Elevated Levels of Endogenous Adenosine Alter Metabolism and Enhance Reduction in Contractile Function During Low-flow Ischemia: Associated Changes in Expression of Ca2+-ATPase and Phospholamban

Adenosine has several potentially cardioprotective effects including vasodilatation, reduction in heart rate and alterations in metabolism. Adenosine inhibits catecholamine-induced increase in contractile function mainly through inhibition of phosphorylation of phospholamban (PLB), the main regulatory protein of Ca2+-ATPase in sarcoplasmic reticulum (SR), and during ischemia it reduces calcium (Ca2+) overload. In this study we examined the effects of endogenous adenosine on contractile function and metabolism during low-flow ischemia (LFI) and investigated whether endogenous adenosine can alter expression of the Ca2+-ATPase/PLB-system and other Ca2+-regulatory proteins. Isolated blood-perfused piglet hearts underwent 120 min 10% flow. Hearts were treated with either saline, the adenosine receptor blocker (8)-sulfophenyl theophylline (8SPT, 300 μ mol/l) or the nucleoside transport inhibitor draflazine (1 μ mol/l). During LFI, 8SPT did not substantially influence metabolic or functional responses. However, draflazine enhanced the reduction in heart rate, contractile force and MVO2, with less release of H+and CO2. Before LFI there were no significant differences between groups for any of the proteins (Ca2+-ATPase, ryanodine-receptor, Na+/K+-ATPase) or mRNAs (Ca2+-ATPase, PLB, calsequestrin, Na+/Ca2+-exchanger) measured. At end of LFI mRNA-level of PLB was higher in draflazine-treated hearts compared to both other groups (P<0.01 vs both). Also, at end of LFI protein-level of Ca2+-ATPase was lower in draflazine-treated hearts (P<0.05vs both), and a parallel trend towards a lower mRNA-level was seen (P=0.11 vs saline andP =0.43 vs 8SPT). During LFI tissue Ca2+tended to rise in saline- and 8SPT-treated hearts but not in draflazine-treated hearts (at end of LFI, P=0.01 vs 8SPT). We conclude that the amount of adenosine normally produced during LFI does not substantially influence function and metabolism. However, increased endogenous levels by draflazine enhance downregulation of function and reduce signs of anaerobic metabolism. At end of LFI associated changes in expression of PLB and Ca2+-ATPase were seen. The functional significance was not determined in the present study. However, altered protein-levels might influence Ca2+-handling in sarcoplasmic reticulum and thus affect contractile force and tolerance to ischemia.