Copper Stimulation of LHRH Release from Median Eminence Explants

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Abstract
We have previously shown that chelated copper stimulates LHRH release from explants of the median eminence area (MEA). Characteristics of this release process are: ligand and metal specificity, the involvement of a limited number of copper interactive sites, and a lack of dependence on extracellular calcium. Since chloride transport is essential for exocytosis of peptides and biogenic amines, we wished to ascertain if chloride transport is essential for the process of CuHis-stimulated release of LHRH. MEA explants were incubated for 15 min with 100 .mu.M CuHis (phase I) and then for 15 min in copper-free medium (phase II) and LHRH released into the medium was evaluated by RIA. In the presence of 136 mM Cl, CuHis stimulated the release of LHRH from a basal level of 5 .+-. 0.4 pg/15 min per MEA to 17 .+-. 0.9 pg during phase I and to 30 .+-. 1.2 pg during phase II. In the absence of Cl-, the CuHis-stimulated release of LHRH during phases I and II was inhibited by 80 and 90%, respectively. In the presence of 136 mM Cl- and the anion transport inhibitor SITS (4-acetamido-4''-isothiocyanostilbene-2,2''-disulfonic acid) the stimulated release was completely inhibited in both phases. When the selectivity of this release process for monovalent anions was tested, the effectiveness of the anions in supporting CuHis-stimulated LHRH release was in this decreasing order: Cl- > Br- > SCN- = acetate > I- = isethionate. In addition, we noted that CuHis-stimulated release was a saturable function of [Cl-]; the saturating [Cl-] for phase I was about 40 mM and the shape of the curve was hyperbolic; the saturating [Cl-] for phase II was about 100 mM and the curve was sigmoid. These results indicate that the process of CuHis-stimulated release of LHRH (1) is dependent on chloride transport, (2) exhibits a selectivity for monovalent anions and (3) involves a limited number of Cl- transport sites. The hyperbolic and sigmoid shapes of the [Cl-] curves are consistent with a single population of Cl- transport sites involved in LHRH release during the exposure to CuHis and with multiple populations after the exposure to CuHis. We propose that the process of CuHis-stimulated release of LHRH operates by compound exocytosis in which an influx of chloride leads to LHRH release by chemiosmotic lysis.