Role of inositol trisphosphate-sensitive calcium stores in the regulation of adrenocorticotropin secretion by perifused rat anterior pituitary cells.
Intracellular Ca2+ (Cai2+) stores contribute significantly to Ca2+ signaling in many types of cells. We studied the role of inositol trisphosphate (InsP3)-sensitive Ca2+ stores, a principal Cai2+ store that presumably is within the endoplasmic reticulum (ER), in cell signaling by examining the effect of thapsigargin (Tg), an ER Ca2+ pump inhibitor that depletes the ER Ca2+ pool, on ACTH secretion. Preincubation for 6-24 h with 2-20 nM Tg had no effect on the resting cytosolic free Ca2+ concentrations ([Cai2+]) but inhibited the ionomycin-stimulated spike-type increase in [Cai2+], which is mediated by InsP3-independent Cai2+ release from the ER, in a dose-dependent (IC50, 4 nM) and time-dependent manner. In ER Cai(2+)-depleted cells, the spike phase (initial 5 min) of the ACTH secretory response to arginine vasopressin (AVP), which is mediated by InsP3-induced Cai2+ release, was also attenuated (IC50, 7.3 nM). However, the spike phase of the ACTH secretory response to AVP was inhibited to a much greater degree than the spike-type response to ionomycin, suggesting that ER Cai2+ stores might have functions other than simply providing Ca2+ for InsP3-stimulated Cai2+ release. Tg pretreatment (IC50, 12 nM) also markedly inhibited the sustained plateau (final 15-min) phase of the ACTH secretory response to AVP, which is mediated by diacylglycerol-induced activation of protein kinase C and subsequent influx of extracellular Ca2+ via L-type voltage-sensitive Ca2+ channels (VSCC), but had no effect on the sustained (full 20 min) response to dioctanoylglycerol that directly activates protein kinase C. Tg had no effect on specific cell binding of [125I]AVP or on specific cell binding of [3H]phorbol 12,13-dibutyrate (except at 20 nM Tg), an index of protein kinase C concentration, or on protein kinase C activity. AVP significantly stimulated inositol trisphosphate accumulation, but pretreatment with Tg completely abolished this effect of AVP, whereas [3H]myoinositol incorporation into membrane-associated inositol lipids and inositol phosphates was unaffected. Thus, Tg-induced depletion of ER Cai2+ stores inhibited both the spike and plateau phases of the ACTH secretory response to AVP, presumably by inhibiting phospholipase C activity and the resulting generation of InsP3 and diacylglycerol. Preincubation with Tg inhibited, in a dose-dependent (IC50, 13 nM) and time-dependent manner, the sustained ACTH secretory response to corticotropin-releasing hormone (CRH) that is mediated by cAMP-induced activation of protein kinase A and Cae2+ influx via L-type VSCC, and the sustained response to forskolin, which directly activates adenylate cyclase.(ABSTRACT TRUNCATED AT 400 WORDS)
Won
J G
JG
Department of Medicine, Veterans General Hospital-Taipei, Taiwan, Republic of China.
Orth
D N
DN
D000324:Adrenocorticotropic Hormone / Q000378:metabolism*; D000818:Animals; D001127:Arginine Vasopressin / Q000378:metabolism; D002118:Calcium / Q000502:physiology*; D003346:Corticotropin-Releasing Hormone / Q000378:metabolism / Q000494:pharmacology; D000242:Cyclic AMP / Q000378:metabolism; D015544:Inositol 1,4,5-Trisphosphate / Q000502:physiology*; D015759:Ionomycin / Q000494:pharmacology; D008297:Male; D010477:Perfusion; D015240:Phorbol 12,13-Dibutyrate / Q000378:metabolism; D010903:Pituitary Gland, Anterior / Q000378:metabolism*; D051381:Rats; D017207:Rats, Sprague-Dawley; D013729:Terpenes / Q000494:pharmacology; D019284:Thapsigargin
D013729:Terpenes; D001127:Arginine Vasopressin; D015240:Phorbol 12,13-Dibutyrate; D015759:Ionomycin; D019284:Thapsigargin; D015544:Inositol 1,4,5-Trisphosphate; D000324:Adrenocorticotropic Hormone; D003346:Corticotropin-Releasing Hormone; D000242:Cyclic AMP; D002118:Calcium