Heterogeneous increases of cytoplasmic calcium: distinct effects on down-regulation of cell surface sodium channels and sodium channel subunit mRNA levels

1 Long-term (greater than or equal to 12 h) treatment of cultured bovine ad renal chromaffin cells with A23187 (a Ca2+ ionophore) or thapsigargin (TG) [an inhibitor of sarco(endo)plasmic reticulum Ca2+-ATPase (SERCA)] caused a time- and concentration-dependent reduction of cell surface [H-3]-saxitoxi n (STX) binding capacity, but did not change the K-D value. In A23187- or T G-treated cells, veratridine-induced Na-22(+) influx was reduced (with no c hange in veratridine EC50 value) while it was enhanced by a-scorpion venom, beta -scorpion venom, or Ptychocliscus brevis toxin-3, like in nontreated cells. 2 The A23187- or TG-induced decrease of [H-3]-STX binding was diminished by BAPTA-AM. EGTA also inhibited the decreasing effect of A23187. A23187 caus ed a rapid, monophasic and persistent increase in intracellular concentrati on of Ca2+ ([Ca2+](i)) to a greater extent than that observed with TG. 2,5- Di-(t-butyl)-1,4-benzohydroquinone (DBHQ) (an inhibitor of SERCA) produced only a rapid monophasic increase in [Ca2+](i), without any effect on [H-3]- STX binding. 3 Reduction in [H-3]-STX binding capacity induced by A23187 or TG was atten uated by Go6976 (an inhibitor of conventional protein kinase C) or calpasta tin peptide tan inhibitor of calpain). When the internalization rate of cel l surface Na+ channels was measured in the presence of brefeldin A (an inhi bitor of vesicular exit from the trans-Golgi network), A23187 or TG acceler ated the reduction of [H-3]-STX binding capacity. 4 Six hours treatment with A23187 lowered Na+ channel alpha- and beta (1)-s ubunit mRNA levels, whereas TG had no effect. 5 These results suggest that elevation of [Ca2+](i) caused by A23187, TG or DBHQ exerted differential effects on down-regulation of cell surface funct ional Na+ channels and Na+ channel subunit mRNA levels.