title: Na(+)-ionophore, monensin-induced rise in cytoplasmic free calcium depends on the presence of extracellular calcium in FRTL-5 rat thyroid cells.
abstract: Calcium is an important regulator of cell function, and may be influenced by the intracellular sodium content. In the present study, the Na(+)-ionophore, monensin, was used to investigate the interrelationship between changes in intracellular Na+ concentration ([Na+]i) and elevation of cytosolic Ca2+ concentration ([Ca2+]i) in FRTL-5 thyroid cells. Cytoplasmic Ca2+ levels were measured using the fluorescent dye, indo-1. Monensin induced a dose-dependent increase in [Ca2+]i in FRTL-5 cells. Inhibitors of intracellular Ca2+ release, TMB-8 and ryanodine, were unable to prevent the monensin effect on [Ca2+]i. The alpha 1-receptor antagonist, prazosin, did not block the monensin-stimulated increase in [Ca2+]i. In the absence of extracellular calcium there was a marked diminution in the monensin effect on [Ca2+]i, yet calcium channel antagonists (nifedipine, diltiazem and verapamil) did not inhibit the response. Replacement of Na+ by choline chloride in the medium depressed the monensin-evoked rise in [Ca2+]i by up to 84%. Furthermore, addition of the Na(+)-channel agonist, veratridine, elicited an increase in [Ca2+]i, even though less dramatic than that caused by monensin. Ouabain increased the resting cytosolic Ca2+ concentration as well as the magnitude of the monensin effect on [Ca2+]i. The absence of any effect on the Na(+)-ionophore evoked increase in [Ca2+]i upon addition of tetrodotoxin (TTX) excluded a possible involvement of TTX-sensitive Na+ channels. These data show that the rise in [Ca2+]i induced by increasing [Na+]i is largely dependent on both external Na+ and Ca2+. Calcium entry appears not to involve voltage-dependent or alpha 1-receptor sensitive Ca2+ channels, but may result from activation of an Na(+)-Ca2+ exchange system.
authors.affiliation: Department of Clinical Physiology, Walter Reed Army Institute of Research, Washington, DC 20307-5100.
authors.forename: C
authors.initials: C
authors.lastname: Ambroz
authors.forename: H G
authors.initials: HG
authors.lastname: Fein
authors.forename: R C
authors.initials: RC
authors.lastname: Smallridge
mesh_terms: D000818:Animals; D002118:Calcium / Q000378:metabolism*; D002478:Cells, Cultured; D003593:Cytoplasm / Q000378:metabolism*; D005707:Gallic Acid / Q000031:analogs & derivatives / Q000494:pharmacology; D007476:Ionophores; D008985:Monensin / Q000494:pharmacology*; D011224:Prazosin / Q000494:pharmacology; D051381:Rats; D011916:Rats, Inbred F344; D012964:Sodium / Q000378:metabolism*; D013961:Thyroid Gland / Q000378:metabolism; D014701:Veratridine / Q000494:pharmacology
chemical_list: D007476:Ionophores; C006014:8-(N,N-diethylamino)octyl-3,4,5-trimethoxybenzoate; D005707:Gallic Acid; D014701:Veratridine; D008985:Monensin; D012964:Sodium; D002118:Calcium; D011224:Prazosin