Endogenous glycogen prevents Ca2+overload and hypercontracture in harp seal myocardial cells during simulated ischemia

Abstract

The purpose of this study was to determine if elevated myocardial glycogen content could obviate Ca2+ overload and subsequent myocardial injury in the setting of low oxygen and diminished exogenous substrate supplies. Isolated harp seal cardiomyocytes, recognized as having large glycogen stores, were incubated under conditions simulating ischemia (oxygen and substrate deprivation) for 1 h. Rat cardiomyocytes were used for comparison. Freshly isolated seal cardiomyocytes contained approximately 10 times more glycogen than those from rats (479 ± 39 vs. 48 ± 5 nmol glucose/mg dry weight (dry wt), mean ± S.E., n = 6), and during ischemia lactate production was significantly greater in seal compared to rat cardiomyocytes (660 ± 99 vs. 97 ± 14 nmol/mg dry wt), while glycogen content decreased both in seal (from 479 ± 39 to 315 ± 58 nmol glucose/mg dry wt) and rat cardiomyocytes (from 48 ± 5 to 18 ± 5 nmol glucose/mg dry wt). Cellular ATP was well maintained in ischemic seal cardiomyocytes, whereas it showed a 65% decline (from 31 ± 3 to 11 ± 1 nmol ATP/mg dry wt) in rat cardiomyocytes. Similarly, total seal cardiomyocyte Ca2+ content was not affected by ischemia, while Ca2+ increased from 8.5 ± 2.0 to 13.3 ± 2.0 nmol/mg dry wt in ischemic rat myocytes. Rat cardiomyocytes also showed a notable decline in the percentage of rod-shaped cells in response to ischemia (from 66 ± 4% to 30 ± 3%), and cell morphology was unaffected in seal incubations. Addition of iodoacetate (IAA, an inhibitor of glycolysis) to seal cardiomyocytes, on top of substrate and oxygen deprivation, reduced the cellular content of ATP by 52.9 ± 4.4% (from 25 ± 4 to 11 ± 2 nmol ATP/mg dry wt) and the percentage of rod-shaped myocytes from 51 ± 3% to 28 ± 4%, while total Ca2+ content was unchanged by these conditions. Seal cardiomyocytes thus tolerate low oxygen conditions better than rat cardiomyocytes. This finding is most likely due to a higher glycolysis rate in seals, fueled by larger myocardial glycogen stores.