Kiang, Juliann Gong

Background Monensin, a Na + ionophore, can increase cytosolic Ca 2+ ([Ca 2+ ] i ) by reversing the Na + /Ca 2+ exchange mechanism. This study provided additional insights into the mechanism of this Na + ionophore-induced increase in [Ca 2+ ] i , and emphasized the critical role of phospholipase C (PLC) in amplifying Na + /Ca 2+ exchange-induced Ca 2+ influx and subsequent arachidonic acid (AA) release in FRTL-5 thyroid cells. The possible involvement of protein kinase C (PKC), mitogen-activated protein kinase (MAPK), and GTP-binding (G) protein in mediating monensin-induced AA release was also explored. Methods FRTL-5 thyroid cells were maintained in Coon's modified Ham's F-12 medium supplemented with a 6-hormone (6H) mixture. Cytosolic Ca 2+ was measured by using indo-1 AM and a dual-wave-length spectrofluorometer. Release of 3 H-labeled inositol trisphosphates and arachidonic acid were determined by a scintillation counter. Results In Hank's balanced salt solution with Ca 2+ (HBSS+), monensin (100 µmol/L) induced a 2.3-fold sustained Ca 2+ increase associated with IP 3 generation and a 6-fold increase in AA release. Deletion of extracellular Ca 2+ , or replacement of Na + by choline chloride in the medium, reduced the [Ca 2+ ] i increase by 77% and completely prevented the monensin-induced rise in AA release. Similar inhibitory effects were observed in cells pretreated with a Na + channel blocker, or Na + /Ca 2+ exchange inhibitors. In HBSS without Ca 2+ (HBSS-), monensin induced a 1-fold transient [Ca 2+ ] i increase but did not increase the AA. This Ca 2+ increase was not suppressed by U-73122, a PLC inhibitor. In HBSS+, U-73122 did not affect the monensin-induced initial transient peak increase of [Ca 2+ ] i , but reduced the sustained second phase of [Ca 2+ ] i from 400 nmol/L to 250 nmol/L, and completely blocked AA release. A phospholipase A 2 (PLA 2 ) inhibitor blocked the monensin-induced AA release without affecting the [Ca 2+ ] i increase. Inhibition of PKC prevented 87% to 94% of the monensin-stimulated AA release. The monensin-induced AA release was also inhibited 94% by pertussis and 51% by a MAP kinase cascade inhibitor. Conclusions The results suggest that monensin initiates an increase in [Ca 2+ ] i via a Na + /Ca 2+ exchange mechanism that triggers more pronounced and sustained [Ca 2+ ] i increase via activation of PLC and Ca 2+ influx. The PLC activation, followed by sustained Ca 2+ influx and PKC activation, is a prerequisite for PLA 2 -mediated processes in monensin-challenged FRTL-5 thyroid cells. Key words: FRTL-5 thyroid cells • phospholipases • protein kinase C • MAP kinase • cell calcium