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

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
Externally publishedYes

Fingerprint

Monensin
Ionophores
Rats
Thyroid Gland
Calcium
Tetrodotoxin
Veratridine
Ryanodine
Diltiazem
Prazosin
Calcium Channel Blockers
Ouabain
Nifedipine
Verapamil
Choline
Fluorescent Dyes
Ion exchange
Sodium
Chemical activation
Electric potential

Keywords

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

ASJC Scopus subject areas

  • Biochemistry
  • Biophysics
  • Cell Biology

Cite this

Na+-ionophore, monensin-induced rise in cytoplasmic free calcium depends on the presence of extracellular calcium in FRTL-5 rat thyroid cells. / Ambroz, Clara; Fein, Henry G.; Smallridge, Robert C.

In: BBA - Biomembranes, Vol. 1028, No. 3, 19.10.1990, p. 229-235.

Research output: Contribution to journalArticle

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N2 - 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.

AB - 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.

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