The following statements regarding growth hormone are true, except
First, I need to recall the core concepts of growth hormone (GH). GH is produced by the anterior pituitary, regulated by the hypothalamus, and has various effects on metabolism and growth. Its secretion is pulsatile and influenced by factors like sleep, exercise, and nutrition. It acts via the GH receptor, stimulating IGF-1 production in the liver, which is crucial for growth.
Next, the correct answer is the one that's incorrect. Let's think about common misconceptions. For example, GH doesn't directly stimulate skeletal muscle growth; that's more about IGF-1. Also, GH has catabolic effects in certain contexts, like during fasting, where it promotes lipolysis and gluconeogenesis. Another point is that GH levels decline with age, contributing to the somatopause phenomenon.
Now, the wrong options. Let's say one option states that GH is secreted in a diurnal pattern with peaks during sleep. That's actually true. Another might claim that GH stimulates protein synthesis in skeletal muscle directly. But wait, GH's effect on muscle is indirect via IGF-1. So that's a false statement. Another incorrect option could be about GH causing sodium retention; GH is actually involved in sodium excretion. Also, GH deficiency in adults leads to reduced muscle mass and increased fat, so if an option says it increases muscle mass, that's false.
Clinical pearl: The main mediator of GH's growth effects is IGF-1. Remember that GH's direct actions are fewer compared to its indirect actions through IGF-1. Also, in adults, GH deficiency is associated with metabolic syndrome features.
Putting it all together, the correct answer is the option that incorrectly states GH directly stimulates skeletal muscle growth. The other options are true. Need to check each distractor and explain why they're wrong based on the core concepts.
**Core Concept**
Growth hormone (GH) is secreted by the anterior pituitary, regulated by hypothalamic growth hormone-releasing hormone (GHRH) and somatostatin. It stimulates somatomedin C (insulin-like growth factor 1, IGF-1) in the liver, which mediates longitudinal bone growth. GH also has direct metabolic effects on fat, muscle, and glucose.
**Why the Correct Answer is Right**
The false statement is that GH **directly stimulates skeletal muscle growth**. GH primarily acts indirectly via IGF-1, which enhances protein synthesis in skeletal muscle. GH itself promotes lipolysis and gluconeogenesis but does not directly drive muscle hypertrophy. Direct GH effects include increased amino acid uptake in muscle, but growth is mediated through IGF-1.
**Why Each Wrong Option is Incorrect**
**Option A:** "GH secretion is pulsatile and stimulated by sleep" is true—GH is released in episodic bursts, with peaks during deep sleep.
**Option B:** "GH decreases insulin sensitivity" is true—GH antagonizes insulin by increasing hepatic glucose production and reducing peripheral glucose uptake.
**Option C:** "GH deficiency in adults causes decreased muscle mass" is true—adult-onset GH deficiency leads to reduced lean body mass and increased adiposity.
**Clinical