A 26-year-old woman presents to the emergency department with shortness of breath and lightheadedness for 6 hours. She delivered a baby 5 days ago and has had heavy vaginal bleeding since delivery. She takes no medications. Temperature is 36.8°C (98.2°F), blood pressure is 84/52 mm Hg, pulse is 128/min, and respiratory rate is 24/min. Physical examination shows cool extremities and delayed capillary refill. Laboratory studies show hemoglobin 7.1 g/dL. After rapid infusion of isotonic saline, her blood pressure improves to 102/64 mm Hg. Which of the following changes is most likely to occur in the afferent arteriole of this patient’s kidneys during the initial hypotensive period (before fluids are given)?