The time course and extracellular Ca2+ involvement of growth hormone (GH) releasing factor-induced GH secretion in perifused dispersed rat pituitary cells.
The time course of GH secretion in response to hpGRF and its dependency on the extracellular Ca2+ concentration were studied in perifused dispersed anterior pituitary cells. The onset of GH secretion in response to 1 nM hpGRF was relatively rapid (within 5 s) but removal of hpGRF after 10-min application further increased the rate of secretion (off-response). The threshold and maximum concentrations of hpGRF in stimulatory secretion were 10(-12) and 10(-8) M respectively. Between these two concentrations, the responses showed dose dependency. A reduction in the extracellular Ca2+ concentration to 0.25 mM or to nominally zero reduced hpGRF-induced GH secretion to 64.4% or to 1.9%, respectively, of the control response in the presence of 2.5 mM Ca2+. Two mM Co2+, known as a strong calcium channel blocker, completely suppressed hpGRF-induced GH secretion. The removal of Ca2+ from the perifusion buffer immediately after the offset of 1 min-applied 1 nM hpGRF accelerated the falling phase of GH secretion, which is parallel to the decline in [Ca2+]o in the perifusion chamber. Under nominal Ca2+-free conditions, hpGRF produced no increase in GH secretion. However, 10 min after the offset of 1 min-applied hpGRF under Ca2+-free conditions, the introduction of normal buffer containing 2.5 mM Ca2+ substantially restored GH secretion, although after 20 min the introduction of normal buffer produced only a slight increase in GH secretion. In perifusion experiment of 10(6) cells, intracellular cyclic AMP (cAMP) content was raised from the basal value of 4 to 26 pmol by 2-min application of 1 nM hpGRF. After cessation of hpGRF application, cAMP content decreased to 8.7 pmol at 11 min and returned to the basal value by 20 min. The same tendency was observed in Ca2+-free buffer. In conclusion, the extracellular Ca2+ was essential for hpGRF-induced GH secretion. This indicates the importance of the influx of Ca2+ in response to hpGRF. The time course of hpGRF-induced rise and fall in cAMP content was roughly parallel to the GH secretion. The possible explanations of the off-response and the restoration of GH secretion by reintroducing normal buffer were discussed.
D000818:Animals; D002118:Calcium / Q000502:physiology*; D003035:Cobalt / Q000494:pharmacology; D000242:Cyclic AMP / Q000502:physiology; D004305:Dose-Response Relationship, Drug; D005110:Extracellular Space / Q000502:physiology; D013006:Growth Hormone / Q000378:metabolism*; D013007:Growth Hormone-Releasing Hormone / Q000494:pharmacology / Q000502:physiology*; D066298:In Vitro Techniques; D010477:Perfusion; D010903:Pituitary Gland, Anterior / Q000378:metabolism*; D011189:Potassium Chloride / Q000494:pharmacology; D051381:Rats; D012636:Secretory Rate / Q000187:drug effects; D013997:Time Factors
D003035:Cobalt; D011189:Potassium Chloride; D013006:Growth Hormone; D013007:Growth Hormone-Releasing Hormone; D000242:Cyclic AMP; D002118:Calcium