Regulation of subcellular localization of muscle FBPase in cardiomyocytes. The decisive role of calcium ions.

ABSTRACT

Glyconeogenesis, the synthesis of glycogen from carbohydrate precursors like lactate, seems to be an important pathway participating in replenishing glycogen in cardiomyocytes. Fructose-1,6-bisphosphatase (FBPase), an indispensible enzyme of glyconeogenesis, has been found in cardiomyocytes on the Z-line, in the nuclei and in the intercalated discs. Glyconeogenesis may proceed only when FBPase accumulates on the Z-line. Searching for the mechanism of a FBPase regulation we investigated the effects of the calcium ionophore A23187, a muscle relaxant dantrolene, glucagon, insulin and medium without glucose on the subcellular localization of this enzyme in primary culture of neonatal rat cardiomyocytes. Immunofluorescence was used for protein localization and the intracellular calcium concentration was measured with Fura. We found that the concentration of calcium ions was the decisive factor determining the localization of muscle FBPase on the Z-line. Calcium ions had no effect on the localization of the enzyme in the intercalated discs or in the nuclei, but accumulation of FBPase in the nuclei was induced by insulin.