['Although recent studies indicate that renal ischemia preconditioning (IPC) protects the kidney from ischemia-reperfusion (I/R) injury, the precise protection mechanism is still unknown. It has been widely recognized that the activity of nuclear factor-kappaB (NF-kappaB) is altered upon I/R injury. However, few studies have focused on the compositional change of NF-kappaB complexes. In the current study, we observed different NF-kappaB dimers in I/R and IPC, and postulated that p50 homodimers might represent the predominant NF-kappaB activation in renal IPC.', '24 male SD rats were treated with a sham operation, I/R or IPC. While the right kidney was removed in all animals, I/R was induced by left renal artery clamping for 45 min, and IPC was induced by left renal artery clamping for 2 min, reperfused for 5 min, and repeated for 3 cycles before 45 min occlusion. After 24 h of reperfusion, we assessed NF-kappaB activities, composition of NF-kappaB, and expression of IL-18.', 'Compared to the I/R group, NF-kappaB activity decreased significantly in the IPC group. Supershift assays were then performed to examine the NF-kappaB subunits in the binding complexes. In both the I/R and IPC groups, the composition of NF-kappaB contained p65 and p50. The densities of these two supershift bands were almost equivalent in the I/R group, suggesting the NF-kappaB composition was p50/p65 dimers. However, the densities of p50 bands were more than twice that of p65 in the IPC group and the levels of Bcl-3 increased in parallel in these rats, suggesting that p50/p50 homodimers might dominate the NF-kappaB activities in renal IPC. Finally, the expression of IL-18, a marker of acute kidney injury, was significantly increased in the area of tubulointerstitium in the I/R group and attenuated in the IPC group.'] Is renal protection by ischemic preconditioning associated with p50/p50 homodimers?