Is a functional sarcoplasmic reticulum necessary for preconditioning?

Several studies have shown that the protective effect of ischemic precondit ioning (PC) is associated with decreased calcium release from the sarcoplas mic reticulum (SR). However, no study has yet demonstrated whether these ch anges are essential in the mechanism of PC. In order to investigate whether a functional SR was necessary for PC, we manipulated SR calcium handling u sing (i) 0.1 mu M ryanodine (RY), a concentration known to lock the SR calc ium release channel in the open state and (ii) 50 mu M cyclopiazonic acid ( CPA), a specific inhibitor of the SR calcium ATPase. Initial experiments co nfirmed that both RY and CPA eliminated the ability of the SR to accumulate calcium. Isolated rat hearts (n = 6-7/group) were perfused normoxically fo r 30 min prior to either a further 40 min of perfusion [control (C)] or 4 x [5 min ischemia (I) + 5 min reperfusion JR)] (PC). All hearts were then su bjected to a further 40 min I Jr 40 min R. The C and PC protocols were then repeated in the presence of RY or CPA, introduced after 10 min of perfusio n. P-31-NMR was used to measure ATP, PCr, P-i and intracellular pH. RY and CPA decreased developed pressure (DP) by 75% and 59%, respectively. Percent age recovery of LVDP was significantly higher in PC (72 +/- 8%), PC + RY (7 2 +/- 7%) and PC + CPA (49 +/- 7%) groups compared with their respective co ntrols (43 +/- 7%, 47 +/- 7% and 10 +/- 4%) (P<0.05). Thus, PC remains prot ective in the presence of a SR unable to accumulate calcium, suggesting tha t the changes in SR calcium release are not essential in the mechanism of p reconditioning. (C) 2000 Academic Press.