Kinetic actions and interactions of arginine vasopressin, angiotensin-ii, and oxytocin on adrenocorticotropin secretion by rat anterior pituitary cells in the microperifusion system

Abstract

We have examined the actions and interactions of arginine vasopressin (AVP), angiotensin-II (All), and oxytocin (OT) on the ACTH secretory response of dispersed rat anterior pituitary cells in a microperifusion system. There was a dose-dependent ACTH secretory response to a 3-min perifusion of All which reached its maximum 10 sec after the cells were exposed to AH and fell rapidly to baseline within 2 min, despite continued infusion of AIL This brief spike type of pattern is similar to that produced by AVP, but different from the sustained plateau response induced by CRF. The threshold stimulating concentration of All was about 10−9 M; the maximally stimulating concentration was not defined, but was 10−6 M or more. The initial ACTH response to OT was similar, but fell to a plateau 2 min after the cells were exposed to OT and remained constant until perifusion with OT was stopped, after which it fell rapidly to baseline. The threshold stimulating concentration of OT was 10−8 M; the maximally stimulating concentration was not defined, but was 10−6 M or more. The ACTH secretory response to 10−8 M All was greatly diminished when cells were exposed to 10−6 AVP or 10−6 M OT before All infusion. However, prior exposure to All had no effect on the magnitude of the ACTH secretory response to either AVP or OT. The effects of simultaneous perifusion of All and AVP and of All and OT were additive. When AVP and OT were perifused sequentially, the ACTH secretory response to the peptide that was infused second was completely abolished. Furthermore, the combination of AVP and OT stimulated no greater response than either agent alone. When cells were perifused with the combination of 10−7 M OT and 10−7-to 10−5-M concentrations of two potent AVP Vi receptor antagonists, [1-(β-mercapto-β,β-cyclopentamethylenepropionic acid),2-(O-methyl)tyrosine]-Arg8-vasopressin and [1-deaminopenicillamine-2-(O-methyl)tyrosine]-Arg8vasopressin, both phases of the response to OT were progressively and almost completely inhibited. The initial spike phase was inhibited at lower antagonist concentrations than the sustained plateau phase. These observations suggest 1) that ACTH secretagogues which presumably activate Ca2+/phospholipid-dependent protein kinase-C, such as AVP, All, and OT, induce a transient spike type of ACTH secretory response; 2) that AVP and OT share a common intracellular signal transduction pathway with All; 3) that AVP and OT also appear to activate an additional intracellular signal transduction mechanism not influenced by All; 4) that OT stimulates both phases of the ACTH response by binding to a single receptor, possibly the AVP V1 receptor; and 5) that the initial phase of the response to OT and AVP is mediated by greater occupancy of the V1 receptor than is required for sustained plateau phase activation. © 1989 by The Endocrine Society.