Molecular and Cellular Biochemistry 267: 31–37, 2004. c  2004 Kluwer Academic Publishers. Printed in the Netherlands. Hyperglycemia does not alter state 3 respiration in cardiac mitochondria from type-I diabetic rats Ossama Lashin and Andrea Romani Department of Physiology and Biophysics, Case Western Reserve University, Cleveland, OH, USA Received 16 January 2004; accepted 16 May 2004 Abstract Cardiovascular complications are the primary cause of death for diabetic patients. Clinically, the development of dysfunctional cardiomyopathy is one of the main complications of diabetes. Experimental evidence indicates that the mitochondrion is one of the main sites implicated in the development of cardiac dysfunction. Yet, the precise cause and mechanisms involved in the process are largely debated. We report here that heart mitochondria from streptozotocin-induced diabetic Sprague-Dawley rats present a gradual reduction in state 3 oxygen consumption that reaches 35% by the fourth week following diabetes onset. Rats presenting a level of hyperglycemia similar to diabetic animals, but not showing the marked weight loss or appearance of urinary ketones typical of the later group present no decline in state 3 mitochondrial oxygen consumption, the values being indistinguishable from those of mitochondria from control animals. Mitochondria from hyperglycemic non-ketotic rats, however, show a 15–20% increase in state 4 respiration, but only when glutamate is used as energetic substrate. Mitochondria from diabetic rats, instead, show a 40–50% increase in state 4 respiration with glutamate and 20–25% with succinate as energetic substrate. Interestingly, hyperglycemic non-ketotic animals present a level of serum insulin intermediate between those of controls and diabetic animals. These functional modifications are unrelated to the time elapsed since the onset of diabetes, as they are observed at 2, 4, 6 as well as 8 and 12 weeks after diabetes onset. Taken together, these data argue against hyperglycemia per se being a direct cause of the decline in state 3 oxygen consumption observed in cardiac mitochondria of type-I diabetic rats. Rather, they point to insulin level and subsequent metabolic alterations as a possible cause for the insurgence of mitochondrial dysfunction. (Mol Cell Biochem 267: 31–37, 2004) Key words: hyperglycemia, type-I diabetes, mitochondria respiration, streptozotocin, insulin Introduction Atherosclerosis and cardiomyopathy are two main compli- cations of diabetes and represent a primary cause of death for diabetic patients. Several factors have been implicated in the development of diabetic cardiomyopathy, including metabolic, biochemical and ultra-structural changes within the cardiac myocyte [1–3]. Mitochondria are the primary source of ATP production in the myocardium. Therefore, dis- ruption of mitochondrial respiratory function is regarded as a key factor in the development of pathologic complications in heart and other tissues in diabetic patients. However, no gen- eral consensus is there about the occurrence of mitochondrial defects under diabetic conditions. Liver mitochondria iso- lated from rats rendered diabetic by alloxan treatment showed Address for offprints: A. Romani, Department of Physiology and Biophysics, School of Medicine, Case Western Reserve University, 10900 Euclid Avenue, Cleveland, OH, 44106-4970, U.S.A. (E-mail: amr5@po.cwru.edu) a deficiency in the ability to synthesize ATP and a low ADP/O ratio [4], an observation that supports the occurrence of a deficit in mitochondrial respiratory function under diabetic conditions. Yet, other investigators have reported no signifi- cant differences in state 3 and 4 respiration of liver mitochon- dria from streptozotocin-induced diabetic rats, although they reported a decrease in ADP/O ratio [5]. A decrease in protein synthesis in addition to a decline in state 3 and uncoupler- stimulated respiration has been observed in mitochondria from skeletal muscles of diabetic rat [6]. Similarly, the as- sessment of respiratory function in mitochondria isolated from streptozotocin-induced diabetic rat hearts showed a de- crease in state 3 but not state 4 respiration [7]. From these observations it appears that controversy still exists about the possible effect of diabetes on mitochondrial respiratory