['Elevated plasma homocysteine (Hcy) levels have been identified as a pathogenic factor causing a variety of pathological changes in different cells and tissues. In vertebrates, Hcy is produced solely from S-adenosylhomocysteine (AdoHcy) through the catalysis of AdoHcy-hydrolase. The direction of AdoHcy-hydrolase activity is determined by its cytosolic substrate concentrations, thereby controlling intracellular AdoHcy levels. Most S-adenosylmethionine (AdoMet)-dependent methyltransferases are regulated in vivo by the ratio of AdoMet/AdoHcy, which is termed "methylation potential" (MP). To test whether high rates of erythropoietin (EPO) expression is reduced by a low MP in vivo we choosed the model of increased EPO production following carbon monoxide (CO) exposure in rats in which high transcriptional activity is responsible for renal EPO production.', 'To induce a sustained hyperhomocysteinemia in rats, we infused i.v. a low or high dose of Hcy resulting in Hcy plasma levels of 87.4+/-6.2 and 300.8+/-23.7 mumol/l, respectively. Renal tissue contents of AdoHcy, AdoMet, and adenosine (Ado) were measured after freeze clamp by means of HPLC. Within 4h of CO exposure EPO serum levels increased from 13.6+/-0.4 (control) to 2254.8+/-278.3 mIU/ml. Only high dose of Hcy reduces both, the MP from 40.8+/-2.0 to 8.2+/-1.0 in the kidney as well as EPO serum levels by 40% compared to control rats.'] Is hyperhomocysteinemia associated with decreased erythropoietin expression in rats?