Study on the functionality and molecular properties of the AT(4) receptor

Whereas the role of angiotensin II (Ang II) has been clarified in numerous tissues and cell types, the physiological relevance of its C-terminal (3-8) degradation fragment, angiotensin IV (Ang IV), is unclear. Previously, we characterized a specific binding site for Ang IV in the bovine adrenal cort ex and on bovine aortic endothelial cells (BAEC). In the present study, we tried to assess the functionality and mechanism of action of this receptor for Ang IV (AT(4) receptor). Our results revealed that none of the classica l second messengers (i.e., cAMP, Ca2+, inositol phosphates, nitric oxide or arachidonic acid derivatives) was modified significantly during acute (les s than 1 h) stimulation of cells with Ang IV. Under normal culture conditio ns, BAEC efficiently internalized I-125-Ang IV. After a 2 h incubation at 3 7 degrees C, acid-resistant binding corresponded to about 50% of total cell -associated radioactivity. This rapid internalization process suggests that the AT(4) receptor is a functional protein. With a photoaffinity labeling approach, we revealed some properties of the AT(4) receptor that are consis tent with those of a growth factor or cytokine receptor.