Angiotensin II binding to human mononuclear cells: receptor or free fluid endocytosis?

It has recently been claimed that there are angiotensin II (ANG II) receptors on human mononuclear cells and on platelets and this has been used for investigating the regulation of the renin-angiotensin system in hypertension. We here show the following. Binding kinetics of 125I-labelled ANG II and [3H]ANG II to mononuclear cells were slow (maximum at 90 min) and the same as for [3H]-inulin. As with [3H]inulin there was no binding at 4 degrees C. Release from the cells was slow and incomplete (about 30% after 15 min, 60% after 60 min). Binding was not saturable over a range from 10(-12) to 10(-6) mol of ANG II/l, about 8% of offered peptide being bound at all concentrations. Various inhibitors of free fluid endocytosis exhibited the same inhibition pattern of ANG II binding to mononuclear cells. Therefore uptake of ANG II into mononuclear cells displayed all the features of free fluid endocytosis. ANG II was degraded by carboxypeptidase A. When this degradation was prevented by D-phenylalanine, no binding occurred. In platelet preparations contaminated by 0.3-5% of mononuclear cells, 125I-labelled ANG II was degraded as well. Free fluid endocytosis of the degradation product strongly depended on the percentage of contaminating mononuclear cells. We conclude that there are no ANG II receptors on human mononuclear cells and that their presence on human platelets is doubtful.