Is a Functional Sarcoplasmic Reticulum Necessary for Preconditioning?

Several studies have shown that the protective effect of ischemic preconditioning (PC) is associated with decreased calcium release from the sarcoplasmic reticulum (SR). However, no study has yet demonstrated whether these changes are essential in the mechanism of PC. In order to investigate whether a functional SR was necessary for PC, we manipulated SR calcium handling using (i) 0.1μ ryanodine (RY), a concentration known to lock the SR calcium release channel in the open state and (ii) 50μ cyclopiazonic acid (CPA), a specific inhibitor of the SR calcium ATPase. Initial experiments confirmed that both RY and CPA eliminated the ability of the SR to accumulate calcium. Isolated rat hearts (n=6–7/group) were perfused normoxically for 30 min prior to either a further 40 min of perfusion [control (C)] or 4×[5 min ischemia (I) + 5 min reperfusion (R)] (PC). All hearts were then subjected to a further 40 min I + 40 min R. The C and PC protocols were then repeated in the presence of RY or CPA, introduced after 10 min of perfusion.31P-NMR was used to measure ATP, PCr, Piand intracellular pH. RY and CPA decreased developed pressure (DP) by 75% and 59%, respectively. Percentage recovery of LVDP was significantly higher in PC (72±8%), PC+RY (72±7%) and PC+CPA (49±7%) groups compared with their respective controls (43±7%, 47±7% and 10±4%) (P<0.05). Thus, PC remains protective in the presence of a SR unable to accumulate calcium, suggesting that the changes in SR calcium release are not essential in the mechanism of preconditioning.