The invention provides a synthetic polypeptide of Formula I′: X1-R-X3-R-L-X6-X7-K-X9-P-X11-X12-X13 or an amide, an ester, a salt thereof, or a bioconjugate thereof, wherein X1, X3, X6, X7, X9, X11, X12 and X13 are defined herein. The polypeptides and bioconjugates are agonist of the APJ receptor. The invention also relates to a method for manufacturing the polypeptides and bioconjugates of the invention, and its therapeutic uses such as treatment or prevention of acute decompensated heart failure (ADHF), chronic heart failure, pulmonary hypertension, atrial fibrillation, Brugada syndrome, ventricular tachycardia, atherosclerosis, hypertension, restenosis, ischemic cardiovascular diseases, cardiomyopathy, cardiac fibrosis, arrhythmia, water retention, diabetes (including gestational diabetes), obesity, peripheral arterial disease, cerebrovascular accidents, transient ischemic attacks, traumatic brain injuries, amyotrophic lateral sclerosis, burn injuries (including sunburn) and preeclampsia. The present invention further provides a combination of pharmacologically active agents and a pharmaceutical composition.