Arginine vasopressin and adrenocorticotropin release: correlation between binding characteristics and biological activity in anterior pituitary dispersed cells.

The present studies were designed to evaluate the binding characteristics of arginine vasopressin (AVP), using rat anterior pituitary dispersed cells, in correlation with the biological activity of the peptide. Synthetic AVP released ACTH from dispersed anterior pituitary cells in a concentration-dependent fashion, with a minimal effective dose of 10(-10) M. [3H] AVP, the ligand for binding studies, showed full biological activity at different concentrations. Saturable and high affinity binding sites for [3H]AVP were obtained using dispersed anterior pituitary cells. Specific binding reached equilibrium by 180 min at 22 C, and rapid and complete dissociation was obtained after the addition of excess AVP. Scatchard analysis of the data indicated a single class of binding sites, with an apparent Kd of 1.63 nM and a binding capacity (Bmax) of 10.7 fmol/10(6) cells, at 37 C. When cells were incubated at 22 C, Kd (7.63 nM) and Bmax (39 fmol/10(6) cells) values were within the same order of magnitude. AVP effectively inhibited [3H]AVP binding with an IC50 of 1.5 X 10(-7) M, while oxytocin showed a somewhat higher IC50 (0.9 X 10(-6) M) and did not achieve complete inhibition of binding. An AVP analog with a ring substitution ( Asu1 ,6 AVP) showed decreased displacement capacity. Both oxytocin and the AVP analog showed weak ACTH-releasing activity compared to synthetic AVP. This indicates that modifications in the tail and/or ring portion of the AVP molecule reduce both the binding affinity and ACTH-releasing activities of these peptides. Other structurally unrelated peptides with intrinsic ACTH-releasing activity, such as rat corticotropin-releasing factor and angiotensin II, had no affinity for AVP-binding sites. The results indicate that specific AVP-binding sites in anterior pituitary cells are closely correlated with the intrinsic ability of the peptide to elicit ACTH release.