The aim of this work was to study whether changes in
fructose 2,6-bisphosphate concentration are correlated with variations of the glycolytic flux in the isolated working rat heart. Glycolysis was stimulated to different extents by increasing the concentration of
glucose, increasing the workload, or by the addition of
insulin. The glycolytic flux was measured by the rate of detritiation of [2-3H]- and [3-3H]
glucose. Under all the conditions tested, an increase in
fructose 2,6-bisphosphate content was observed. The
glucose- or
insulin-induced increase in
fructose 2,6-bisphosphate content was related to an increase in the concentration of
fructose 6-phosphate, the substrate of
6-phosphofructo-2-kinase. An increase in the workload correlated with a 50% decrease in the Km of
6-phosphofructo-2-kinase for
fructose 6-phosphate. Similar changes in Km have been observed when purified heart
6-phosphofructo-2-kinase was phosphorylated in vitro by the
cyclic AMP-dependent protein kinase or by the
calcium/calmodulin-dependent protein kinase. Since the concentration of
cyclic AMP was not affected by increasing the workload, it is possible that the change in Km of
6-phosphofructo-2-kinase, which was found in hearts submitted to a high load, resulted from phosphorylation by
calcium/
calmodulin protein kinase; other possibilities are not excluded.
Anoxia decreased the external work developed by the heart, stimulated glycolysis and glycogenolysis, but did not increase
fructose 2,6-bisphosphate.