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

Monensin, a Na+ ionophore, can increase cytosolic free Ca2+ concentration ([Ca2+]i) in many cell types, but no studies have investigated the mechanism underlying a monensin-induced increase in [Ca2+]i in immune cells. In view of this, we investigated the effect of monensin on [Ca2+]i and cytosolic free Na+ concentration ([Na+]i) in mouse splenic lymphocytes using a fluorescence Ca2+ indicator, fura-2, and a fluorescence Na+ indicator, sodium-binding benzofuran isophthalate (SBFI), respectively. Monensin (1–100 μM) caused transient and sustained increases in [Ca2+]i and [Na+]i, respectively, in a concentration-dependent manner. The monensin-induced increase in [Ca2+]i was abolished by the omission of extracellular Ca2+ or 1-[β-[3-(4-methoxyphenyl)propoxy]-4-methoxyphenethyl]-1H-imidazole hydrochloride (SKF-96365, 100–150 μM), and was largely inhibited by Ni2+ (2–5 mM). The omission of extracellular Na+ failed to inhibit the monensin-induced increases in [Ca2+]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 [Ca2+]i. Monensin-induced Mn2+ influx into splenic lymphocytes. The Mn2+ influx was completely inhibited by SKF-96365. These results suggest that monensin transiently increases [Ca2+]i in mouse splenic lymphocytes by stimulating Ca2+ entry via non-selective cation channels in a Na+-independent manner.