ACTIVE TRANSPORT OF ION AND SUBSTRATE IN ISOLATED CELLS AND REGULATION MECHANISM OF ENERGY METABOLISM.

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Effects of caffeine on gluconeogenesis and ureagenesis of rat isolated hepatocyte were investigated in the presence of various hormonal agonists. Gluconeogenesis and urea synthesis from 4 mM glutamine were stimulated 1.7 fold and 1.9 fold (compared to control) by 10 <micro> M phenylephrine, respectively. The addition of caffeine at various concentrations with 10 <micro> M phenylephrine induced biphasic effects depending its concentration, i.e., stimulation ( <10^(-4)> - <10^(-3)> M), and inhibition (above 5 mM caffeine). Caffeine at 1 mM stimulated gluconeogenesis and urea synthesis 2.1 fold and 2.4 fold. Effects of caffeine were dependent upon the action of phenylephrine. Caffeine without phenylephrine did not affect both syntheses. Effects of caffeine required the presence of extracellular <Ca^(2+)> , and were not observed in the absence of extracellular <Ca^(2+)> . The stimulations of both syntheses by phenylephrine were also eliminated in the absence of extracellular <Ca^(2+)> .The respiratory rate was enhanced by 13 % by caffeine, indicating that the combined ATP requirement for the increase in urea synthesis and gluconeogenesis caused by caffeine is 1.73 <micro> mol/min/g wet weight. This value corresponded 44 % of total ATP produced by enhanced total oxygen uptake caused by phenylephrine and caffeine. The phosphorylation potentials were constant, 1.7 x <10^3> - 2.4 x <10^3> <M^(-1)> under all of these conditions. The two pathways of glucose and urea syntheses have equal access to the cellular ATP supply, since both syntheses showed very similar patterns. Intramitochondrial and cytochrome c redox state were more reduced by caffeine.