Lactate-dependent gluconeogenesis and atractyloside-sensitive flux through pyruvate carboxylase are reduced during smoltification of atlantic salmon (Salmo solar)

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We sought to determine whether limited gluconeogenesis might contribute to depletion of liver glycogen during parr-smolt transformation. In a comparison of smolts with parr, we found smolts to exhibit reductions in blood glucose (48%), liver glycogen (97%), hepatic production of glucose from lactate (43%), and mitochondrial pyruvate carboxylase (PC) activity (75%). The relationship between PC flux, assayed as pyruvate-dependent fixation of [14C]HCO3- by iso-lated mitochondria, and liver glycogen content was examined with monthly measurements for 1 year. During the 4 months leading to smoltification, PC flux declined by 94%, in concert with depletion of liver glycogen. Atractyloside, an inhibitor of mitochondrial ATP translocase, greatly amplified seasonal differences in PC flux, stimulating flux ∼ 10-fold in mitochondria from January parr, but not at all in mitochondria from smolts. Measurements of mitochondrial ATP efflux suggested improved retention of ATP in the mechanism, but assays of ATP content did not. In the months following smoltification, glycogen repletion was initiated well in advance of the rise in PC flux. We conclude that: (1) a reduced capacity for gluconeogenesis from lactate contributes to depressed blood glucose and liver glycogen during smoltification, (2) reduced PC flux likely accounts for impaired gluconeogenesis from lactate, (3) factors in addition to PC flux play a role in glycogen repletion upon reversion to parr status, and (4) unexplained seasonal changes render PC flux in mitochondria from parr, but not from smolts, very sensitive to stimulation by atractyloside. © 1996 Wiley-Liss, Inc.

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Journal of Experimental Zoology