Introduction: Fasting for 24 hours protects glucose perfused hearts from myocardial ischemia. Prior studies have shown that fasting increases glycogen stores prior to ischemia and glycogen use during ischemia. Since glycogen utilization is primarily regulated by the relative activities of glycogen phosphorylase (GP) a to b (a/b ratio), we hypothesized that fasting would, under ischemic conditions, result in a relative increase in the active form, secondary to increased phosphorylase kinase (phos kinase) activity and/or reduced end-product (G6P) inhibition. Methods: Hearts from fed or 24 hr fasted rats received 15 min of normal perfusion (baseline) followed by total global ischemia of 1 or 10 min, after which the tissue was freeze clamped and biochemically analyzed for enzyme activities and glycolytic intermediates. Results: The a/b ratio was significantly greater in hearts from fasted animals than fed animals (0.29±.04 vs 0.20±.02 [baseline], 0.40±.02 vs 0.27±.02 [1 min ischemia], 0.16±.02 vs 0.12±.01 [10 min ischemia]; all p<0.05). The maximal phos kinase activity was similar in these 2 groups at all time points and at physiologic Ca2+ concentrations. Conclusions: Increased glycogen utilization during ischemia in hearts from fasted animals results from a relative increase in the active form of GP. Since the activity of glycogen phos kinase is not altered in hearts from fasted animals, the relative increase in GPa is not consistent with either increased phosphorylation by PKA or increased sensitivity of the δ sub-unit of phos kinase (calmodulin) during ischemia.
|Original language||English (US)|
|State||Published - Mar 20 1998|
ASJC Scopus subject areas
- Agricultural and Biological Sciences (miscellaneous)
- Biochemistry, Genetics and Molecular Biology(all)
- Cell Biology