An autoradiographical study of binding sites for vasopressin located on corticotrophs in rat and sheep pituitary glands

Abstract

ABSTRACT Arginine vasopressin (AVP) carried by hypophysial portal blood acts in the pituitary gland, in synergy with corticotrophin-releasing factor (CRF), to induce ACTH secretion. The relative importance of AVP and CRF in this secretory response depends upon the nature and intensity of stressful stimuli, and perhaps also on the species. The aim of the present work was to study, in isolated rat and sheep pituitary glands, the topography of binding sites for AVP and to establish whether they are indeed associated with corticotrophs. To this end, we performed contact autoradiography on tritium-sensitive film using 1·5–2·0 nmol [3H]AVP/l as ligand. ACTH immunoreactivity was detected on sections immediately adjacent to those used for autoradiography. In both species, specific binding sites for AVP were only present in the anterior lobe; the intermediate lobe was not labelled and the neural lobe showed non-specific labelling. Autoradiograms from experiments using [3H]AVP in competition with different synthetic structural analogues showed that, in rat and sheep anterior pituitary glands, receptors differ from the V1 and V2 subtypes. Specific [3H]AVP-binding sites formed an irregular, patchy pattern throughout the anterior lobe. In both species, this pattern was strikingly similar to that formed by cell clusters showing ACTH immunoreactivity, indicating that the AVP-binding sites are associated with the corticotrophs. Immunoreactive cells in the intermediate lobe had no [3H]AVP-binding sites. [3H]AVP binding was more intense in the sheep than in the rat anterior lobe, suggesting that AVP may be particularly important for ACTH secretion in the sheep. Journal of Endocrinology (1991) 129, 197–203