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

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.