The invention includes a method of reducing urea concentration in a subject's serum. Such a method comprises administering to the subject (e.g., a mammal such as a human) a composition comprising an oligopeptide (or oligopeptides) having activity in reducing urea concentration in the subject's serum as determined by a mouse renal reperfusion test, wherein the oligopeptide comprises the sequence AQG or MTRV (SEQ ID NO: 1), AQGV (SEQ ID NO:2) or LAGV (SEQ ID NO:4).