The ability of LHRH to induce Ca2+ mobilization and production of inositol phosphates in rat anterior pituitary tissue in vitro was investigated in relation to the self-priming effect of LHRH. Prior exposure to LHRH (which caused a characteristic potentiation of subsequent secretory responses) specifically enhanced LHRH-induced inositol phosphate production and mobilization of intracellular Ca2+ stores. LHRH-induced influx of Ca2+ through dihydropyridine-sensitive Ca2+ channels was unaltered, as was ligand binding to LHRH receptors. These data suggest that a novel facilitation of signalling may occur in the phospho-inositide-Ca2+ mobilization response mechanism during LHRH priming, and that this may represent an important means of regulating cellular responsiveness in gonadotrophs.

The LHRH receptor has been solubilized from male rat anterior pituitary glands, using the zwitterionic detergent 3-((3-cholamidopropyl)-dimethylammonio)-1-propanesulphonate in the presence of a high concentration of sodium chloride. This method gave high yields (up to greater than 70%) of the LHRH-binding site from the membrane preparation. Ligand binding studies using LHRH analogues were carried out to determine dissociation constants for LHRH receptors both in situ in the membrane preparation and for solubilized LHRH receptors. For all the analogues the binding characteristics were similar in both preparations, suggesting that the solubilization procedure left the LHRH receptor undenatured. Gel filtration revealed an apparent molecular weight for the LHRH receptor of 100,000-160,000, with the mean value being approximately twice that found by others using sodium dodecyl sulphate-polyacrylamide gel electrophoretic techniques. The results indicate that the LHRH receptor probably exists in gonadotroph membranes as a large complex of more than one subunit.

Calcium-mobilizing receptors appear to induce calcium movements from both intracellular and extracellular sites.' The mechanism of the Ca2+ influx component remains enigmatic, being probably independent from voltage-sensitive Caz+ channels because it occurs in both excitable and nonexcitable cell types. Several hypotheses for the mechanism have been proposed, including actions of inositol tetrakis-and tris-phosphates, Ca2+-activated Ca2+ channels, and the Na+ /Ca2+One further possibility is that the concomitant activation of protein kinase C (PKC) by such receptors may have a role in signaling Ca2+ influx.Luteinizing hormone-releasing hormone ( LHRH) and thyrotrophin-releasing hormone (TRH) both act through this class of receptor, causing calcium mobilization and phosphoinositide hydrolysis. The Ca-influx processes activated by these hormone receptors in anterior pituitary tissue can be measured by a rapid "Ca2+ influx assay, involving quenching, filtration, and washing with EGTA-containing medium.' The 4sCa2+ influx induced by LHRH, TRH, or K + was maximal by 30 seconds and was concentration dependent, with peak increases over basal "Ca2+ accumulation of 83 2 8% (100 nM LHRH), 162 * 14% (300 nM TRH), and 232 2 30% (60 mM

No abstract

No abstract