[Mechanism of suppressed prolactin secretion due to medium hyperosmolarity--the importance of Ca2+ influx].
It is still unknown how extracellular hyperosmolarity suppresses exocytosis. To evaluate the possibility that extracellular hyperosmolarity affects one of the most important second messenger system, Ca2+ signal, we evaluated the effect of hyperosmolarity on the thyrotropin releasing hormone (TRH)-induced changes in both intracellular Ca2+ concentration ([Ca2+]i) and prolactin (PRL) secretion in GH4C1 cells. TRH caused two phases of [Ca2+]i: an initial high-amplitude phase (first phase), which was not inhibited by Ca2+ free medium, and a sustained low-amplitude phase (second phase), which was abolished by Ca2+ free medium. Medium hyperosmolarity (isotonic = 300mOsm, hypertonic = 338, 375, 450, and 600mOsm) suppressed both TRH-induced phases of [Ca2+]i in a dose dependent manner, however, the suppressive effect was clearly stronger in the second phase of [Ca2+]i than in the first phase of [Ca2+]i. Low doses of medium hyperosmolarity (338 and 375mOsm) suppressed PRL secretion, which was dependent on Ca2+ influx. However, high doses of medium hyperosmolarity (450 and 600mOsm) also blocked PRL secretion, which was dependent on Ca2+ mobilized from cytosolic Ca2+ pools. These data indicate that in GH4C1 cells medium hyperosmolarity may inhibit PRL secretion by both blocking Ca2+ influx and a mechanism unrelated to Ca2+.
Sato
N
N
First Department of Internal Medicine, Gunma University.
Greer
M A
MA
Shimomura
Y
Y
Shimizu
H
H
Uehara
Y
Y
Takahashi
M
M
Ohshima
K
K
Kobayashi
I
I
Mori
M
M
D002118:Calcium / Q000378:metabolism* / Q000502:physiology; D002478:Cells, Cultured; D005260:Female; D006801:Humans; D007477:Ions; D009994:Osmolar Concentration; D011388:Prolactin / Q000378:metabolism*; D015290:Second Messenger Systems / Q000502:physiology; D013973:Thyrotropin-Releasing Hormone / Q000494:pharmacology
D007477:Ions; D013973:Thyrotropin-Releasing Hormone; D011388:Prolactin; D002118:Calcium