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Membrane permeability is regulated by the insertion or removal of carrier proteins, which facilitate the transport of specific molecules across the cell membrane. In the case of glucose, the insulin-sensitive cells use GLUT4 proteins to regulate permeability. When insulin is present, GLUT4 proteins are inserted into the cell membrane, increasing glucose uptake. In the absence of insulin, GLUT4 proteins are removed, reducing glucose permeability. Water permeability is regulated through aquaporin-2 proteins in kidney's collecting duct cells. When blood osmolality increases, the antidiuretic hormone (ADH) triggers the insertion of aquaporin-2 proteins into the cell membrane, increasing water permeability and reabsorption. In the absence of ADH, aquaporin-2 proteins are removed, decreasing water permeability and promoting urine production.