When the hormone epinephrine binds to receptors on the surface of a heart muscle cell, a new molecule, called cyclic AMP, is produced. Cyclic AMP then causes the heart muscle cells to contract more strongly, increasing blood flow throughout the body. In this example, cyclic AMP is acting as a(n)
A) target cell. B) intercellular enzyme. C) energy carrier. D) second messenger. E) hormone.