Which observation suggested to Sutherland the involvement of a second messenger in epinephrine's effect on liver cells?
A. Receptor studies indicated that epinephrine was a ligand.
B. Glycogen breakdown was observed only when epinephrine was administered to intact cells.
C. Enzymatic activity was proportional to the amount of calcium added to a cell-free extract.
D. Epinephrine was known to have different effects on different types of cells.
E. Glycogen breakdown was observed when epinephrine and glycogen phosphorylase were combined.
Answer: B