Objective: It was previously shown that glucose 6-phosphate (G-6-P) may be used in vitro to support the ATP formation by the reaction catalyzed by hexokinase (FEBS Lett. 1992, 197-201; Ann NY Acad. Sci. 1992, 19-30; J. Biol. Chem. 1992,1829-33). Calorimetric measurements showed that this reaction absorbs heat from the reaction medium (J. Biol. Chem. 2001, 25078-87; J. Membrane Biol. 2002, 1-9). Thus, we decided to measure if the reversal reaction of hexokinase occurs in isolated heart.
Results: During the heart perfusion in Langendorff model with solution without glucose was observed a glucose accumulation of 83 + 20 mmol/g in the perfusate (p<0.005) with was decreased to 38 + 11 mmol/g by N-acetylglucosamine (NAG, a specific inhibitor of hexokinase, p<0.05) and to 18 + 6 mmol/g by LiCl (inhibitor of phosphoglicomutase and fructose-1,6-bisphosphatase, p<0.01). The myocardial temperature also decreased 0,27 + 0,05 ^oC (p<0,01). The temperature decrease was abolish by NAG (p<0.01) and LiCl (p<0.02). The lack of glucose also increased the concentration of lactate in the perfusate (4.19 + 0.30 to 11.64 + 1.99 mmol/g, p<0.0005) and decreased the cardiac content of G-6-P (1.19 + 0.10 to 0.98 + 0.05 mmol/g, p<0.025) and glycogen (3.41 + 0.19 to 0.81 + 0.20 mmol/g, p<0.0005). In presence of NAG or LiCl the cardiac content of G-6-P was similar to control (1.19 + 0.10 to 1.22 + 0.10 mmol/g, p<0.025 and to 1.54 + 0.24 mmol/g, p<0.02, respectively) while the glycogen concentration was smaller than hearts perfused without glucose (0.81 + 0.20 to 0.30 + 0.04 mmol/g, p<0.01 and to 0.27 + 0.03 mmol/g, p<0.01, respectively). The lactate dehydrogenase (LDH) activity in the perfusate increased in the hearts perfused with glucose free solution (0.09 + 0.01 to 0.31 + 0.05 mmol/min, p<0.0005) and NAG did not alter it (0.33 + 0.05 mmol/min). LiCl decreased the LDH activity in the perfusate (0.31 + 0.05 to 0.21 + 0.02 mmol/min, p<0.025) and the efflux of lactate in the perfusate (11.64 + 1.99 to 5.78 + 0.80 mmol/g, p<0.01).
Conclusions: These results suggest that in the heart perfused with glucose free solution the G-6-P from both glycogen and glyconeogenesis could be used to synthesized ATP by the reversal reaction catalyzed by hexokinase leading to glucose accumulation and a paradoxal glucose efflux.
Supported by: CNPq, FAPERJ, PRONEX