Immunoreactive Somatostatin in Rat Hypophysial Portal Blood*

Abstract

Somatostatin levels have been determined by RIA in hypophysial portal blood of pentobarbital-anesthetized male rats. In most animals, immunoreactive somatostatin (SRIF) levels were higher in hypophysial portal blood than in systemic blood. In euthyroid rats, the mean level was 158 +/- 27 pg/ml (n = 8); SRIF was undetectable (less than 30 pg/ml) in systemic blood of these rats. It is suggested that endogenous SRIF was not degraded during the collection of stalk blood, since synthetic SRIF is stable when incubated in rat serum during 4 min at 37 c and 2 h at 0 C, i.e. under the conditions the blood was kept during the collection. SRIF in hypophysial portal plasma had the same immunoreactivity with a specific antiserum against SRIF as did synthetic SRIF. Gel filtration of hypophysial portal plasma revealed two immunoreactive peaks, the major one corresponding to synthetic SRIF, the smaller one representing a larger molecular form. Thyroidectomy and excess of T4 did not modify the levels of SRIF in hypophysial portal blood, suggestinc SRIF is stable when incubated in rat serum during 4 min at 37 C and 2 h at 0 C, i.e. under the conditions the blood was kept during the collection. SRIF in hypophysial portal plasma had the same immunoreactivity with a specific antiserum against SRIF as did synthetic SRIF. Gel filtration of hypophysial portal plasma revealed two immunoreactive peaks, the major one corresponding to synthetic SRIF, the smaller one representing a large molecular form. Thyroidectomy and excess of T4 did not modify the levels of SRIF in hypophysial portal blood, suggesting that the feedback of thyroid hormones on TSH secretion does not involve changes in the secretion of SRIF by the hypothalamus.

Somatostatin Inhibits the Isoproterenol-Stimulated Adenylate Cyclase in the Intermediate Lobe of the Male Rat Pituitary Gland

“…Other n eu ro p e p tid es (m et-an d leu-enkephalin, (3-endorphin, a-m elan o cy te-stim u latin g h o rm o n e, A C T H , n eu ro ten sin , substance P, vasoactive intestinal polypeptide, and vasopressin) were without effect. Our results suggest a role for pituitary and/or brain somatostatin in the physiological regulation of the intermediate pituitary lobe, and the possibility of an interaction between somatostatins and catecholamines on the regulation of intermediate lobe adenylate cyclase.Somatostatin is located in neurosecretory granules of the median eminence [17] from which it is released to the hypo physeal portal circulation [7] to inhibit anterior pituitary growth hormone release [2], an effect correlated with its ability to inhibit adenylate cyclase activity in this organ [1], Somatostatin terminals were described in the intermedi ate lobe of the rat pituitary [18], High somatostatin levels have been recently detected by radioimmunoassay in this tissue [13]. The functions of somatostatin in the intermedi ate pituitary lobe, however, are unknown.…”

Primary structure of ovine hypothalamic somatostatin-28 and somatostatin-25.

“…The identical tetradecapeptide was subsequently isolated and characterized from porcine hypothalamus (2) as well as pigeon (3) and anglerfish (4) pancreases. In addition, several laboratories (2,3,(5)(6)(7)(8) have observed, in the hypothalamic and pancreatic extracts, bioactive and immunoreactive somatostatin-like substances having molecular weights larger than the tetradecapeptide. Pradayrol et al (9) reported the isolation (from porcine intestinal extracts) and primary structure of an NH2-terminally extended somatostatin containing 28 amino acids which was designated SS-28.…”

Administration of Antisomatostatin Serum to Rats Reverses the Inhibition of Pulsatile Growth Hormone Secretion Produced by Injection of Metergoline but not Yohimbine

“…A large body of data suggests that one such hy pothalamic hormone may be somatostatin, a tetradecapeptide found in the hypothalamus and pituitary portal blood [7,9,14] that can inhibit GH release from pituitary cells, both in vivo [6,19] and in vitro [7], Other peptides in the hypothalamus can also influence GH secretion (e.g., sub stance P [cf. 15].…”