Intracellular calcium stores are involved in growth hormone-releasing hormone signal transduction in rat somatotrophs

• Published in: Canadian journal of physiology and pharmacology Vol. 77; no. 7; pp. 520 - 528
• Main Authors: Petit, Audrey, Bleicher, Catherine, Lussier, Benoît T
• Format: Journal Article
• Language: English
• Published: Ottawa, Canada NRC Research Press 01.07.1999; National Research Council of Canada; Canadian Science Publishing NRC Research Press
• Subjects: Animals; Biological and medical sciences; Caffeine - pharmacology; Calcium; Calcium - metabolism; Fundamental and applied biological sciences. Psychology; Growth Hormone-Releasing Hormone - pharmacology; Growth hormones; Hormones and neuropeptides. Regulation; Hypothalamus. Hypophysis. Epiphysis. Urophysis; Male; Pituitary gland; Pituitary Gland, Anterior - metabolism; Rats; Rats, Sprague-Dawley; Ryanodine - pharmacology; Signal Transduction - drug effects; Thapsigargin - pharmacology; Vertebrates: endocrinology; Endocrine gland; Calcium; Rat; Somatotropin releasing factor; Rodentia; Adenohypophysis; Hypothalamic hormone; Signal transduction; Vertebrata; Mammalia; Pituitary gland; Intracellular; Hormone releasing factor; Somatotropic cell
• ISSN: 0008-4212; 1205-7541
• DOI: 10.1139/y99-048

In rat pituitary somatotrophs, the stimulation of growth hormone secretion by growth hormone-releasing hormone (GHRH) is a Ca 2+ -dependent event involving Ca 2+ influx. The presence of calcium-induced calcium release (CICR) Ca 2+ stores has been suggested in these cells. The aim of our study was to demonstrate the presence of CICR stores in rat somatotrophs and to determine their function in GHRH Ca 2+ signalling. To this end we measured cytosolic free Ca 2+ concentration ([Ca 2+ ] i ), using indo-1 in purified rat somatotrophs in primary culture, while altering intracellular Ca 2+ stores. Ionomycin (10 µM) or 4-bromo-A23187 (10 µM), used to mobilise organelle-bound Ca 2+ , raised [Ca 2+ ] i in the absence of extracellular Ca 2+ . Caffeine (5 to 50 mM), used to mobilise Ca 2+ from CICR stores, transiently raised [Ca 2+ ] i in 65% of cells tested. The response to 40 mM caffeine was abolished when Ca 2+ stores were depleted, with 1 µM thapsigargin or with 10 µM ryanodine. All cells that responded to 40 mM caffeine responded to 10 nM GHRH. The [Ca 2+ ] i response to 10 nM GHRH was reversible and repeatable. However, the second response was 38% smaller than the first. Ryanodine treatment abolished the reduction in the second [Ca 2+ ] i response, while thapsigargin increased the reduction by 67%. We conclude that rat somatotrophs possess CICR Ca 2+ stores and that they account for 30-35% of the GHRH-induced increase in [Ca 2+ ] i , and that their partial depletion is involved in somatotroph desensitization.Key words: somatotrophs, growth hormone-releasing hormone, intracellular calcium, calcium stores, calcium-induced calcium release.