Na+-ionophore, monensin-induced rise in cytoplasmic free calcium depends on the presence of extracellular calcium in FRTL-5 rat thyroid cells

Clara Ambroz, Henry G. Fein, Robert C. Smallridge

Research output: Contribution to journalArticle

18 Scopus citations

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 α1-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 α1-receptor sensitive Ca2+ channels, but may result from activation of an Na2+-Ca2+ exchange system.

Original languageEnglish (US)
Pages (from-to)229-235
Number of pages7
JournalBBA - Biomembranes
Volume1028
Issue number3
DOIs
StatePublished - Oct 19 1990

Keywords

  • (Rat thyroid cell)
  • Calcium ion mobilization
  • Ionophore
  • Monensin
  • Sodium-calcium ion exchange

ASJC Scopus subject areas

  • Biophysics
  • Biochemistry
  • Cell Biology

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