Abstract Introduction. Heme oxygenase (HO) isoforms, HO-1, and HO-2, are responsible for heme breakdown to iron and carbon monoxide (CO). HO may respond to oxidative stress and may modulate the expression of vasoactive factors like nitric oxide (NO). Since diabetes induced oxidative stress may change HO, the present study examined whether dia- betes is associated with HO alterations, its relationship with NO, endothelin-1(ET-1) and the functional significance. Materials & Methods. Male SD rats with Streptozotocin induced diabetes were investigated after six-weeks. Poorly controlled diabetic animals were randomized to one of three treatment groups (n = 6 each group); a) untreated, b) HO-1 inhibitor SnPP-IX (50 mmol/kgIP/day), c) NO donor mol- sidomine (120mg/L PO/day) and were compared with age and sex matched non diabetic control animals with or without SnPP-IX treatment. Color Doppler ultrasound analysis was used to determine retinal resistivity index (RI). mRNA for HO-1, HO-2, ET-1, eNOS and iNOS were analyzed with competitive RT-PCR. HO distribution in the retina was inves- tigated by immunocytochemistry. Results. Diabetic animals expressed lower body weight, higher blood glucose and increased glycated hemoglobin levels. HO-1 and HO-2 immuno-reactivity were identified in the retina. Diabetes induced increased RI was associated with up-regulation of both ET-1 and HO-1 mRNA expression but not eNOS or iNOS mRNA. Both SnPP-IX and molsido- mine treatments prevented a diabetes increase of RI, in spite of increased ET-1 expression and were associated with increased iNOS mRNA. Conclusions. The present data suggests that the HO system is up-regulated in short term diabetes leading to HO and NO interactions which may modulate vascular function in the retina. Keywords: heme oxygenase; retina; diabetes; mRNA; blood flow Introduction Diabetes is recognized as a worldwide problem reaching near epidemic levels. 1 Associated with both type 1 and 2 diabetes is the development of secondary diabetic complications, which are linked to increased morbidity and mortality. 2 Dia- betic retinopathy predominantly affects the microvasculature of the retina. 3 The Diabetes Control and Complications Trial identified hyperglycemia as one of the main initiating factors of diabetic complications. 4 During the early stages of the disease, hyperglycemia causes increased vascular permeabil- ity and blood flow changes. 5 Alterations of several vasoac- tive molecules and cytokines such as decreased nitric oxide (NO), increased angiotensin II, endothelin (ET) and vascu- lar endothelial growth factor (VEGF) have been demon- strated in the retina in early diabetes. 6,7 Hyperglycemia has also been demonstrated to increase oxidative stress. 8 Associated with oxidative stress are specific biochemical changes including protein kinase C (PKC) activation, the non-enzymatic glycation of proteins and increased polyol pathway activity. 3,9 Oxidative stress may further lead to increased expression of heme oxygenase (HO). HO catalyses the conversion of heme to biliverdin with the release of carbon monoxide (CO) and free iron. Biliverdin is subsequently converted to bilirubin by the enzyme biliverdin reductase. 10 To date, three isoforms (HO- 1, HO-2, and HO-3) have been characterized with the great- est research completed with HO-1 and HO-2. HO-1 is also known as heat shock protein 32 (hsp 32). The protein is extremely sensitive to both oxidative and cellular stresses. 11 HO-2 is expressed constitutively by endothelial cells, neural Received: April 28, 2003 Accepted: August 1, 2003 Correspondence: Subrata Chakrabarti, MD, PhD, FRCP(C), University of Western Ontario, Department of Pathology, Dental Sciences Building, London, Ontario, Canada N6A 5C1. Tel: 519-661-2030; Fax: 519-661-2930; E-mail: schakrab@uwo.ca Heme oxygenase in the retina in diabetes Mark Cukiernik 1 , Suranjana Mukherjee 1 , Donal Downey 2 and Subrata Chakabarti 1,3 Departments of 1 Pathology, 2 Diagnostic Radiology and 3 Microbiology & Immunology, University of Western Ontario, London, Ontario, Canada Current Eye Research 0271-3683/03/2705-301$16.00 2003, Vol. 27, No. 5, pp. 301–308 © Swets & Zeitlinger