A 32-year-old man presents with polyuria and polydipsia after sustaining a traumatic brain injury. Laboratory studies reveal a high serum osmolality and a low urine osmolality. He is diagnosed with central diabetes insipidus and is treated with intranasal desmopressin (DDAVP). Following treatment, water permeability in the cortical collecting ducts increases dramatically. Which of the following mechanisms best explains the increase in water permeability in response to DDAVP?