Thyrotropin-releasing hormone-mediated Mn2+ entry in perifused rat anterior pituitary cells.
Receptor-mediated Ca2+ influx has been shown to exist in several cell types. Thyrotropin-releasing-hormone (TRH)-stimulated Ca2+ entry has also been postulated to exist in rat anterior pituitary cells, but direct evidence has been lacking. We have measured the fluorescence quenching of indo-1 caused by Mn2+ at a Ca(2+)-insensitive wavelength to investigate the actions of TRH on cation entry in dispersed perifused anterior pituitary cells. In indo-1-loaded cells perifused with Ca(2+)-free medium, Mn2+ caused fluorescence quenching in unstimulated cells; TRH caused further quenching. TRH-stimulated Mn2+ entry was transient, and levelled off within a few minutes in the presence of continuous TRH infusion. TRH-stimulated Mn2+ entry was dependent on the concentration of Mn2+ (50 microM-1 mM). TRH (1 microM) caused a larger effect than TRH (10 nM). La3+ and Ni2+ blocked the quenching stimulated by TRH. The rate of basal quenching was not blocked by dopamine, but TRH-stimulated Mn2+ entry was partially blocked by 1 microM-dopamine and almost completely abolished by 10 microM-dopamine. Thapsigargin (1-5 microM), a tumour promotor which depleted intracellular Ca2+ stores, had little effect on Mn2+. F- (20 mM), which activates G-proteins, also had little effect on Mn2+ entry. We conclude that TRH can transiently stimulate Ca2+ entry through a channel than can pass Mn2+ and be inhibited by dopamine. Depleting Ca2+ stores alone is not sufficient to stimulate Ca2+ entry, and so TRH must do so by other mechanisms.
Cui
Z J
ZJ
Department of Pharmacology, Yale University School of Medicine, New Haven, CT 06510.
Dannies
P S
PS
D000818:Animals; D001692:Biological Transport / Q000187:drug effects; D004298:Dopamine / Q000494:pharmacology; D005260:Female; D005456:Fluorescent Dyes; D066298:In Vitro Techniques; D007211:Indoles; D007811:Lanthanum / Q000494:pharmacology; D008345:Manganese / Q000378:metabolism*; D009532:Nickel / Q000494:pharmacology; D004369:Pentetic Acid / Q000494:pharmacology; D010477:Perfusion; D010903:Pituitary Gland, Anterior / Q000187:drug effects / Q000378:metabolism*; D051381:Rats; D011919:Rats, Inbred Strains; D012969:Sodium Fluoride / Q000494:pharmacology; D013050:Spectrometry, Fluorescence; D013973:Thyrotropin-Releasing Hormone / Q000494:pharmacology*
D005456:Fluorescent Dyes; D007211:Indoles; D008345:Manganese; D013973:Thyrotropin-Releasing Hormone; D007811:Lanthanum; D004369:Pentetic Acid; D009532:Nickel; D012969:Sodium Fluoride; C048960:indo-1; D004298:Dopamine