SECRETION OF ACTH BY PERIFUSED ISOLATED RAT ANTERIOR-PITUITARY-CELLS - PULSES OF SECRETAGOGUE ENHANCE THE SECRETORY RESPONSE AND MODIFY THE EFFECT OF ATRIOPEPTIN

The aim of the present study was to characterize the inhibitory action of atriopeptin on secretagogue-evoked ACTH release in vitro. Perifused isolated rat anterior pituitary cells were exposed to repeated pulses of 41-residue corticotrophin-releasing factor (CRF-41) or arginine vasopressin (AVP). The net ACTH secretory response to both neurohormones increased progressively with the number of pulses applied, until a maximum hormonal response was reached which was stable for the subsequent period of observation (2-3h). The maximal secretagogue-evoked hormone release eventually achieved was 4 and 1.7 times greater than the initial response to AVP and CRF-41 respectively. The size of ACTH response elicited by 50 pmol CRF-41/l and 500 pmol AVP/l (CRF/AVP) given together also underwent progressive enhancement. The number of secretagogue pulses required to reach the maximal response to a particular stimulus depended upon the concentration of the secretagogue peptides, higher concentrations favoured a more rapid development of the stable secretory response. The potency of 103-126 residue atriopeptin to inhibit CRF/AVP-induced ACTH release varied by about 1000-fold depending upon the prior treatment of the cells. In general, cells not previously exposed to secretagogues appeared largely resistant, those under a moderate secretagogue drive were strongly inhibited, and those under intense stimulation were again refractory to inhibition by atriopeptin. In contrast, corticosterone suppressed stimulated ACTH release regardless of the state of the cells. The data demonstrate that the conditions of cell maintenance are pivotal determinants of the inhibitory effect of atriopeptin on secretagogue-stimulated ACTH release in vitro. The in-vivo correlate of these findings may be that the sensitivity of corticotrophs towards atriopeptin is determined by requirements encoded through the pattern of release of hypothalamic CRF-41 and AVP.