Which observation suggested to Sutherland the involvement of a second messenger in epinephrine's effect on liver cells?
a. Enzymatic activity was proportional to the amount of calcium added to a cell-free extract.
b. Receptor studies indicated that epinephrine was a ligand.
c. Glycogen breakdown was observed only when epinephrine was administered to intact cells.
d. Glycogen breakdown was observed only when epinephrine and glycogen phosphorylase were mixed.

Verified step by step guidance

1. Understand the role of epinephrine: Epinephrine is a hormone that triggers various physiological responses, including the breakdown of glycogen in liver cells.
2. Identify the concept of a second messenger: A second messenger is a molecule that relays signals received at receptors on the cell surface to target molecules inside the cell, leading to a physiological response.
3. Analyze the options: Consider each observation and determine which one implies the involvement of a second messenger.
4. Focus on intact cells: The observation that glycogen breakdown occurs only when epinephrine is administered to intact cells suggests that the signal from epinephrine requires a cellular component to be effective, indicating the involvement of a second messenger.
5. Conclude the reasoning: The requirement for intact cells implies that the signal from epinephrine is not directly causing glycogen breakdown but is instead triggering a cascade of events inside the cell, likely involving a second messenger.

Key Concepts

Second Messenger System

The second messenger system is a method of cellular signaling where the binding of a ligand to a receptor triggers the release of a secondary molecule inside the cell. This secondary molecule, or 'second messenger,' amplifies the signal and initiates various cellular processes. In the context of epinephrine's effect on liver cells, the second messenger is crucial for transmitting the signal from the cell surface to the interior, leading to glycogen breakdown.

Epinephrine and Glycogen Breakdown

Epinephrine, also known as adrenaline, is a hormone that plays a key role in the 'fight or flight' response, including the breakdown of glycogen in liver cells. Glycogen breakdown is a process where glycogen is converted into glucose, providing energy for the body. The observation that glycogen breakdown occurs only in intact cells suggests the involvement of a second messenger, as the hormone itself does not directly interact with glycogen but rather initiates a signaling cascade.

Cell Signaling and Ligand-Receptor Interaction

Cell signaling involves the interaction between a ligand, such as epinephrine, and a receptor on the cell surface. This interaction triggers a series of intracellular events leading to a specific cellular response. The receptor studies indicating epinephrine as a ligand highlight its role in initiating the signaling pathway that results in glycogen breakdown, emphasizing the importance of receptor-mediated processes in cellular communication.