Question

Which of the following hormones do not act by a second messenger system? (a) GH (b) Epinephrine (c) \(\mathrm{ICSH}\) (d) Oestrogen

Short answer

Oestrogen doesn't act by a second messenger system.

Step-by-step solution

- \(\mathrm{GH}\)

GH, or Growth Hormone, is a peptide hormone that triggers growth, cell reproduction, and cell regeneration. It operates both through a second messenger system and direct effects.

- Epinephrine

Epinephrine, also known as adrenaline, acts through the second messenger system, leading to the activation of intracellular enzymes.

- \(\mathrm{ICSH}\)

Intercellular Stimulation Hormone (ICSH) is responsible for regulation of Leydig cell function as it does act directly with its receptors, hence it also works with a second messenger mechanism.

- Oestrogen

Oestrogen is a steroid hormone. Steroid hormones generally cross the cell membrane, bind to intracellular receptors and act at level of gene transcription, hence they do not use second messenger system.

Key concepts

Second Messenger System

Hormones often need to communicate messages to cells without entering them directly. This is where the second messenger system comes into play. It involves a series of events where a hormone binds to a receptor on a cell's surface, which then causes a chain reaction inside the cell. This reaction typically involves molecules like cyclic AMP (cAMP) or calcium ions, which act as secondary messengers. They relay signals quickly and efficiently inside the cell to produce the desired effect. Here are some characteristics:

• Allows rapid cellular response without the hormone entering the cell.
• Utilizes conformational change of a receptor protein as a starting point.
• Common examples include epinephrine which activates enzymes that increase glucose production in the liver.

This method is ideal for hormones that need a fast response such as epinephrine. If not using a second messenger, a hormone must find other ways to affect cellular change.

Steroid Hormones

Steroid hormones have a unique way of getting their message across, different from peptide hormones. Being lipid-soluble, they can diffuse through the cell membrane. Once inside, they bind to specific receptors either in the cytoplasm or nucleus of the target cell. This complex then influences gene expression. This process takes longer compared to the second messenger system because it involves directly altering DNA transcription. Key points include:

• Lipid solubility allows them to cross cell membranes easily.
• Regulate processes that require long-term changes like growth and metabolism.
• Examples include sex hormones like oestrogen, which coordinates reproductive functions.

This ability to enter cells and act directly on DNA means they don't rely on a second messenger system.

Growth Hormone

Growth Hormone (GH) is a peptide hormone critical for growth, development, and cellular repair. While it acts through second messenger systems for many of its actions, it also exerts direct effects via its receptor on target cells, such as cartilage, muscle, and bone. GH's effects include promoting growth in tissues by increasing protein synthesis and cell division. Important features are:

• Stimulates growth and regeneration.
• Uses the second messenger when it acts indirectly through insulin-like growth factors (IGFs).
• Can directly influence tissues involved in growth and metabolic regulation.

This dual mechanism allows GH to have powerful effects on physical development and maintenance.

Epinephrine

Epinephrine, also known as adrenaline, is a fast-acting hormone that prepares the body for 'fight-or-flight' responses. It acts primarily through the second messenger system. Specifically, it binds to beta-adrenergic receptors on cells, which then activate a cascade involving cAMP. This process results in various bodily changes, such as increased heart rate and glucose release from the liver. Understanding epinephrine's actions includes:

• Triggers quick responses necessary during stress.
• Works efficiently through secondary messengers like cAMP.
• Orchestrates multiple cellular responses simultaneously.

Through these mechanisms, epinephrine ensures that the body can react promptly to sudden changes in the environment.