Recycling of glucose and determination of the Cori Cycle and gluconeogenesis

We have derived equations, by employing [U-C-13] glucose and mass isotopome r analysis, to determine the pathways of glycogen synthesis (J. Katz, W. P. Lee, P. A. Wals, and E. A. Bergner. J. Biol. Chem. 264: 12994-13004, 1989) . More recently, by use of these methods we have derived equations to deter mine the rate of glucose recycling and of gluconeogenesis [Tayek and Katz. Am. J. Physiol. 270 (Endocrinol. Metab. 33): E709-E717, 1996 and 272 (Endoc rinol. Metab. 35): E476-E484, 1997, and Katz and Tayek. Am. J. Physiol. 275 (Endocrinol. Metab. 38): E537-E542, 1988]. The former equations have been criticized and challenged by C. Des Rosiers, B. R. Landau, and H. Brunengra ber [Am. J. Physiol. 259 (Endocrinol. Metab. 22): E757-E762, 1990], and the latter recently by B. R. Landau, J. Wahren, S. F. Previs, G. K. Ekberg, D. Yang, and H. Brunengraber [Am. J. Physiol. 274 (Endocrinol. Metab. 37): E9 54-E961, 1998]. Landau et al. claimed that our equations were in error and "corrected" them. Their analysis, and their values for recycling and glucon eogenesis (GNG) differ markedly from ours. We show here our equations and e stimates of recycling and GNG to be correct. We present here a theoretical analysis of recycling and discuss the determination of the Cori Cycle and G NG. We illustrate by numerical examples the difference in parameters of glu cose metabolism calculated by the methods of Katz and Landau. J. Radziuk an d W. N. P. Lee [Am. J. Physiol. 277 (Endocrinol Metab. 40): E199-E207, 1999 ] and J. K. Kelleher [Am. J. Physiol. 277 (Endocrinol. Metab. 40): E395-E40 0, 1999] present a mathematical analysis that, although differing in some r espects from Landau's, supports his equation for GNG. We show in the APPEND IX that their derivation of the equation for GNG is incorrect.