Characteristics of the ACTH response to repeated pulses of corticotrophin-releasing factor and arginine vasopressin in vitro

Abstract

A multi-column perifusion system was used to investigate the dynamics of the dose-response relationships of ACTH release by ovine pituitary cells when stimulated by both corticotrophin-releasing hormone (CRF) and arginine vasopressin (AVP) given alone and in combination. A dose-response relationship was obtained when 10-min pulses were given at 60-min intervals over the range of 0.002-2000 nmol CRF/1 and 1-2000 nmol AVP/1, with a minimum effective concentration of 0.02 nmol CRF/1 or 1 nmol AVP/1. When AVP was given together with CRF, the expected potentiation of the ACTH response occurred when compared with the summed response of these secretagogues given separately. At the higher concentrations of CRF and AVP used, the ACTH responses to repeated pulses decreased with time during the experiment. The rate of this loss of responsiveness was significantly correlated to the size of the response to the first pulse (for CRF: r = 0.89, P less than 0.01; for AVP: r = 0.95, P less than 0.01), being greatest when the response was potentiated by adding the secretagogues together (for CRF plus AVP: r = 0.95, P less than 0.01). Reduced availability of receptors or changes in intracellular transduction processes may contribute to this desensitization. Reduced levels of secretable ACTH do not appear to be implicated because desensitization to pulses of one secretagogue did not cause equivalent desensitization to the other. In addition, cells stimulated continuously with submaximal levels of either secretagogue showed desensitization while more ACTH was still available for release to higher levels of stimulant.(ABSTRACT TRUNCATED AT 250 WORDS)

Desensitization of the adrenocorticotrophin responses to arginine vasopressin and corticotrophin-releasing hormone in ovine anterior pituitary cells

“…In the sheep, CRH pulses during acute stress rarely last more than 30 min, with maximal portal plasma concentrations being less than 0·4 nM (Caraty et al 1988(Caraty et al , 1990. Our data are inconsistent with evidence provided by Evans et al (1988) indicating that the minimal concentration required for CRH-induced desensitization to repetitive 10 min CRH pulses in sheep anterior pituitary cells is 0·2 nM. This concentration corresponds with the peak CRH level seen in portal blood during acute haemorrhage, and is less than the level seen with insulin-induced hypoglycaemia.…”

“…Earlier in vitro studies reported desensitization to AVP following stimulation with AVP at either high concentrations or for long periods of time (Holmes et al 1984, Antoni et al 1985, Evans et al 1988, Castro 1993. In contrast, secretion of AVP into the hypophyseal portal circulation is highly pulsatile, with the AVP concentration of pulses normally being below 6 nM and secretory events lasting less than 30 min (Caraty et al 1988, Engler et al 1989, Battaglia et al 1998, Dadoun et al 1998.…”

“…Similarly a 1 h treatment with 100 nM CRH caused substantial desensitization to a subsequent stimulation with 100 nM CRH (Reisine & Hoffman 1983). In perifused sheep anterior pituitary cells repeated stimulation with 10 min AVP pulses (100 or 2000 nM) every 60 min for 4 h reduced the ACTH responsiveness of the cells, and successive 10 min pulses of CRH (0·2-2000 nM) also caused decreased ACTH release with time (Evans et al 1988).…”

“…Additionally, prolonged infusion of either CRH (Wynn et al 1988) or AVP (Antoni et al 1985, Koch & Lutz-Bucher 1985 results in down-regulation of their respective receptors. In vitro studies have shown that prior exposure to either AVP or CRH can result in a reduction in ACTH response, or desensitization, to a subsequent stimulation with the same neuropeptide (Reisine & Hoffman 1983, Holmes et al 1984, Hoffman et al 1985, Evans et al 1988, 1996, Thomson et al 1990, Castro 1993. The treatments used in these in vitro studies to induce desensitization were either of high concentration, long duration or a combination of both and did not closely match the characteristics of endogenous CRH and AVP pulses.…”

Reverse Haemolytic Plaque Assay Study of Corticotropin‐Releasing Hormone and Arginine Vasopressin Interaction in Ovine Corticotropes

“…Evans et al (21), using a multi-column perifusion system, have shown that AVP is a less potent stimulus for ACTH release than C R H in ovine pituitary cells. However, evidence has been provided by other workers that the reverse occurs, that is, AVP is a more potent secretagogue than CRH in stimulating ACTH secretion in cultured ovine pituitary cells (1 8) and also in vivo (15,17).…”