AJP - Cell Physiology, Vol 256, Issue 4 C712-C718, Copyright © 1989 by American Physiological Society
Effect of Li+ substitution for extracellular Na+ on GRF-induced GH secretion from rat pituitary cells
M. Kato and M. Suzuki
Department of Physiology, Gunma University, Maebashi, Japan.
Several observations have been made on the mechanism of human growth
hormone-releasing factor (hGRF)-induced growth hormone (GH) secretion. 1)
hGRF activates adenylate cyclase and the production of adenosine
3',5'-cyclic monophosphate (cAMP). 2) Extracellular Ca2+ is indispensable
in both hGRF- and excess K+-induced GH secretion. 3) Extracellular Na+ is
also essential in hGRF-induced but not in excess K+-induced GH secretion.
4) Both Ca2+ and Na+ are required in dibutyryl adenosine 3',5'-cyclic
monophosphate (DBcAMP)-induced GH secretion. Thus hGRF may increase Na+
conductance via cAMP, which in turn depolarizes the somatotrophs and
activates voltage-sensitive Ca2+ channels, thereby promoting Ca2+ entry and
GH secretion. To further examine this possibility, replacement of Na+ with
Li+ (an alkali metal ion permeant to Na+ channel) was studied in perifused
dispersed rat anterior pituitary cells. Li+ substitution for extracellular
Na+ did not suppress but augmented hGRF-and DBcAMP-induced GH secretion,
whereas the rise in cellular cAMP content produced by hGRF was greatly
attenuated in Na+-free, Li+ medium. This hGRF-induced GH secretion in
Na+-free, Li+ medium was almost completely nullified by removing
extracellular Ca2+. Thus Li+ was able to replace Na+, which further
suggests the involvement of Na+ channels in hGRF-induced GH secretion. The
possible mechanism of the augmented response in Na+-free, Li+ medium is
discussed.
