Glucose-induced changes in somatostatin-14 and somatostatin-28 released from rat hypothalamic fragments 'in vitro'

Effect of oral glucose on the late growth hormone rise and growth hormone responses to GHRH in normal subjects

A late rise in serum GH occurs 3-5 h following oral glucose in man. In order to investigate the mechanisms through which this occurs we have studied the late GH rise after oral glucose during administration of a supramaximal dose of GHRH. In eight normal subjects, oral glucose (100 g) greatly enhanced the GH responses to a supramaximal dose of GHRH (50 micrograms bolus, followed immediately by 100 micrograms/h infusion for 3 h) given 3.5 h after the glucose. GH peak (mean +/- SEM) elicited by GHRH (bolus + infusion) rose from 55.2 +/- 20.4 to 133.4 +/- 29.6 mU/l (P less than 0.02) after glucose pretreatment. In conclusion, it is likely that the late rise in GH secretion induced by oral glucose occurs via a non-GHRH-dependent mechanism. These data are consistent with the hypothesis that the delayed GH response to glucose is a consequence of reduced release of somatostatin from the hypothalamus.

Effects of Glucose on Thyrotrophin-Releasing Hormone, Growth Hormone-Releasing Hormone, Somatostatin and Luteinizing Hormone-Releasing Hormone Release from Rat Hypothalamus in vitro

Abstract Increasing concentrations of D-glucose (1 to 25 mM) inhibited somatostatin, thyrotrophin-releasing hormone (TRH) and growth hormone-releasing hormone (GHRH) release from incubated adult rat hypothalami in a stereospecific manner. In contrast, the effects of D- and L-glucose on luteinizing hormone-releasing hormone release were virtually identical. Increasing concentrations of D-glucose also inhibited somatostatin release following depolarization with high K(+), but had no obvious effect on depolarization-induced TRH or GHRH release when compared with L-glucose. In conclusion, D-glucose exerts a potent, dose-related modulatory action on the release of rat hypothalamic TRH and GHRH as well as somatostatin in vitro. Further studies are required to establish any physiological relevance of glucose in the modulation of these hypothalamic neuropeptides.

Stimulation of Alpha-Adrenoceptors Facilitates the Release of Growth Hormone-Releasing Hormone from Rat Hypothalamus in vitro

Abstract While extensive evidence suggests that adrenoceptors play an important role in the control of growth hormone in the rat, there are few studies involving the direct measurement of growth hormone-releasing hormone (GHRH). We have therefore developed a radioimmunoassay for rat GHRH, and used it to investigate the modulation of GHRH release by noradrenaline from incubated rat hypothalamus in vitro. The GHRH radioimmunoassay had no significant cross-reactivity with other hypothalamic or GHRH-related peptides, and was sensitive to 4 pg/tube; intra- and interassay coefficients of variation were 6% and 12% respectively. Single incubated rat hypothalami produced a stable and readily measurable output of GHRH in successive 20 min incubations after an initial 60 min preincubation; the release of GHRH was increased in the presence of 56 mM KCI, but did not respond to KCI-depolarization when calcium was excluded from the medium. Stimulated GHRH release was identical to synthetic rat GHRH(1-43) on high-performance liquid chromatography and Sephadex G-75 chromatography. Noradrenaline stimulated GHRH secretion in a dose-dependent manner in the concentration range 10(-10)- 10(-6)M, with a plateau in response at 10(-7)M. Stimulation with noradrenaline 10(-7)M was blocked by idazoxan 10(-5)M and attenuated by thymoxamine 10(-5)M, but was unaffected by timolol 10(-5)M. Both the alpha(2)-adrenoceptor agonist guanfacine, and the alpha(1)-adrenoceptor agonist methoxamine, specifically stimulated GHRH secretion. It is concluded that noradrenaline stimulates the release of GHRH at both alpha(1) and alpha(2)-adrenoceptors.

Further studies on the effect of glucose concentrations and other oxidizable substrates upon ionic gradients and in vitro somatostatin release from rat mediobasal hypothalamus

During in vitro incubation of rat mediobasal hypothalamus (MBH), potassium and sodium gradients were high in the presence of glucose, pyruvate, lactate or the mixture glucose and pyruvate; in the absence of substrate, the ionic gradients were markedly lowered and corresponding somatostatin release from MBH was maximal. The specific effect of glucose on somatostatin release from MBH was tested under normal tissue polarization, i.e. in the presence of pyruvate. Under these more physiological conditions, somatostatin release was submaximal and inversely related to glucose concentrations (within the range 0-7 mM).