Abstract
The uptake, metabolism, and action of cAMP, captured within phospholipid vesicles, in H-35 hepatoma cells were studied. Sonication of lipids in buffer containing cAMP resulted in the formation of 300-A unilamellar lipid vesicles, capturing cAMP in the internal aqueous cavity. Incubation of H-35 hepatoma cells with vesicles containing cAMP (vesicle-cAMP) resulted in rapid incorporation of the vesicle content; apparent saturation of uptake was reached after approximately 30 min of incubation at 37 degrees C. Uptake of vesicle-cAMP was linear over a 10-fold vesicle concentration range. Pretreatment of cells with combined inhibitors of glycolysis and respiration inhibited vesicle uptake by 27%, suggesting vesicle fusion with the cell membrane as a predominant pathway of vesicle uptake. Studies on the metabolism of incorporated cAMP indicated that greater than 50% of the cell-associated radioactivity, derived from vesicle-[3H]cAMP, was preserved as cAMP at the end of a 20-min incubation at 37 degrees C. The incorporation of vesicle-cAMP by H-35 hepatoma cells resulted in increased tyrosine aminotransferase (TAT) activity. The concentration of vesicle-cAMP needed to produce a half-maximal increase in TAT activity was 10 microM, appro...Continue Reading
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