Which of the following statements regarding pituitary hormones is false?

Correct Answer: B

Rationale: Step 1: Antidiuretic hormone (ADH) is released by the posterior pituitary, and its main function is to increase water reabsorption in the kidneys, leading to decreased urine volume and increased blood volume. Step 2: If ADH causes urine volume to increase and blood volume to decrease, it would be counterproductive to its known physiological role. Step 3: Therefore, Choice B is false as it contradicts the established function of ADH. Summary: - Choice A is true because oxytocin and antidiuretic hormone are indeed produced in the hypothalamus and stored in the posterior pituitary. - Choice C is true as luteinizing hormone does trigger ovulation and stimulates the production of progesterone and some estrogens. - Choice D is true because hyposecretion of follicle-stimulating hormone or luteinizing hormone can lead to infertility in both males and females.

Correct Answer: A

Rationale: The correct answer is A: Thyroid hormone. Thyroid hormone acts on its target tissues by a steroid hormone mechanism of action because it is a lipid-soluble hormone that can diffuse through cell membranes and bind to intracellular receptors. These hormone-receptor complexes then directly regulate gene expression. In contrast, choices B, C, and D do not follow a steroid hormone mechanism of action. Parathyroid hormone (PTH) acts through G-protein coupled receptors, Antidiuretic hormone (ADH) acts through V2 receptors, and β1 adrenergic agonists act through beta-adrenergic receptors, which are all distinct from the mechanism of action of steroid hormones.

Correct Answer: D

Rationale: The correct answer is D because growth hormone promotes long bone growth during the formative years by stimulating the growth plates in bones. This hormone plays a crucial role in skeletal growth and development. Choice A is incorrect because somatostatin is a different hormone that inhibits growth hormone release. Choice B is incorrect because growth hormone is primarily regulated by neural mechanisms. Choice C is incorrect because growth hormone actually promotes muscle growth and reduces fat mass.

Correct Answer: C

Rationale: The correct answer is C: G protein acts as the link between first and second messengers. Rationale: 1. Peptide hormones cannot enter the cell membrane due to their large size. 2. Hormones do not directly stimulate genes; they act through signaling pathways. 3. G proteins are key in transmitting signals from hormone receptors to second messengers, facilitating cellular responses. 4. The hormone receptor complex typically stays on the cell membrane to initiate signaling cascades.

Correct Answer: A

Rationale: The correct answer is A because cellular responses to hormones initiating second-messenger systems can activate various pathways. Different hormones can utilize different second messengers such as cAMP, cGMP, IP3, and DAG. This diversity allows for a wide range of responses in target cells. Choice B is incorrect because cyclic AMP phosphodiesterase degrades cAMP, not forms an active second messenger. Choice C is incorrect as it describes the mechanism of a specific protein kinase, which is not a characteristic of second-messenger systems. Choice D is incorrect since hormone binding to intracellular receptors typically leads to gene transcription regulation, not direct activation of second-messenger systems.