Monensin causes transient calcium ion influx into mouse splenic lymphocytes in a sodium ion-independent fashion

Abstract

Monensin, a Na + ionophore, can increase cytosolic free Ca 2+ concentration ([Ca 2+] i) in many cell types, but no studies have investigated the mechanism underlying a monensin-induced increase in [Ca 2+] i in immune cells. In view of this, we investigated the effect of monensin on [Ca 2+] i and cytosolic free Na + concentration ([Na +] i) in mouse splenic lymphocytes using a fluorescence Ca 2+ indicator, fura-2, and a fluorescence Na + indicator, sodium-binding benzofuran isophthalate (SBFI), respectively. Monensin (1–100 μM) caused transient and sustained increases in [Ca 2+] i and [Na +] i, respectively, in a concentration-dependent manner. The monensin-induced increase in [Ca 2+] i was abolished by the omission of extracellular Ca 2+ or 1-[β-[3-(4-methoxyphenyl)propoxy]-4-methoxyphenethyl]-1H-imidazole hydrochloride (SKF-96365, 100–150 μM), and was largely inhibited by Ni 2+ (2–5 mM). The omission of extracellular Na + failed to inhibit the monensin-induced increases in [Ca 2+] i. Furthermore, tetrodotoxin (1–10 μM), 5-( N, N-dimethyl)-amiloride (DMA, 10–20 μM), 2-[4-[(2,5-difluorophenyl)methoxy]phenoxy]-5-ethoxyaniline (SEA0400, 3–10 μM), verapamil (10–200 μM), nifedipine (10–200 μM), ω-agatoxin IVA (0.2–10 μM), ω-conotoxin GVIA (1–10 μM), ω-conotoxin MVIIC (0.5–10 μM), and nordihydroguaiaretic acid (NDGA, 1–10 μM) had no effect on the increases in [Ca 2+] i. Monensin-induced Mn 2+ influx into splenic lymphocytes. The Mn 2+ influx was completely inhibited by SKF-96365. These results suggest that monensin transiently increases [Ca 2+] i in mouse splenic lymphocytes by stimulating Ca 2+ entry via non-selective cation channels in a Na +-independent manner.