試題詳解

21. At very low concentrations, epinephrine causes an artery to dilate. At higher concentrations epinephrine causes the same artery to constrict. How can these different effects be explained?
(A) There is one type of epinephrine receptor that uses two second-messenger systems.
(B) There are two types of epinephrine receptors with different affinities for epinephrine that use two different second-messenger systems.
(C) There are two types of receptors for epinephrine that use the same second-messenger system.
(D) At higher concentrations epinephrine can pass through the plasma membrane and directly stimulate contraction within the cell.