Characterization of Ca²⁺ flows essential in ornithine decarboxylase induction by L-asparagine in rat hepatoma cells using Ca²⁺ flow inhibitors

L-Asparagine stimulates bi-directional Ca²⁺ flows and induces ornithine decarboxylase in Reuber H-35 hepatoma cells. Previously it has been shown that these effects are completely, but reversibly inhibited by lanthanum chloride. In this study we examined the role(s) of Ca²⁺ flows using more specific Ca²⁺ flow inhibitors. It was shown that ornithine decarboxylase induction was inhibited by CdCl₂, and verapamil at concentrations above 1 μM and 100 μM respectively, but was unaffected by as much as 300 μM NiCl₂, 1 mM nifedipine, or 10 μM ω-conotoxin. Enzyme induction was blocked by the Ca²⁺-ATPase pump antagonists vanadate and Compound 48/80 in a dose-dependent manner. These results, taken together with the observations that extracellular Ca²⁺ is essential for enzyme induction but a substantial elevation of cytoplasmic [Ca²⁺] is not, suggest that Ca²⁺ inflow independent of the receptor-activated Ca²⁺ channels, and the Ca²⁺ ATPase mediated Ca²⁺ out-flow, are both important factors in the action of L-asparagine.