolved, but there is increasing evidence that its anabolic actions are mediated by the insulin-like growth factors. Recent studies of measurement of insulin-like growth factor-1 and -2 in the ruminant and the role of growth hormone, nutrition, insulin, and sex steroids in their regulation are reviewed. The relative role of the two factors and the multiple forms of their receptors remain to be resolved. It is well-documented that growth hormone is galactopoietic. The evidence that this effect is largely due to enhanced nutrient supply to the mammary gland is not convincing. Effects of growth hormone are indirect and may be mediated by the insulin-like growth factors. The potential is considerable for manipulating the growth hormone insulin-like growth factor axis to enhance lactation.

Insulinlike growth factor I regulation of growth hormone gene transcription in primary rat pituitary cells

We have previously shown that insulinlike growth factor I (IGF-I) inhibits growth hormone (GH) secretion and messenger RNA (mRNA) levels in pituitary cells. The effects of IGF-I on new GH mRNA synthesis rates in primary monolayer rat pituitary cells were therefore examined by nuclear runoff transcription assays. IGF-I (1.3 nM) treatment for 1 h inhibited GH gene transcription to 60% of controls. IGF-I (3.25 nM) maximally suppressed GH gene transcription to 30% of control values after 4 h. After 24 h treatment, GH transcription was suppressed to 48% of controls by 3.25 nM IGF-I. IGF-I (3.25 nM) also inhibited the twofold growth hormone-releasing hormone (GHRH) (10 nM)-stimulated GH gene transcription by 30% after 4 h. Transcription of the prolactin (PRL) gene was not suppressed in these cells by IGF-I. Relatively high doses of insulin (200 nM) also suppressed GH gene transcription, but epidermal growth factor and fibroblast growth factor did not change GH mRNA synthesis. The results show that IGF-I exerts a rapid and selective suppression of basal and GHRH-stimulated GH gene transcription. These data indicate a role for IGF-I in negative feedback of GH gene expression and provide evidence for the direct transcriptional regulation of the GH gene by IGF-I in primary rat anterior pituitary cells.

Somatomedin C/insulin-like growth factor I production by human fetal lung tissue maintained in vitro

The production of somatomedin C/insulin-like growth factor I (Sm-C/IGFI) by human fetal lung tissue maintained in vitro was examined in the present study. We have shown that epithelial cells in human fetal lung explants maintained in vitro differentiate into type II cells within 4-6 days. During the first 24 h of culture, the fetal lung explants released 2.74 +/- 0.14 ng Sm-C/IGFI/mg tissue protein into the culture medium. At this time the explants contained 0.24 +/- 0.02 ng Sm-C/IGFI/mg tissue protein. During the next 4 days of culture, explant Sm-C/IGFI content and the rate of Sm-C/IGFI secretion into the medium declined by approximately 50%. Sm-C/IGFI secretion was inhibited significantly when fetal lung explants were cultured in media that contained cortisol (10(-7) M), a hormone that is known to stimulate fetal lung type II cell differentiation. The effect of cortisol was both concentration- and time-dependent. While insulin, bovine prolactin, and human growth hormone had no apparent effect on Sm-C/IGFI production by the explants, human prolactin and human placental lactogen both decreased Sm-C/IGFI production. These findings are unprecedented and are suggestive that Sm-C/IGFI synthesis may be regulated in a unique fashion in the fetal lung. The decline in Sm-C/IGFI production by fetal lung tissue temporally correlates with the initiation of fetal lung type II cell differentiation in the human fetal lung explants.

Potential therapeutic indications for growth hormone and growth hormone-releasing hormone in conditions other than growth retardation

Growth hormone (GH) previously was available in limited supply and only for the treatment of GH-deficient children. The recent production of GH by recombinant DNA technology has provided a potential surfeit of this hormone and raises the possibility of its use in other conditions. In addition, the isolation, characterization, and synthesis of GH-releasing hormone (GRH) provides an opportunity to use this peptide in conditions in which increased circulating levels of GH are desired. Both GH and GRH have potential therapeutic uses in conditions other than growth retardation.

Laboratory diagnosis of hypopituitarism

Hypopituitarism can be caused by failure or loss of one or more of the eight identifiable hormones in the anterior lobe of the pituitary gland. The endocrine manifestations of hypopituitarism are related to the type and degree of hormonal deficiency and the stage in life during which the deficiency occurs. In patients with suspected hypopituitarism, the diagnostic approach consists of determining the extent and the cause of the hormonal loss. Specific provocative tests for the diagnosis of hypopituitarism are reviewed in detail in this article.

Disorders of prolactin and growth hormone secretion

A large range of tests is now available to help us understand, diagnose and manage GH-related growth disorders. The traditional provocative tests of GH secretion will identify short children with severe GH deficiency. However, evidence is emerging that these pharmacological tests may not be sufficiently sensitive to identify some subjects with GH deficiency arising from neurosecretory disturbance of GH release. There is a need for a simple sensitive test that will detect subtle GH secretion of this type. hGRF administration is a reliable test of GH reserve and, when used in combination with conventional tests, may help to identify GH-deficient children with hypothalamic GRF deficiency. Whether the GH responses following GRF administration reflects physiological GH secretory activity needs to be established. The diagnosis of acromegaly is made on clinical grounds. The abnormal GH responses to glucose and TRH support the diagnosis, but by themselves should not be considered to be diagnostic of acromegaly. An elevated Sm C level also helps to establish the diagnosis, although Sm C concentrations may be elevated to the same degree in pregnancy and during puberty. The use of Sm C to monitor disease activity remains to be established. Circulating GRF levels should be measured in patients with acromegaly so that ectopic production of GRF can be identified.