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 vasodi latation, reduction in heart rate and alterations in metabolism. Adenosine inhibits catecholamine-induced increase in contractile function mainly thro ugh inhibition of phosphorylation of phospholamban (PLB), the main regulato ry protein of Ca2+-ATPase in sarcoplasmic reticulum (SR), and during ischem ia it reduces calcium (Ca2+) overload. In this study we examined the effect s of endogenous adenosine on contractile function and metabolism during low -flow ischemia (LFI) and investigated whether endogenous adenosine can alte r expression of the Ca2+-ATPase/PLB-system and other Ca2+-regulatory protei ns. Isolated blood-perfused piglet hearts underwent 120 min 10% now. Hearts were treated with either saline, the adenosine receptor blocker (8)-sulfop henyl theophylline (8SPT, 300 mu mol/l) or the nucleoside transport inhibit or draflazine (1 mu mol/l). During LFI, 8SPT did not substantially influenc e metabolic or functional responses. However, draflazine enhanced the reduc tion in heart rate, contractile force and MVO2, with less release of H+ and CO2. Before LFI there were no significant differences between groups for a ny of the proteins (Ca2+-ATPase, ryanodine-receptor, Na+/K+-ATPase) or mRNA s (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 C a2+-ATPase was lower in draflazine-treated hearts (P<0.05 vs both), and a p arallel trend towards a lower mRNA-level was seen (P=0.11 vs saline and P=0 .43 vs 8SPT). During IPI tissue Ca2+ tended to rise in saline- and 8SPT-tre ated 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 LF I does not substantially influence function and metabolism. However, increa sed endogenous levels by draflazine enhance downregulation of function and reduce signs of anaerobic metabolism. At end of LFI associated changes in e xpression 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 contract ile force and tolerance to ischemia. (C) 1999 Academic Press.