RELEASE OF THYROTROPHIN IN VIVO AND IN VITRO BY SYNTHETIC NEUROHYPOPHYSIAL HORMONES

Abstract

The influence of synthetic oxytocin and synthetic lysine-8-vasopressin on the release of thyrotrophin (TSH) from slices of the "basophilic" zone of bovine anterior pituitary glands was determined. Up to 10-fold stimulation of TSH release occurred in the presence of the peptide hormones at low concentrations (approximately 10⁻¹¹ to 10⁻⁹ M). Concentrations greater than 10⁻⁹ M were less stimulatory, ineffective, or inhibitory. In general, vasopressin stimulated at lower concentrations than did oxytocin. The dose–response curve of oxytocin began to descend at lower concentrations than did that of vasopressin.Stimulation of I¹³¹ discharge from the thyroids of propylthiouracil (PTU)-treated, day-old chicks was produced by the intraperitoneal injection of as little as 4 ng vasopressin or 25 ng oxytocin. As the injected dose of either peptide was increased beyond an optimal level, there was less enhancement of I¹³¹ discharge, and, with further increases, inhibition. The decreasing response began with lower doses of oxytocin than of vasopressin. The similarities of the dose–response curves of thyroid I¹³¹ discharge and of in vitro release of TSH indicate that the in vivo effects of injected neurohypophysial peptides are mediated through the release of endogenous TSH.