Mg2+- and MgATP-inhibited and Ca2+/calmodulin-sensitive TRPM7-like current in hepatoma and hepatocytes

Abstract

Although understood to be ubiquitously expressed, the functional identification and significance of Mg2+-inhibited, nonspecific cation currents has been established in only a few cell types. Here we identified an outwardly rectifying nonspecific cation current in quiescent rat hepatocytes and the proliferating and polarized rat hepatoma, WIF-B. Under whole cell recording conditions in which cells were bathed and dialyzed with Na-gluconate solutions, the latter Ca2+and Mg2+free, current reversed close to 0 mV, was time independent, and was greater than 10 times higher at +120 mV compared with −120 mV. Outward current at −120 mV developed slowly, from 17.7 ± 10.3 pA/pF at patch rupture to 106.6 ± 15.6 pA/pF at 12 min in WIF-B cells, and 4.9 ± 2.7 to 20.6 ± 5.6 pA/pF in rat hepatocytes. The nonspecific TRP channel inhibitor, 2-aminoethoxyphenylborate (2-APB), inhibited current (IC50= 72 ± 13 μM) and caused apoptotic cell death in WIF-B cells. Rat hepatocyte survival was more resistant to 2-APB. Dialysis of WIF-B cells with physiological concentrations of Mg2+and Mg-ATP, but not ATP alone, inhibited current development, suggesting that Trpm7 rather than Trpm6 underlies this current. RT-PCR demonstrated that both Trpm6 and Trpm7 are expressed at similar levels in both cell types, suggesting that the functional differences noted are not transcript dependent. Intracellular Ca2+(IC50= 125 ± 35 nM) also inhibited current development, and this could be partially relieved by the calmodulin and Ca2+/calmodulin-dependent kinase inhibitors W-7, staurosporine, KN-93, or calmodulin kinase II (CaMKII) inhibitory peptide. To summarize, our results show that in addition to their established Mg2+sensitivity, Trpm7-like channels are inhibited by cytosolic Ca2+in a CaMKII-dependent manner and may support hepatocellular survival during proliferation.