Conversion of angiotensin II into active fragments by an endosomal pathway in bovine adrenal medullary cells in primary culture.

We previously demonstrated that angiotensin II (AII) is internalized in primary cultures of bovine adrenal medullary cells by receptor-mediated endocytosis. In the present work, we followed internalized AII in these cells by means of confocal laser scanning microscopy combined with analytical subcellular fractionation techniques and compared its fate with that of transferrin and horseradish peroxidase. The integrity of [125I]AII was investigated by chromatography. With pulse-chase experiments, internalized AII could only be detected in endosomes using either fluorescence microscopy or fractionation studies. With chase, most of the radioactivity initially associated with the cells was rapidly released into the medium, as converted fragments (> 60%), essentially as AIV (80% of the fragments). Fragments efficiently bound to bovine adrenal medullary and cortical cells, a binding that was specifically displaced by AIV > AIII = AII. These results indicate that AII is taken up in bovine adrenal medullary cells and can be rapidly converted in the endosomal pathway into fragments that bind specifically to putative angiotensin receptors. These fragments are presumably biologically active and could act on either the chromaffin cell itself (autocrine) or the cortical cells (paracrine).