The time course and extracellular Ca2+ involvement of growth hormone (GH) releasing factor-induced GH secretion in perifused dispersed rat pituitary cells.

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Abstract
The time course of GH secretion in responses 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%, repectively, 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 accelarated 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 106 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.