Epinephrine inhibits salivary gland activity and muscle cell contraction in the airways, yet stimulates glycogen release in muscle cells and heart muscle cell contraction. How could one hormone have opposing effects on different organs? Only responsive tissues are exposed to the epinephrine in the blood stream. Cells in tissues that are not responsive contain enzymes that break down epinephrine when it enters the cell. Cells in responsive tissues have signal transduction pathways. Cells in the different tissues would have epinephrine receptors coupled to different signaling pathways. Cells in the tissues that are suppressed do not express an epinephrine receptor.

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