CRUCIAL ROLE OF INTRACELLULAR EFFECTORS ON GLYCOGENOLYSIS IN THE ISOLATED RAT-HEART - POTENTIAL CONSEQUENCES ON THE MYOCARDIAL TOLERANCE TOISCHEMIA

The role played by glycogenolysis in the ischemic heart has been recen tly put into question because it is suspected that a slowing down of t his process could be beneficial for the tolerance of the myocardium to ischemia. The role of the intracellular effecters that control the ra te of glycogenolysis has therefore regained interest, We aimed to unde rstand the role played by those intracellular effecters which are dire ctly related to the energy balance of the heart. To this end, we revie w some of the previously published data on this subject and we present new data obtained from P-31 and C-13 NMR spectroscopic measurement on isolated rat heart. Two conditions of ischemia were studied: 15 min g lobal no-flow and 25 min low-flow ischemia. The hearts were isolated e ither from control animals or from rats pre-treated with isoproterenol (5 mg.kg(-1) b.w.i.p.) 1 h before the perfusion in order to C-13 labe l glycogen stores. Our main results are as follows: (1) the biochemica lly determined glycogenolysis rate during the early phase of ischemia (up to 10-15 min) was larger in no-flow ischemia than in low-flow cond itions for both groups, (2) direct measurement of the glycogenolysis r ate, as determined by C-13 NMR, after labelling of the glycogen pool i n the hearts from isoproterenol-treated rats, confirms the estimations from the biochemical data, (3) glycogenolysis was slower in the heart s from pre-treated animals than in control hearts for both conditions of ischemia, (4) the total activity of glycogen phosphorylase (a + b) increased, by 50%, after 5 min no-flow ischemia, whereas it decreased by 42% after the same time of low-flow ischemia. However, the ratio ph osphorylase a/a + b was not altered, whatever the conditions, (5) the concentration of inorganic phosphate (Pi) increased sharply during the first minutes of ischemia, to values above 8-10 mM, under all conditi ons studied. The rate of increase was larger during no-flow ischemia t han during low-flow ischemia. The concentration of Pi was thereafter h igher in controls than in the hearts from isoproterenol-treated animal s. The calculated cytosolic concentration of free 5' AMP increased sha rply at the onset of ischemia, reaching in a few minutes values above 30 mu M in controls and significantly lower values, around 15 mu M, in the hearts from isoproterenol-treated rats. (6) The hearts from isopr oterenol-treated rats displayed a reduced intracellular acidosis, when compared to controls, under both conditions of ischemia. We conclude that the intracellular effecters, mainly free AMP, play an essential r ole in the control of glycogenolysis via allosteric control of phospho rylase b activity. The alteration in the concentration of free Pi, the substrate of both forms of phosphorylase, can also be considered as d eterminant in the control of the rate of glycogenolysis. The attenuati on of ischemia-induced intracellular acidosis in the hearts from isopr oterenol-treated rats could be a consequence of a reduced glycogenolyt ic rate and is likely to be related to a better resumption of the mech anical function on reperfusion.