The response of normal subjects and patients with idiopathic growth hormone deficiency to continuous infusion of human pancreatic growth hormone releasing factor 1-44

Synthetic growth hormone-releasing hormone (GHRH 1-44) in the differential diagnosis between hypothalamic and pituitary GH deficiency

GH response to an iv bolus of growth-hormone-releasing hormone (GHRH 1-44, 2 micrograms/kg iv) was evaluated in 17 prepubertal children with total GH deficiency (GHD), 6 with partial GHD and in 7 prepubertal normal but short children tested as control. GH response to conventional pharmacological (insulin and arginine or L-Dopa) and physiological stimuli (sleep test) and to GHRH test was also compared. GHRH-induced GH peak occurred at variable times and marked heterogeneity in magnitude of the individual responses to this peptide was observed in GHD patients and in controls. GH increases after GHRH with a peak greater than 10 ng/ml suggested an hypothalamic origin of the GHD in 12 patients (8 with total GHD and 4 with partial GHD). A significant difference (p less than 0.025) of GH peak mean following GHRH administration between total GHD children and normal short children was found. GH response to GHRH injection was usually higher than to conventional stimuli. Nevertheless GH peak following GHRH administration was lower than GH peak following conventional stimuli in 6 children (2 partial GHD children and 4 normal ones). A normal short child failed to respond to GHRH test performed twice. GHRH test is an important diagnostic tool in order to point out hypothalamic GHD.

Growth hormone releasing hormone

Human growth hormone releasing hormone (GHRH) was originally extracted from two pancreatic tumours in patients with acromegaly, and is now known to consist of a 44 residue amidated peptide or its C-terminal-shortened derivatives. The sequence of rat GHRH has also been determined; this 43 residue peptide shows approximately 70% homology with human GHRH, and is located mainly in the arcuate nucleus of the hypothalamus. Pulsatile GH release in the rat is principally a consequence of the pulsatile release of hypothalamic GHRH, although this appears to be associated with a transient suppression of somatostatin release. Exogenous GHRH specifically increases circulating GH in many species, and in the long term may increase growth. In normal man, several analogues of GHRH have been shown to be safe, sensitive and specific stimuli to GH release; although there may be a variable prolactin response, this is usually of small magnitude. Continuous infusion of GHRH leads to a decrement in responsiveness, due at least in part to changes in hypothalamic somatostatin. The GH response to GHRH is also modulated by obesity, blood sugar, free fatty acids, and GH itself. Many children with 'GH deficiency' (idiopathic, radiation-induced, or secondary to hypothalamopituitary tumours) respond to intravenous GHRH with an acute rise in serum GH. Early studies also indicate that long-term therapy with subcutaneous GHRH may increase growth velocity in some of these children. It is concluded that analogues of GHRH are useful in the investigation of the hypothalamopituitary axis, and may be important in the therapy of short stature.

The measurement of insulin-like growth factor I: clinical applications and significance

Of the somatomedins so far measured, the selective quantitation of insulin-like growth factor I (IGF-I) appears to have the greatest potential in clinical diagnosis. There have been two approaches to the development of immunoassay systems. One type uses antibodies raised against synthetic fragments of IGF-I which exhibit cross-reactivity with the whole hormone. Such assay systems may be adequate for measuring normal adult plasma IGF-I levels, but the potential for the higher sensitivity required for detecting sub-normal plasma levels in young children is apparent only in methods using antibodies raised against the complete hormone. IGF-I in plasma exists as part of a high molecular weight complex in which it is bound to carrier proteins. The binding proteins may interfere with plasma IGF-I measurements by radioligand assays. Direct analysis of untreated plasma samples is claimed to be possible using disequilibrium assay conditions but in order to maximise assay sensitivity it is necessary to employ an initial extraction stage in order to eliminate binding protein interference. Whether the measurement of plasma IGF-I can or should be used in addition to, or as a replacement for, plasma growth hormone (GH) measurement in the clinical assessment of growth disorders remains a controversial issue. Available evidence indicates that a single, random plasma IGF-I level provides an accurate reflection of GH secretion. Adequate discrimination between the elevated levels in acromegaly and normal reference values has been demonstrated. However, in the investigation of growth-retarded children available radioimmunoassay (RIA) methods have proved only partially successful because of the age-related nature of normal plasma IGF-I concentrations. Existing assays appear capable of identifying sub-normal plasma levels after the age of approximately 4 years. In younger subjects an improvement in assay sensitivity is required in order to establish with greater accuracy the relevant normal ranges. Improvements in the identification of the particular lesion responsible for retarded growth in a child can be achieved by measurement of both plasma GH and IGF-I concentrations. The predictive value of the acute plasma IGF-I response to single-dose GH therapy may identify patients who will respond to long-term GH therapy. Better, more informed decisions on subsequent treatment may therefore be made. Apart from GH control, several other factors influence circulating IGF-I levels. Nutritional status can be assessed through reference to IGF-I analysis, overall catabolic or anabolic processes being associated with decreasing or increasing plasma IGF-I levels respectively.

Does growth hormone releasing factor desensitize the somatotroph? Interpretation of responses of growth hormone during and after 10-hour infusion of GRF 1-29 amide in man

It is unclear whether growth hormone releasing factor (GRF) administration in vivo may desensitize the somatotroph. To investigate this possibility we have examined the effects of 10-h infusion of the equipotent 1-29 amide analogue of hpGRF on serum GH levels and on the subsequent GH response to a bolus dose of GRF (1-29). Four normal adult males received an intravenous infusion of 1-29 GRF (1 microgram/kg/h) from 0800 to 1800 h, with blood samples taken at 10 min intervals. A 100 micrograms intravenous bolus dose of GRF was given at 1800 h, and sampling continued for a further 90 min. GH levels were near or below the limit of detection (0.5 mU/l) throughout the control 10 h period. During GRF infusion there was increased GH release with pulses of irregular frequency and amplitude (up to 80 mU/l) continuing throughout the entire infusion period. There was no apparent reduction in total GH released towards the latter part of the infusion. On the control day, 100 micrograms GRF bolus increased mean (+/- SEM) GH from 0.82 +/- 0.21 mU/l to a peak of 59.0 +/- 44.8 mU/l (P less than 0.002). Following 10-GRF infusion, responses to bolus injection of GRF were reduced, but variable. In two subjects a small rise in GH levels occurred (basal 6.4 and 7.2 rising to peak values of 11.2 and 23.0 mU/l respectively). In the other two subjects, GH levels fell but in these the GRF bolus had coincided with a GH peak. The loss of GRF responsiveness after GRF infusion may be due to 'desensitization'.(ABSTRACT TRUNCATED AT 250 WORDS)

The interaction of human pancreatic growth hormone releasing factor 1-44 with somatostatin in vivo in normal man

The interaction between the stimulatory effects of hpGRF 1-44 and the inhibitory effects of somatostatin on GH release have been investigated in six normal male subjects receiving continuous 4 h infusions of these peptides alone and in combination. hpGRF 1-44 0.3 microgram/kg/h alone produced a peak GH response of 27.0 +/- 7.6 mU/l (mean +/- SEM). Somatostatin 1.0 microgram/kg/h markedly inhibited the GH response to hpGRF 1-44 with mean levels less than 4.0 mU/l during the infusion, though a rebound rise in GH levels to 26.1 +/- 9.0 mU/l was observed at the end of the infusion period. Somatostatin 0.2 microgram/kg/h inhibited the GH response to hpGRF 1-44 to a lesser degree (peak GH during the infusion 11.7 +/- 2.5 mU/l) and the rebound rise in GH levels (maximum 13.2 +/- 4.3 mU/l) was less than that observed with high dose somatostatin. During somatostatin 1.0 microgram/kg/h alone GH levels were suppressed less than 1.0 mU/l followed by a rebound at the end of the infusion in only two subjects. These data demonstrate a dose-dependent inhibition of hpGRF 1-44 by somatostatin in vivo in man.