A 63-year-old man is in stable condition after an acute myocardial infarction when he became hypotensive for 3 hours before paramedical personnel arrived. Over the next week, the serum urea nitrogen level increases to 48 mg/dL, the serum creatinine level increases to 5 mg/dL, and the urine output decrease. He undergoes hemodialysis for the next 2 weeks and then develops marked polyuria, with urine output of 2 to 3 L/day. His renal function gradually returns to normal. Release of which of the following substances most likely participated in the elevation of BUN, creatinine, and reduced urinary output?