Hypertension is the main side effect developing in patients suffering from renal anemia who are treated with recombinant human erythropoietin (rHuEPO). We investigated the effect of rHuEPO on the vascular tone of isolated rabbit aorta and carotid artery under isometric conditions. The production of prostacyclin and the vasoconstrictor prostanoids PGF2 alpha and TXB2 was investigated in arterial rings incubated with rHuEPO. Endothelial cells from human umbilical veins (HUVECs) were isolated and cultured in flasks (37 degrees C, 5% CO2). After incubation with rHuEPO, the formations of prostacyclin (as its stable metabolite 6-keto-PGF1 alpha), PGF2 alpha, PGE2, thromboxane (TX) B2 and of ET-1 were measured by radioimmunoassays. rHuEPO had no direct vasoconstrictor effect, but it enhanced noradrenalin-induced contractions. This effect was more prominent in rings with intact endothelium than in rings from which the endothelium had been mechanically removed, indicating that endothelial vasoactive factors might be involved. Relaxations to acetylcholine (ACh, 1 microM) were unaltered in the presence or absence of rHuEPO, suggesting that the endothelial NO-cGMP pathway was not impaired by rHuEPO. Incubation with rHuEPO (20 to 200 U/ml) increased the release of the vasoconstrictor mediators ET-1, PGF2 alpha and TXB2, and decreased prostacyclin formation in isolated rabbit arterial rings and in HUVECs, respectively. The cyclooxygenase inhibitor indomethacin abolished the rHuEPO-induced increase in vasoconstrictor prostanoid production. ET-1 formation by HUVECs was also increased by rHuEPO in a dose-dependent manner (maximal effect +90% by rHuEPO 200 U/ml, P < 0.05). Indomethacin and the selective ETA receptor antagonist BQ123 each partly inhibited the enhancement of vascular responsiveness to noradrenalin induced by rHuEPO in rabbit carotid artery, but simultaneous administration of rHuEPO with both antagonists completely abolished the force increment. In conclusion, these studies show that a dose-dependent shift in the balance of constrictor and relaxing prostanoids as well as an increased synthesis of ET-1 induced by rHuEPO lead to the enhanced vascular responsiveness to noradrenalin in isolated rabbit arteries. The increased vascular responsiveness to noradrenalin, which is in line with clinical observations, may contribute to the hypertensive side effect associated with rHuEPO therapy in patients with chronic renal failure.