Which of the following does not act as a second messenger in second-messenger systems of hormone action?

Correct Answer: B

Rationale: In the context of the endocrine system and second-messenger systems of hormone action, understanding the role of different molecules as second messengers is crucial. In this question, the correct answer is B) calmodulin. Calmodulin is not a second messenger itself but rather a calcium-binding protein that plays a role in transmitting the signal initiated by the second messenger. A) cyclic AMP is a classic example of a second messenger that mediates the action of various hormones like epinephrine and glucagon. It activates protein kinase A, leading to intracellular responses. C) cyclic GMP is another second messenger, particularly associated with signaling pathways involving nitric oxide. It regulates processes like smooth muscle relaxation and neurotransmission. D) Inositol triphosphate (IP3) is also a second messenger that is produced upon activation of certain receptors. IP3 triggers the release of calcium from intracellular stores, leading to various cellular responses. Educationally, this question helps reinforce the importance of understanding the specific roles of different molecules in signaling pathways within the endocrine system. It highlights the diversity of second messengers and their distinct functions in mediating cellular responses to hormonal stimuli. Students need to grasp these concepts to comprehend how hormones exert their effects on target cells and tissues.

Correct Answer: B

Rationale: Step 1: Negative feedback is the primary mechanism in endocrine system regulation to maintain homeostasis. Step 2: When hormone levels reach a certain set point, negative feedback signals the gland to stop producing that hormone. Step 3: This ensures that hormone levels remain within a specific range. Step 4: Positive feedback would lead to further hormone secretion, disrupting homeostasis. Step 5: Neural feedback is not a common mechanism for regulating endocrine secretions.

Correct Answer: D

Rationale: The correct answer is D: ovary. The anterior pituitary gland secretes follicle-stimulating hormone (FSH) and luteinizing hormone (LH) that regulate the function of the ovaries. FSH stimulates the growth of ovarian follicles, while LH triggers ovulation and stimulates the production of progesterone and estrogen. The pancreas (A) is mainly controlled by insulin and glucagon from the pancreas itself. The parathyroid gland (B) is regulated by parathyroid hormone (PTH) secreted by the parathyroid glands. The adrenal medulla (C) is controlled by the sympathetic nervous system, not by anterior pituitary hormones.

Correct Answer: B

Rationale: The correct answer is B because parathyroid hormone (PTH) actually increases blood calcium levels. PTH achieves this by releasing calcium from bone (A), promoting reabsorption of calcium in the kidneys (C), and activating Vitamin D for increased calcium absorption in the intestines (D). By decreasing blood calcium levels, it would go against the primary function of PTH. This choice is incorrect as PTH is known for its role in increasing blood calcium levels through the mentioned mechanisms.

Correct Answer: C

Rationale: Rationale: C is correct because insulin does not stimulate the breakdown of glycogen. Insulin promotes the storage of glucose as glycogen, not its breakdown. A, B, and D are incorrect because insulin does stimulate cellular absorption of glucose, usage of glucose for energy, and lowers blood glucose levels, respectively. Insulin's main role is to facilitate the uptake and storage of glucose, not its breakdown.