The purpose of this study is to test the hypothesis that endothelin antagonist (ETA) receptor blockade using ambrisentan is safe, tolerable, and improves kidney function/albuminuria in patients with sickle cell disease (SCD). The investigators anticipate a reduction in proteinuria, a decrease in tricuspid regurgitation jet (TRJ) velocity, reduction in inflammatory markers, improvement in forearm blood flow, and improvement in nociception/pain.