Hormone secretagogues increase cytosolic calcium by increasing cAMP in corticotropin-secreting cells.

Abstract

Corticotropin (ACTH)-releasing factor, vasoactive intestinal peptide, and catecholamines-hormones that stimulate ACTH secretion and cAMP generation-increased cytosolic calcium in AtT-20 cells. The increase in intracellular calcium is presumably a consequence of the stimulated cAMP synthesis, since forskolin, an activator of the catalytic unit of adenylate cyclase, and the cAMP analog 8-bromoadenosine 3',5'-cyclic monophosphate (8Br-cAMP) also increased the cytosolic levels of this ion. Pretreatment with somatostatin, a neuropeptide that inhibits stimulation of the adenylate cyclase system and the secretion of ACTH blocked the increase of cytosolic calcium. The effect of 8Br-cAMP, which bypasses the cyclase, was not inhibited by somatostatin pretreatment. The source of the increased calcium appears to be mainly extracellular. This is indicated by the inability of the secretagogues to increase cytosolic calcium in a medium deprived of this ion or in the presence of blockers of voltage-gated calcium channels. The involvement of calcium channels in the calcium rise evoked by the secretagogues was supported by experiments using the whole-cell patch-clamp technique. In these experiments 8Br-cAMP increased voltage-dependent calcium currents. These results suggest the following chain of events in the receptor-mediated elevation of cytosolic calcium and the concomitant release of ACTH from hormone-receptor binding --cAMP synthesis --protein kinase activation calcium channel activation -+ increase in cytosolic calcium many steps --ACTH release. Phorbol myristate acetate, a compound which does not stimulate cAMP generation but enhances the release of ACTH in AtT-20 cells, decreased the cytosolic calcium level.The corticotropin (ACTH)-releasing factor (CRF) stimulates the secretion of ACTH and increases the synthesis of cAMP in anterior pituitary cells (1). Extracellular calcium is essential for the stimulation of ACTH release (2). Studies on the involvement of cAMP and calcium in ACTH secretion from the pituitary are hindered by the difficulty of preparing a pure population of ACTH-secreting cells. The mouse pituitary tumor AtT-20 provides a homogeneous corticotrophic cell line that makes such studies more feasible. AtT-20 cells secrete ACTH in response to a variety of secretagogues, some of which, including CRF, vasoactive intestinal peptide (VIP), isoproterenol, and forskolin, increase the levels of cAMP by either a receptor-mediated stimulation of the adenylate cyclase complex or by a direct activation of the catalytic unit (3). The neuropeptide somatostatin blocks the secretagogue-induced increase in cAMP by acting on the GTP-dependent inhibitory protein component (N.) of the adenylate cyclase complex and, consequently, suppresses the release of ACTH induced by secretagogues (3). Removal of calcium from the medium prevents the ACTH release from AtT-20 cells induced by the above secretagogues (3). We now report that the secretagogues that generate cAMP also increase cytosolic calcium and that cAMP analogs incre...

Glucocorticoids rapidly inhibit oxytocin-stimulated adrenocorticotropin release from rat anterior pituitary cells, without modifying intracellular calcium transients.

“…(8,37,38). It has been proposed that calcium channels in a variety of cells are activated by CAMP (39,40). A study with permeabilized corticotrope tumor cells (AtT-20) provides evidence for requirement of calcium and no direct effect of CAMP on exocytotic release of ACTH (16).…”

MicroRNA 375 Mediates the Signaling Pathway of Corticotropin-releasing Factor (CRF) Regulating Pro-opiomelanocortin (POMC) Expression by Targeting Mitogen-activated Protein Kinase 8

“…It has long been acknowledged that the corticotropin-releasing factor (CRF) from the hypothalamus acts as a key regulator of POMC gene expression in vivo and in vitro (7,8). CRF enhances POMC transcription (8,9) and ACTH secretion (10) by binding to the G protein-coupled CRF receptor, thus increasing intracellular cAMP (11) and calcium levels (12) and activating protein kinase A (PKA). PKA stimulates ERK1/2 activity and increases the transcriptional activity of the POMC promoter (13).…”

Control of action potentials and Ca2+ influx by the Ca(2+)‐dependent chloride current in mouse pituitary cells.

“…Although it is clear that some Ca21 influx is important for secretion from pituitary cells ( Fig. 8B; Luini et al 1985;Holl et al 1988), the present studies suggest that secretion does not depend on the occurrence of rhythmic, long-duration action potentials. This is not unexpected, since bathing cells in high extracellular [K+] or Caa2+ ionophores stimulates secretion (Axelrod & Reisine, 1984 …”

“…For example, somatostatin, which inhibits secretion (Patel & Srikant, 1986), depresses intracellular Ca2+ transients that are associated with spontaneous action potentials (Schlegel et al 1987). Conversely, fi-adrenergic agonists, which stimulate secretion and cause a rise in intracellular Ca21 (Luini et al 1985), have been reported to increase the frequency of action potential firing (Suprenant, 1982;Adler et al 1983). …”

“…Hormone secretion from anterior pituitary cells is usually accompanied by a rise in intracellular calcium, and is prevented by Ca2+ channel blockers or removal of extracellular Ca2+ (Luini, Lewis, Guild, Corda & Axelrod, 1985;Holl, Thorner & Leong, 1988). Both acutely dissociated and clonal anterior pituitary cells spontaneously fire Ca2+ action potentials (Kidokoro, 1975;Taraskevich & Douglas, 1987; Suprenant, 1982;Adler, Wong, Sabol, Busis, Jackson & Weight, 1983;Mollard, Vacher, Guerin, Rogowski & Dufy, 1987;Schlegel, Winiger, Mollard, Vacher, Wuarin, Zahnd, Wollheim & Dufy, 1987).…”