(D-Trp8)-Somatostatin-14

The effect of the somatostatin analog (GHRIH-A) D-Trp8, D-Cys14 on plasma levels of growth hormone, pancreatic glucagon, insulin and glucose was studied in four acromegalic patients and in four maturity-onset mild diabetics. Acromegalics received a bolus iv injection of 25 microgram of GHRIH-A, followed by a continuous infusion of 25 microgram in saline over an hour. Mild diabetics were submitted in two different days to two tests: arginine (30 g in 30 min) +/- GHRIH-A (bolus iv injection of 25 microgram followed by an infusion of 25 microgram/h over 120 min) and arginine + saline. GHRIH-A lead to a significant (2 p less than 0.01) fall in GH basal secretion in acromegalics, and significantly reduced the GH response to arginine in maturity-onset diabetics. The inhibitory effect of insulin secretion was less impressive, but significative in both groups. No significant changes in plasma pancreatic glucagon values were noted. In mild diabetics, GHRIH-A infusion induced a small but significant increase in the blood glucose increment due to arginine. Our data suggest that this somatostatin analog may be potentially useful only when GH suppression is the main therapeutic goal to be reached, as in acromegaly and in severe diabetic retinopathy, but not in metabolic control of mild diabetic patients with a good residual insulin secretion.

Somatostatin is a widely distributed hormone localized in the central nervous system, pancreas and gastrointestinal tract. We have investigated the possible influence of somatostatin and a synthetic analogue, (D-Trp8,D-Cys14)-somatostatin, on the intestinal absorption 'in vivo' of D(+)-glucose and D(+)-galactose and also the effect on disaccharidase intestinal activities in hamster. Somatostatin, or its analogue, (12 micrograms/100 g body wt) was administered intraperitoneally 4 or 14 h prior to experiments. The results are compared to control animals. Animals treated with somatostatin and the synthetic analogue showed that there were no significant difference from control animals with respect to intestinal absorption of carbohydrates. Somatostatin produced inhibition of brush-border lactase activity in females only, whereas brush-border sucrase was increased 14 h after treatment in males and females, and brush-border maltase was inhibited in females only 4 h after hormone administration.

The localization and characterization of receptors for somatostatin-14 (S-14) and somatostatin-28 (S-28) were studied in the rat brain using the iodinated agonists [Tyr0,D-Trp8]S-14 and [Leu8,D-Trp22,Tyr25]S-28 as tracers. Slide-mounted frozen sections were used for the radioautographic localization and biochemical characterization of somatostatin receptors. In the latter case counting was performed on scraped off serial sections from rostral regions of the brain. Specificity studies demonstrated that either tracer could be displaced with S-28, S-14 or their agonists. The N-terminus fragment (1-12) of S-28 as well as a number of unrelated peptides were unable to compete with either tracer, indicating that the binding capacity for ligand-receptor recognition is located in the C-terminal portion of S-28. Scatchard analysis of saturation curves gave a one-site interaction with Kd values of 0.42 +/- 0.09 nM and 0.32 +/- 0.04 nM for the S-14 and S-28 iodinated agonists, respectively. By radioautography, the distribution of receptors for both S-14 and S-28 appeared very similar with high levels of binding in the deep layers of the cortex, the cingulate cortex, the claustrum, the locus coeruleus and most structures of the limbic system. Treatment with cysteamine, which caused a somatostatin depletion in the brain, was required to observe labeling in the hypothalamus. In some caudal areas of the brain, especially in the cerebellar nuclei, the solitary tract nucleus and the nucleus of the vagus nerve, only labeling with the S-28 agonist could be detected. This S-28 binding could be displaced by native S-14 (10(-6) M). Generally, there was a correlation between the localization of somatostatin receptors and that of immunoreactive somatostatin, as evaluated by immunocytochemistry. However, in some areas, an inverse correlation between receptor and peptide concentrations was observed. These results are in agreement with previous data suggesting that somatostatin could act as a neurotransmitter or neuromodulator in several brain areas.