Human umbilical vein endothelial cells express high affinity neurotensin receptors which are coupled to phosphoinositide turnover and 45Ca2+ efflux (Schaeffer et al., 1995. J. Biol. Chem. 270, 3409-3413). In order to assess the physiological significance of neurotensin receptor activation in endothelial cells, we have compared the in vitro effect of neurotensin on prostacyclin release and cytosolic free calcium increase ([Ca2+]i) as determined by fura-2 fluorescence experiments to the in vivo effect of neurotensin on blood pressure and haematocrit. Neurotensin increased [Ca2+]i levels at low concentrations (EC50 = 4.2 +/- 0.2 nM, n = 3). At similar concentrations, neurotensin was also able to induce prostacyclin release from human umbilical vein endothelial cells (EC50 = 14 +/- 1 nM, n = 3) as determined by a 6-keto-prostaglandin F1 alpha enzyme immunoassay. The neurotensin (100 nM)-induced [Ca2+]i increase and prostacyclin release were inhibited by the specific non-peptide neurotensin receptor antagonist SR 48692 at similar concentrations (IC50 = 41 +/- 16 nM and 86 +/- 17 nM, respectively, n = 3), confirming that these responses were mediated by high affinity neurotensin receptors. Intravenous injection of neurotensin (1-4 nmol/kg i.v.) in the rat resulted in a drop of blood pressure and increased haematocrit, and nearly doubled the plasma levels of 6-keto-prostaglandin F1 alpha, the stable metabolite of prostacyclin. Whereas indomethacin (10 mg/kg i.v.) pretreatment significantly reduced the effect of neurotensin on blood pressure, it did not alter its effect on haematocrit. These results suggest that prostacyclin release plays a role in the hypotensive effects of neurotensin, but is not involved in its effects on haematocrit.