Several metabolic abnormalities may be triggered secondary to hyperglycemia in diabetes. Some of these abnormalities may alter expression of vasoactive factors in the target organs of diabetic complications. We investigated alterations of endothelin-1 (ET-1) and its receptors, ET(A) and ET(B), and associated structural changes in the myocardium of streptozotocin-induced diabetic rats after 6 months of hyperglycemia. We further assessed the preventive effects of an ET-receptor antagonist bosentan on these changes. Compared to the non-diabetic, age- and sex-matched control animals, diabetic rats showed hyperglycemia, glucosuria, reduced body weight gain and elevated glycated Hb levels. Measurement of ET-1, ET(A) and ET(B) mRNAs by semiquantitative RT-PCR showed significantly increased mRNA levels in the hearts of diabetic rats. Treatment with bosentan failed to reduce ET-1 or ET(B) mRNA expression in diabetes, however ET(A) mRNA expression was reduced. Immunocytochemically, ET-1 was detected in the cardiomyocytes, endothelium and smooth muscle cells of the larger blood vessels and was increased in diabetes. Autoradiographic localization of ET-1 receptors, using (125)I-ET-1, showed increased binding in the endothelium and myocardium of diabetic animals. Histologically, focal fibrous scarring with apoptotic cardiomyocytes, consistent with changes secondary to microvascular occlusion, was only present in the diabetic rats. In keeping with focal fibrosis, myocardium from diabetic rats further showed significantly increased mRNA expression of two extracellular matrix protein transcripts, fibronectin and collagen alpha 1(IV) which were completely prevented by treatment with bosentan. These data suggest that hyperglycemia-induced upregulation of the ET-system in the heart may be important in the pathogenesis of cardiac involvement in diabetes.