BACKGROUND: Mitochondrial structural changes and energy dysmetabolism frequently occur subsequent to cerebral ischemia. Adenosine triphosphate (ATP)-sensitive potassium channel openers exhibit protective effects on cerebral ischemia/reperfusion injury. OBJECTIVE: To validate the effects of cromakalin on mitochondrial structure and function in ischemic penumbra brain tissue in a rat model of middle cerebral artery occlusion (MCAO). DESIGN, TIME AND SETTING: The present single-factor analysis of variance, randomized, controlled, animal experiment was performed at the Institute of Brain Science, Affiliated Hospital of Qingdao University Medical College between October 2007 and March 2008. MATERIALS: Forty male, Wistar rats were randomly divided into four groups, with 10 rats per group: sham-operated, MCAO, MCAO+ATP-sensitive potassium channel opener (cromakalin), and MCAO+eromakalin+ATP-sensitive potassium channel blocking agent (glibenclamide). METHODS: Focal cerebral ischemia/reperfusion injury was induced by MCAO in all groups except the sham-operated group. The MCAO cromakalin group was administered 10 mg/kg cromakalin (i.p.) prior to MCAO induction. The MCAO+cromakalin+glibenclamide group received an injection of 10 mg/kg cromakalin (i.v.), and subsequently an injection of 10 mg/kg cromakalin (i.p.) prior to MCAO induction. MAIN OUTCOME MEASURES: At 24 hours after cerebral ischemia/reperfusion injury, cellular apoptosis was detected by terminal deoxynucleotidyl transferase-mediated deoxyuridine triphosphate (dUTP) nick-end labeling technique. Cytochrome C expression was measured by immunohistochemistry. In addition, mitochondrial swelling, membrane fluidity, membrane phospholipid and malonaldehyde (MDA) contents, as well as Na+-K+-ATPase, Ca2+-ATPase, and superoxide dismutase (SOD) activities were determined. RESULTS: Compared with the sham-operated group, the three ischemia groups exhibited significantly elevated mitochondrial MDA content, reduced membrane phospholipid and ATP contents, down-regulated membrane fluidity, and reduced Na+-K+-ATPase, Ca2+-ATPase, and SOD activities (P < 0.05-0.01 ). In the MCAO+eromakalin group, the number of apoptotic cells decreased, and cytochrome C expression, as well as MDA content, were reduced. However, ATP content and Na+-K+-ATPase, Ca2+-ATPase, and SOD activities significantly increased compared with the MCAO group (P < 0.05-0.01 ). Glibenclamide noticeably antagonized cromakalin protection of mitochondria.CONCLUSION: Pretreatment with the ATP-sensitive potassium channel opener cromakalin increased mitochondrial Na+-K+-ATPase, Ca2+-ATPase, and SOD activities, decreased neuronal apoptosis, and inhibited cytochrome C expression following MCAO.