Endothelium is a source of vasoconstrictor factor also. The best characterized of these are the endothelins. Endothelium produces only ET1. Unlike NO, which is released rapidly in response to vasodilator stimuli and then inactivated in a few seconds, ET1 has actions that lasts minutes to hours. ET1 contributes to the regulation of vascular tone primarily by exerting a tonic vasoconstrictor influence. ET1 is also produced by activated human macrophages.