2. A particular group of similar cells responds to 2 different hormones that are

Question

2. A particular group of similar cells responds to 2 different hormones that are secreted when blood sugar falls. Hormone A is lipid-soluble and the Hormone B is water-soluble. Hormone A is released from its gland directly in response to a drop in blood sugar. Hormone B is released at the end of a pathway that involves the release of 2 other hormones prior to the release of hormone B. Without identifying specific hormones, contrast the homeostatic regulatory pathways that would involve the release of these 2 hormones and the restoration of normal blood sugar. Consider the water vs. lipid solubility and the different ways in which each hormone release is regulated in your answer.

Explanation / Answer

We will talk of Hormone B first (and you will know why in a while). Hormone B is water-soluble. When blood sugar falls, this is sensed by the hypothalamus. The hypothalamus produces a "releasing" hormone that will act on the pituitary to produce a "tropic" hormone. The "tropic" hormone acts on the specific cells to cause the release of pre-synthesized Hormone B. Hormone B, being water-soluble, travels freely in unbound state through blood to reach the target cells and binds to cell-surface receptors. This triggers the activation of a second messenger within the cell, which brings about different cellular changes, for example activation of glycogenase, resulting in restoration of blood sugar. The effects of Hormone B are short-lived.

Hormone A is directly released in response to a drop in blood sugar. However, being lipid-soluble, it cannot be pre-synthesized and stored. Hence, it is produced when required. Also, being lipid-soluble, it requires carriers for traveling through blood before it can reach and enter its target cells. Once inside the cell, the hormone moves in to the nucleus and binds to receptors resulting in the transcriptional changes such that either the production of some new enzymes is started or the production of some house-keeping enzymes gets shut. The effects of Hormone A are long-lived.

(You may, however, please note that the mechanism of release detailed for Hormones A and B in the question is quite the opposite. Lipid-soluble hormones, like hormone A, are usually secreted following a "releasing" hormone from the hypothalamus and a "tropic" hormone from the pituitary, whereas water-soluble hormones, like hormone B, are released directly. You may as well discuss this key concept with your instructor.)

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