<prompt> Vasopressin acts by:Water transpo across collecting ductWater absorption at medullary ductsWater secretion at loop of HenleWater transpo at PCT  <response>: A i.e. Water transpo across collecting ductVasopressin acts by increasing permeability of collecting ductsQ. to water. The key to the action of vasopressin on collecting duct is aquaporin -2 . The overall effect is retention of water in excess of solute which results in decrease effective osmotic pressure 