A parturient presents for an emergency Cesarean section due to fetal distress.
After placement of a spinal anesthetic with hyperbaric bupivacaine and fentanyl, her blood pressure drops from 125/75 mmHg to 70/40 mmHg despite a 500 mL crystalloid co-load.
The fetal heart rate is now 80 bpm.
What is the most appropriate initial pharmacological intervention?