Myocardial subcellular glycogen distribution and sarcoplasmic reticulum Ca2+ handling: effects of ischaemia, reperfusion and ischaemic preconditioning.

Abstract

Ischaemic preconditioning (IPC) protects against myocardial ischaemia-reperfusion injury. The metabolic and ionic effects of IPC remain to be clarified in detail. We aimed to investigate the effect of IPC (2 times 5min ischaemia) on the subcellular distribution of glycogen and Ca2+-uptake and leakiness by the sarcoplasmic reticulum (SR) in response to ischaemia-reperfusion in cardiomyocytes of isolated perfused rat hearts (Wistar rats, 335±25g). As estimated by quantitative transmission electron microscopy, the pre-ischaemic contribution [%, mean (95% CI)] of three sub-fractions of glycogen relative to total glycogen was 50 (39:61) as subsarcolemmal, 41 (31:50) as intermyofibrillar, and 9 (5:13) as intramyofibrillar glycogen. After 25min of ischaemia, the relative contribution (%) of subsarcolemmal glycogen decreased to 39 (32:47) in control hearts (Con) and to 38 (31:45) in IPC. After 15min reperfusion the contribution of subsarcolemmal glycogen was restored to pre-ischaemic levels in IPC hearts, but not in Con hearts. IPC increased the left ventricular developed pressure following ischaemia-reperfusion compared with Con. In saponin-skinned cardiomyocyte bundles, ischaemia reduced the SR Ca2+-uptake rate, with no effect of IPC. However, IPC reduced a SR Ca2+-leakage at pre-ischaemia, after ischaemia and during reperfusion. In conclusion, subsarcolemmal glycogen was preferentially utilised during sustained myocardial ischaemia. IPC improved left ventricular function reflecting reduced ischaemia-reperfusion injury, mediated a re-distribution of glycogen towards a preferential storage within the subsarcolemmal space during reperfusion, and lowered SR Ca2+-leakage. Under the present conditions, we found no temporal associations between alterations in glycogen localisation and SR Ca2+ kinetics.