Fernand Labrie

Physiological role of somatostatin in the control of growth hormone and thyrotropin secretion.

Abstract Intravenous injection of sheep antiserum to somatostatin in the rat not only increases basal plasma TSH levels but also potentiates TSH response following exposure to cold (5° C). Plasma levels of GH rise 2–3 fold during the first 3 h after injection of the antiserum, with a progressive decrease of the effect up to 10 h. Rhythmical change of serum GH levels during a 10-hour period of observation is not altered after antiserum injection. These data indicate that somatostatin plays a physiological role in the control of both TSH and GH secretion and suggest the involvement of GH-releasing hormone, in addition to somatostatin, in the GH release mechanism.

Isolation, structural elucidation and synthesis of a tetradecapeptide with in vitro ACTH-releasing activity corresponding to residues 33–46 of the α-chain of porcine hemoglobin

Abstract A peptide found in acetic acid extracts of porcine hypothalami and capable of stimulating the release of ACTH in vitro was isolated in pure state, structurally identified as Phe-Leu-Gly-Phe-Pro-Thr-Thr-Lys-Thr-Tyr-Pre-Pro-His-Phe and synthesized. This tetradecapeptide, which corresponds to amino acid residues no. 33–46 in the sequence of the α-chain of porcine hemoglobin, probably represents an artefact of extraction or isolation procedures. Since this peptide stimulates ACTH release from rat pituitary fragments and from monolayer cultures of pituitary cells, but not in vivo , caution must be exercised in interpreting the results of in vitro assays for corticotropin releasing factor.

Radioautographic localization of radioactivity in rat brain after intracarotid injection of 125I-α-melanocyte-stimulating hormone

125I-alpha-melanocyte-stimulating hormone (MSH) was injected into the carotid artery of the rat and the radioactivity localized by radioautography. Radioactivity in the areas surrounding the ventricles and blood vessels after administration of 125I-alpha-MSH but not 125I-luteinizing hormone indicated passage of labeled material through the blood brain barrier. A specific concentration of radioactivity was found in the striatum and reticular nucleus of the thalamus. This localization, particularly in the thalamus, could be correlated with the previously reported effects of MSH on the brain of animals and man.

EFFECT OF THYROXINE ON THE INACTIVATION OF THYROTROPHIN‐RELEASING HORMONE BY RAT AND HUMAN PLASMA

The inactivation of synthetic [3H]thyrotrophin‐releasing hormone (TRH) by plasma was studied in rats treated with propylthiouracil (PTU) alone or with PTU and thyroxine. 48 h after the onset of treatment with thyroxine, the capacity of rat plasma to inactivate [3H]TRH was significantly increased. The percentage of deamidation of TRH to TRH‐free acid was increased 2‐fold after 4 days of administration of thyroid hormone. The inactivation of TRH by plasma from hypothyroid patients was compared to that obtained from hyperthyroid patients. Extraction of human plasma incubated with [3H]TRH, followed by thin‐layer electrophoresis, showed that transformation of [3H]TRH into TRH‐free acid was 44% higher in plasma from hyperthyroid than from hypothyroid patients (P < 0.05). These data suggest that the inactivation process of TRH by blood proteins could be an important factor in the regulation of the hypothalamo‐hypophyseal‐thyroid axis in rat and man.

Stimulated release of hypothalamic growth hormone-releasing activity by morphine and pentobarbital.

Two minutes after the intravenous administration of 1 ml of sheep somatostatin antiserum in the rat, plasma growth hormone (GH) levels had reached a maximal 10-15-fold increase which remained approximately constant up to 90 min. The injection of sodium pentobarbital (2 mg/100 g body wt) led to a rapid and transient rise of plasma GH levels of similar magnitude (10-fold) reached 20 to 40 min after injection of the narcotic. The important finding is, however, that the combined administration of somatostatin antiserum and pentobarbital or morphine led to an almost exact additive effect on the plasma GH concentrations. These data, beside providing additional evidence for the existence of GH-releasing activity (GH-RH), indicate that morphine and pentobarbital stimulate GH secretion through increased release of GH-RH.

Inhibition by six somatostatin analogs of plasma growth hormone levels stimulated by thiamylal and morphine in the rat

Administration of sodium thiamylal (50 mg/kg,i.p.) and morphine (3 mg/animal,s.c.) leads to high plasma levels of growth hormone (GH) with a maximum measured approximately 30 min after injection. When the same dose of morphine is administered 60 and 120 min later and small additional doses of thiamylal are injected to maintain the animals deeply anesthetized, constant high levels of plasma GH are maintained up to the last interval studied (3 h). This in vivo model has been used to evaluate the potency and duration of action of somatostatin and of six of its analogs by serial blood sampling of animals bearing a cannula inserted into the right superior vena cava. A significant inhibitory effect of somatostatin (45% inhibition) is observed 15 min after a s.c. injection of 1 mug of the peptide while a near maximal effect (90-95% inhibition) is found at a dose of 25 mug. Both the degree of inhibition and duration of action of somatostatin are dose-dependent. Inhibitory activities equivalent to 1-250 mug of somatostatin can be measured with the model described. [Tyr1] somatostatin, [D-Ala1]somatostatin, [N-acetyl-Cys3] somatostatin and [N-benzoyl-Cys3] somatostatin have activities indistinguishable from somatostatin itself while [D-Lys4] somatostatin and [des-amino1, des-carboxy14] somatostatin have approximately 10% the activity of the natural hypothalamic peptide. This in vivo model offers advantageous characteristics of precision and reproducibility for the evaluation of potency of inhibitors of GH release.

INHIBITORY ACTIVITY OF ANALOGUES OF LUTEINIZING HORMONE‐RELEASING HORMONE (LH‐RH) IN VITRO AND IN VIVO

Improved inhibitors of LH‐RH are those which, beside removal of the histidine residue at position 2 of LH‐RH, include replacement of glycine at position 6 by a d‐amino acid. A still better modification is replacement of the histidine residual at position 2 by d‐phenylalanine. As examples, when tested in pituitary cells in culture, [Des‐His2]LH‐RH, [Des‐His2, D‐Leu6] LH‐RH, [Des‐His2, D‐Phe6]‐LH‐RH, [D‐Phe2]LH‐RH, [D‐Phe2, D‐Leu6]LH‐RH and [D‐Phe2, D‐Phe6]LH‐RH inhibit 50% of LH release induced by LH‐RH at molar ratios (MR50s) of 3000, 500, 60, 1000, 150 and 25, respectively. [D‐Phe2, Phe5, D‐Phe6]LH‐RH have MR50 values of respectively 400, 100, and 75. When evaluated in vivo, some of the mentioned structural modifications permit inhibition of LH‐RH action at molar ratios lower than observed in vitro. At a 500 molar ratio, [D‐Phe2, Phe5, D‐Phe6]‐LH‐RH inhibits the plasma LH rise induced by LH‐RH by 75% up to 5 h after its injection. When administered at 12.00 hours at the dose of 2 mg, this analogue inhibits the sponatenous pro‐oestrus LH surge and ovulation by 85 and 75%, respectively.

Inhibitory activity of four analogs of luteinizing hormone-releasing hormone in vivo.

Four analogs of luteinizing hormone-releasing hormone (LH-RH), [des-His2,D-Ala6]-LH-RH, [des-His2,D-Ala6, des-Gly-NH2(10)1-LH-RH ethylamide, [des-His2,D-Leu6]-LH-RH, and [D-Phe2,D-Leu6]-LH-RH, at 300-fold molar ratios (analog/LH-RH) led to an almost complete inhibition of LH response to LH-RH in anesthetized 4-day cycling rats on the afternoon of proestrus. At a 75-fold molar ratio, [des-His2,D-Ala6]-LH-RH still inhibited the LH-RH-induced LH release by 50%. The ethylamide substitution at the COOH terminus of [des-His2,D-Ala6]-LH-RH did not significantly improve the inhibitory activity of the molecule.