Inhibition by extracellular Na+ replacement of GRF-induced GH secretion from rat pituitary cells.
To further clarify the ionic mechanism of the action of growth hormone (GH)-releasing factor (hGRF) on GH secretion, the involvement of extracellular Na+ was studied in perifused dispersed rat anterior pituitary cells. Replacing extracellular Na+ with mannitol or tris(hydroxymethyl)aminomethane (Tris+) suppressed hGRF- and dibutyryl adenosine 3',5'-cyclic monophosphate (DBcAMP)-induced GH secretion. The peak responses to a 2-min application of 1 nM hGRF were 165.0 +/- 5.6 ng/ml (normal medium, mean +/- SE), 21.2 +/- 1.4 ng/ml (Na+-free, mannitol medium), and 18.0 +/- 1.7 ng/ml (Na+-free, Tris+ medium). GH secretion induced by DBcAMP was also suppressed by Na+ replacement to less than 50% of that in normal medium. However, either 15 or 30 mM KCl-stimulated GH secretion was not markedly affected by replacement of Na+ with either compound. Tetrodotoxin, a voltage-sensitive Na+ channel blocker, had no effect on either hGRF- or excess K+-induced GH secretion. cAMP production by hGRF was not greatly affected by replacing extracellular Na+. Thus extracellular Na+ plays an important role in hGRF-induced GH secretion, especially in the process after cAMP production. The involvement of cAMP-sensitive Na+ channels in hGRF-stimulated GH secretion is discussed.
Kato
M
M
Department of Physiology, Gunma University, Maebashi, Japan.
Hattori
M A
MA
Suzuki
M
M
D000818:Animals; D003994:Bucladesine / Q000494:pharmacology; D013006:Growth Hormone / Q000378:metabolism*; D013007:Growth Hormone-Releasing Hormone / Q000494:pharmacology*; D066298:In Vitro Techniques; D007700:Kinetics; D008297:Male; D008353:Mannitol / Q000494:pharmacology; D010903:Pituitary Gland, Anterior / Q000187:drug effects / Q000378:metabolism*; D011188:Potassium / Q000494:pharmacology; D051381:Rats; D011919:Rats, Inbred Strains; D012964:Sodium / Q000494:pharmacology*; D013779:Tetrodotoxin / Q000494:pharmacology
D008353:Mannitol; D013779:Tetrodotoxin; D003994:Bucladesine; D013006:Growth Hormone; D013007:Growth Hormone-Releasing Hormone; D012964:Sodium; D011188:Potassium