Response to Aliev et al.: PCr and ATP Export Both Participate in Energy Transfer from Mitochondria in Normoxic Heart

This is a response to a letter by Aliev et al. (1) We measured total unidirectional CK flux and probed the distribution of energy transfer directly by ATP and PCr (2). The stability of total unidirectional CK flux over a large range of performance in the normoxic heart is in agreement with Refs. 28–32 in Ref. 2. There is no contradiction with Ref. 3. We observe a stability of total unidirectional CK flux in normoxia whereas Pucar et al. (3) observe a postischemic relation of flux and contractility. As stated in Ref. 3, 18O studies estimate net fluxes and not total unidirectional fluxes, such as total unidirectional CK flux. Stable total unidirectional CK flux does not imply constant net fluxes through CK shuttle. At medium performance, as in Ref. 3, CK shuttle may provide a major link between mitochondria and ATPases (Figs. 7 and 9 in Ref. 2). No evidence of significant AK flux (see “Results”) or glycolytic ATP production (model 6) was observed, in agreement with their low normoxic values (3). At higher performance, the CK shuttle is partially bypassed. In addition, extreme stimulation resulted in 2.5× higher RPP (rate pressure product) than in Ref.3, a new metabolic steady state, a reduction of total unidirectional CK flux. No total or subcellular measurements of CK flux were previously reported at such high performance. As clearly explained (2), we consider this extreme condition to be representative of pathology, and it highlights the limit of the isovolumic perfused heart (well known for its lower performance than the working heart of Ref. 4). However, no leakage of CK was observed (data not shown); the RPP-VO2 relationship remained the same as for other stimulation levels. This suggests that both cells and mitochondrial inner membranes were intact.