Effects of eryrthropoietin on endothelin-1 synthesis and the cellular calcium messenger system in vascular endothelial cells

A rise in blood pressure is the main side effect of erythropoietin (EPO) treatment in patients with renal anemia. The mechanisms, however, by which EPO may cause hypertension are still unclear. We therefore investigated the effects of EPO on endothelin (ET) synthesis and cytosolic free calcium concentration ([Ca²⁺](i)) in vascular endothelial cells. Porcine endothelial cells were isolated from thoracic aorta, pulmonary artery, and vena cava. Studies were performed with cells of the first subculture. ET concentrations were measured radioimmunologically. Changes in [Ca²⁺](i) were determined with the fluorescent probe fura-2. Cytotoxicity was assessed by sodium 3'-[1-(phenyl-amino-carbonyl)-3,4-tetrazolium]-bis(4-methoxy-6-nitro)b enzene sulfonic acid hydrate (XTT) assay. ET synthesis was similar in cells of different vascular origins and was time-dependent, reaching approximately 2 pmol ET/mg protein within 12 h of incubation. EPO (12 to 200 U/mL) stimulated ET release time- and dose-dependently by up to 83.2% (P < .01) within 12 h in the absence of fetal calf serum and heparin. EPO induced an immediate significant rise in [Ca²⁺](i) from 58 ± 12 nmol/L to 495 ± 85 nmol/L (P < .01) with a subsequent slow return to 257 ± 3 nmol/L. During 2 h of incubation, the Ca-ionophore A 23187 (10^-8 mol/L) moderately but significantly stimulated endothelial ET synthesis. However, the Ca-channel blocker verapamil, the intracellular Ca-release blocker TMB-8, and nickel, an unspecific calcium channel blocker, had no consistent effects on [Ca²⁺](i) or ET synthesis. The protein kinase C inhibitor H-7 stimulated basal [Ca²⁺](i) and cellular ET synthesis. The tyrosine kinase inhibitor genistein suppressed the EPO-induced rise in [Ca²⁺](i) and cellular ET synthesis. From these data we conclude that EPO may stimulate ET synthesis in vascular endothelial cells by activation of an EPO-receptor and via intracellular signalling mechanisms that comprise tyrosine kinase activation and a rise in [Ca²⁺](i). Therefore, the systemic hypertensive effects of EPO may be due at least in part to local stimulation of vascular endothelial ET synthesis via calcium mobilization.