Adrenocorticotropin Stimulation of 2-Ketoglutarate Oxidation by Isolated Rat Epididymal Adipose Tissue MEDIATION BY CYCLIC ADENOSINE 3',5'-MONOPHOSPHATE AND RELATIONSHIP TO LIPID METABOLISM

Abstract Adrenocorticotropin (ACTH) stimulated the oxidation of 2-ketoglutarate-1-14C to 14CO2 by isolated rat epididymal adipose tissue. The results of further studies indicated that this effect of ACTH was mediated by an increase in adipose tissue cyclic 3',5'-adenosine monophosphate which is known to result from an action of the hormone. Caffeine, an inhibitor of the phosphodiesterase which degrades cyclic AMP, in high concentration (10 mm), stimulated 2-ketoglutarate oxidation and, in lower concentration (0.5 mm), potentiated the ACTH effect. An acyl derivative of cyclic AMP, N6,O2'-dibutyryl cyclic 3',5'-adenosine monophosphate (dibutyryl-cAMP) stimulated 2-ketoglutarate oxidation. Phentolamine, a drug known to block the lipolytic effect of cyclic AMP, inhibited ACTH-stimulated 2-ketoglutarate oxidation. The relationship between ACTH effects upon 2-ketoglutarate oxidation and lipid metabolism was also studied. ACTH stimulated the incorporation of 3H from glucose-1-3H into total lipid, presumably reflecting enhanced fatty acid esterification known to result from ACTH action. The hormone inhibited incorporation of label into fatty acid. The enhanced synthetic activity could deplete stores of high energy compounds and indirectly stimulate oxidative processes necessary to replete the high energy stores. The findings did not support this possibility. Caffeine and phentolamine each inhibited incorporation of glucose-3H into total lipid, while stimulating 2-ketoglutarate oxidation. Caffeine inhibited the effect of ACTH upon total lipid synthesis, but potentiated the hormone stimulation of 2-ketoglutarate oxidation. Dibutyryl-cAMP stimulated 2-ketoglutarate oxidation in proportion to the nucleotide concentration over the concentration range of 1 to 10 mm. A lower concentration of dibutyryl-cAMP (0.3 mm) stimulated total lipid synthesis, but the effect of the nucleotide diminished as its concentration was increased. Inhibition of lipid synthesis was observed when the dibutyryl-cAMP concentration was 10 mm. 2-Deoxy-d-glucose suppressed lipogenesis, but did not alter 2-ketoglutarate oxidation by ACTHor dibutyryl-cAMP-stimulated tissues. 2,4-Dinitrophenol (DNP) presumably depleted high energy stores by uncoupling oxidative phosphorylation. This was reflected in almost completely inhibited lipogenesis. DNP only moderately stimulated 2-ketoglutarate oxidation, however. ACTH and dibutyryl-cAMP each stimulate lipolysis of adipose tissue triglycerides. Others have suggested that the fatty acids produced by this action might uncouple oxidative phosphorylation and thereby indirectly stimulate oxidative processes. This possibility is also considered unlikely. As noted above, DNP only moderately (in comparison to the other agents tested) stimulated 2-ketoglutarate oxidation. 2-Deoxy-d-glucose inhibited fatty acid reesterification, and presumably caused tissue free fatty acid levels to rise. In spite of this, no increase in 2-ketoglutarate oxidation was observed when 2-deoxy-d-glucose was added alone, or in the presence of ACTH or dibutyryl-cAMP. DNP suppressed the stimulatory effects of ACTH and of dibutyryl-cAMP upon 2-ketoglutarate oxidation.