Effect of Li+ substitution for extracellular Na+ on GRF-induced GH secretion from rat pituitary cells

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.