K~Y words second messengers, luteinizing hormone, luteinizing hormone pleiomorphisrn luteinizing hormone-/I messenger ritmnucleic acid. gonadotrophin-releasing hormone Abstract Several second messenger systems have been implicated in mediating the action of gonadotrophin-releasing hormone on the pituitary gonadotrophs and numeroLis studies have shown that activation of these systems induces luteinizing hormone (LH) secretion. However, it is not known how gonadotrophin-releasing hormone or the second messenger systems induce d e novo LH biosynthesis and post-translational modification of the hormone. In these experiments hemipituitary glands have been perifused with drugs which activate second messengers or stimulate protein kinase C directly. The LH secretory responses have been correlated with measurements of common CI and LH/I mRNA and t h e molecular species of LH which were present in t h e pituitary perifusate after exposure to the drugs.Gonadotrophin-releasing hormone (50 ng/ml, 42 nM), with and without the presence of extracellular Ca2'% the Ca2+ ionophore, A23187 (IOpM), and phorbol 12-myristate (1 pM) all stimulated an increase in LHP mRNA compared with controls and the appearance of a different isoform of LH to that found stored in and released from the unstimulated pituitary gland. Phospholipase C was without effect on LH/I mRNA levels and showed minimal efficacy in inducing the appearance of the different LH isoform.The intcraction of gonadotrophin-releasing hormone (GnKH) with specific receptors on the pituitary gonadotroph ultimately ind iiccs both luteinizing hormone (LH) release and synthesis. Thcse effects are mediated by second messenger systems, many of bcliich have been implicated in the LH secretory process. These include the Ca2+-ealmodulin system and phosphoinositide hydrolycij generating diacylglycerol and inositol trisphosphate and arachidonic acid (I).While it is clear that the action of GnRH involves more than a single second messenger system, the precise role of these second mcsacngers in mediating the initial rapid LH response to GnRH and o r the second phase protein synthesis-dependent rclease (2) has not yet been fully established. Even less is known about thc showcd that phorbol estcrs, like GnRH, can incrcase LHP mRNA levels in pituitary cell cultures and they suggest that protein kinase C may, in part, regulate LH biosynthesis. Furthermore, it has been shown that both CAMP and phorbol esters mimic G n R H in enhancing glycosylation of LH ( 5 , 6) thus indicating that both the transcriptional and post-translational effccts of G n R H arc mediated by similar second messenger pathways.In an attempt to establish intracellular mechanisms which may be involved in stimulating LH biosynthesis and post-translational modifications, we have perifused hemipituitary glands (in thc prcscncc of a protcase inhibitor) with a varicty of drugs known to activatc the Ca2+-calmodulin systcm or protein kinase C. The LH responses of the pituitary glands to these secretagogues have been measured and c...