S Ghosh, N B Standen *, M Galiñanes

Dept Cardiac Surgery and *Cell Physiology ,Glenfield Hosp, Univ. of Leicester Leicester, UK

Background: Ischaemic preconditioning (IP) has been shown to exist in all species studied. There are also data supporting the existence of IP in man, however, the phenomenon has not been fully characterised.

Aim: To characterise the protective effect of IP in the human heart.

Methods and Results: Right atrial appendage obtained from patients undergoing elective cardiac surgery were prepared and superfused with normoxic Krebs-Henseleit solution at 37° C. After 30 min stabilisation, muscles were subjected to various protocols of IP followed by 90 min of normothermic ischaemia and then 120 min of reperfusion. At the end of each protocol, the leakage of creatinine kinase (CK, U/g wet wt) and the reduction of MTT to an insoluble formazan dye (OD/mg), an index of cell viability, were measured. Values are expressed as mean ± SEM of 6 experiments. In study 1, muscles were preconditioned by 2, 3, 5 and 10 min of ischaemia followed by 5 min reperfusion before the 90 min ischaemia. The mean values for CK leakage were: 5.29± 0.47, 3.74± 0.35*, 3.06± 0.31*, 6.20± 1.2 vs 5.56± 0.52 in control and for reduction of MTT were: 0.20± 0.04, 0.40± 0.04*, 0.56± 0.05*, 0.21± 0.03, respectively vs 0.18± 0.04 in control (*p<0.01 vs control). In study 2, muscles were preconditioned with 1 to 3 cycles of 2 or 5 min ischemia interspaced with 5 min reperfusion preceeding the 90 min ischaemia. CK leakage for the 2 min IP protocol was: 5.02± 0.1, 2.89± 0.02* and 4.23± 0.10* vs 7.13 ± 0.05 in control and for the 5 min IP protocol was: 2.85± 0.10*, 5.99± 0.15 and 6.93± 0.16 vs 6.82± 0.01 in control (*p<0.01 vs control). The reduction of MTT for the 2 min IP protocol was: 0.12± 0.05, 0.47± 0.09* and 0.27± 0.11 vs 0.11± 0.09 in control and for the 5 min IP protocol: 0.45± 0.01*, 0.1± 0.01 and 0.01± 0.03 vs 0.10± 0.09 in control (*p <0.01 vs control). In study 3, the muscles were preconditioned with 5 min ischaemia followed by 1, 2, 3, or 4 hours reperfusion before the subsequent 90 min ischaemia. CK leakage was: 2.87± 0.17*, 3.43± 0.22*, 6.30± 0.56 and 5.81± 0.46 vs 6.74± 0.28 in control and the MTT reduction measured: 0.46± 0.09*, 0.37± 0.09*, 0.16± 0.02 and 0.17± 0.02 vs 0.15± 0.03 in control (*p<0.01 vs control), respectively.

Conclusion: These studies show that in the human myocardium maximal protection induced by IP using this model is achieved by a total 4-5 min ischaemic stimulus, that extending this period by increasing the ischaemia time or the number of cycles abolishes protection and that this IP-induced protection is lost beyond two hours of its application.