Test: Endocrine System and Hormonal Regulation - 2 - MCAT MCQ

The parathyroid glands are small endocrine glands located near the thyroid gland in the neck. Their main function is to produce and secrete parathyroid hormone (PTH). PTH plays a vital role in the regulation of calcium and phosphate levels in the body.

When blood calcium levels drop, the parathyroid glands release PTH into the bloodstream. PTH acts on several target organs, including the bones, kidneys, and intestines, to increase blood calcium levels. It stimulates the release of calcium from the bones, enhances calcium reabsorption in the kidneys, and promotes the absorption of calcium in the intestines. PTH also acts on the kidneys to decrease phosphate reabsorption, which indirectly increases serum calcium levels.

Maintaining the appropriate balance of calcium is crucial for various physiological processes, including muscle and nerve function, bone health, and blood clotting. Therefore, the primary role of the parathyroid gland is to regulate serum calcium levels (option B).

The pancreas is responsible for producing insulin, a hormone that regulates blood glucose levels. Destruction of pancreatic cells, specifically the beta cells in the islets of Langerhans, can lead to a decreased or absent production of insulin. Without sufficient insulin, glucose cannot effectively enter cells, leading to elevated levels of glucose in the bloodstream. This condition is characteristic of type 1 diabetes mellitus, an autoimmune disorder in which the immune system mistakenly attacks and destroys the beta cells of the pancreas.

Therefore, the laboratory abnormality that is most likely to be seen in this scenario is an elevated serum glucose level (option B).

Hormones are chemical messengers that are released by endocrine glands into the bloodstream and act on specific target cells or organs in the body. For a hormone to exert its effects on a particular organ or tissue, it must bind to specific receptors present on the surface of or within the target cells. These receptors are specific to each hormone or hormone group.

The interaction between a hormone and its specific receptor triggers a series of biochemical reactions and signaling cascades within the target cells, leading to various physiological responses. The binding of the hormone to its receptor is highly specific, and only cells or organs that possess the appropriate receptor will respond to the hormone's signal.

The classification of a hormone as a corticosteroid or a gonadotropic hormone (option A) does not determine its organ specificity. While different hormone classes may have distinct physiological functions and target different organs, their organ specificity is primarily dictated by the specific receptors they interact with.

The signaling cascade initiated by a hormone (option C) is an intracellular process that occurs after the hormone binds to its receptor and triggers cellular responses. This cascade amplifies the hormone's signal and ultimately leads to the desired physiological effects. However, the signaling cascade itself does not determine the organ specificity of the hormone.

Whether a hormone is lipophilic (lipid-soluble) or lipophobic (water-soluble) (option D) affects its mode of action and how it interacts with receptors, but it is not the sole determinant of organ specificity. Lipophilic hormones can cross cell membranes and bind to intracellular receptors, while lipophobic hormones typically bind to cell surface receptors. Nonetheless, the interaction with specific receptors remains the primary factor determining organ specificity.

Therefore, the most accurate answer is that the organ specificity of a hormone is determined by its ability to interact with a specific receptor (option B).

Paracrine signaling refers to the release of chemical messengers that act locally on nearby cells. In the case of the hypothalamus, it releases hormones that act on adjacent cells within the hypothalamus and nearby regions, such as the pituitary gland. These paracrine hormones, known as releasing hormones or inhibiting hormones, regulate the secretion of hormones from the pituitary gland.

Posterior pituitary hormones do not include direct and tropic hormones (option A). The posterior pituitary gland does not produce hormones itself but stores and releases two hormones synthesized by the hypothalamus: antidiuretic hormone (ADH) and oxytocin.

ADH, also known as vasopressin, is involved in regulating water balance and concentration in the body. It acts on the kidneys to promote water reabsorption, reducing urine output and helping to maintain water balance.

Oxytocin is involved in various reproductive and social behaviors. It plays a role in uterine contractions during labor and childbirth and stimulates milk ejection during breastfeeding. Oxytocin is also associated with social bonding, trust, and emotional attachment.

Both ADH and oxytocin are released from the posterior pituitary gland in response to nerve signals originating from the hypothalamus (option C). These hormones are synthesized in the hypothalamus and transported down the axons of the hypothalamic-hypophyseal tract to be stored and released when appropriate signals are received.

Therefore, the statement that is not true of posterior pituitary hormones is that they include direct and tropic hormones (option A).

Feedback loops play a critical role in maintaining homeostasis by regulating the concentration of hormones in the bloodstream. Hormone regulation involves complex interactions between the endocrine glands, target organs or tissues, and the hypothalamus and pituitary gland (which act as the central control center for hormone regulation).
Positive feedback loops amplify the initial hormone signal, leading to increased hormone production and release. This can occur in certain situations, such as during childbirth, where oxytocin is released, causing uterine contractions that further stimulate oxytocin release.
Negative feedback loops, on the other hand, act to maintain hormone levels within a narrow range by counteracting any deviations from the set point. When hormone levels reach a certain threshold, negative feedback mechanisms are activated to inhibit further hormone production or release. This helps prevent excessive hormone secretion and maintain balance. For example, in the regulation of thyroid hormones, when blood levels of thyroid hormones are high, they inhibit the release of thyroid-stimulating hormone (TSH) from the pituitary gland, which then reduces thyroid hormone production.

Steroid hormones are a class of hormones derived from cholesterol. Examples include hormones such as cortisol, aldosterone, estrogen, and testosterone. Unlike polypeptide hormones, which are typically hydrophilic and cannot easily pass through the cell membrane, steroid hormones are lipophilic and can readily diffuse across cell membranes.

Once inside the target cell, steroid hormones bind to specific receptors that are located in the cytosol or nucleus of the cell (option B). The receptor-hormone complex then translocates into the nucleus, where it acts as a transcription factor, binding to specific DNA sequences and regulating gene expression. This ultimately leads to changes in protein synthesis and cellular function.

Polypeptide hormones, on the other hand, commonly function via specific second messengers (option C). Upon binding to cell surface receptors, they initiate signaling cascades that involve the activation of second messengers such as cyclic adenosine monophosphate (cAMP), inositol trisphosphate (IP3), and calcium ions. These second messengers propagate the hormone signal within the cell, leading to various cellular responses.

Steroid hormones tend to have longer-lasting effects compared to polypeptide hormones because they alter gene expression, resulting in the synthesis of new proteins (option D). This is in contrast to polypeptide hormones, which often exert more rapid but transient effects by modifying existing proteins or activating existing cellular pathways.

Therefore, the statement that is true about steroid hormones in comparison to polypeptide hormones is that steroid hormones are more likely to have an associated receptor in the cytosol or nucleus (option B).

Adenylate cyclase is an enzyme that plays a crucial role in signal transduction pathways, particularly those mediated by G protein-coupled receptors (GPCRs). When a hormone or neurotransmitter binds to a GPCR, it activates a G protein, which in turn activates adenylate cyclase.

Activated adenylate cyclase catalyzes the conversion of adenosine triphosphate (ATP) to cyclic adenosine monophosphate (cAMP) by cyclizing the phosphate groups. cAMP acts as a second messenger that can relay the signal from the hormone or neurotransmitter to the intracellular components of the cell.

The biosynthesis of steroids does involve the binding of pyrophosphate to a terpene molecule. Terpenes are a class of compounds derived from the precursor isoprene units, and they serve as building blocks for the synthesis of various organic compounds, including steroids.

During the biosynthesis of steroids, isopentenyl pyrophosphate (IPP), a terpene precursor, undergoes condensation reactions with other molecules to form larger terpene units. These terpene units are then combined with pyrophosphate, resulting in the binding of pyrophosphate to a terpene. This step is essential for the subsequent enzymatic modifications and cyclization reactions that ultimately lead to the formation of sterols, such as lanosterol and cholesterol.

Steroid hormones are a class of hormones that are derived from cholesterol. They are characterized by their lipid-soluble nature and ability to diffuse across cell membranes. The two main types of steroid hormones are sex hormones and adrenal cortical hormones.

Sex hormones, such as estrogen, progesterone, and testosterone, are responsible for the development and regulation of sexual characteristics and reproductive processes. Estrogen and progesterone are predominantly produced by the ovaries in females, while testosterone is primarily produced by the testes in males. However, small amounts of sex hormones are also produced by the adrenal glands in both sexes.

Adrenal cortical hormones are produced by the outer layer (cortex) of the adrenal glands. These hormones include glucocorticoids (such as cortisol), mineralocorticoids (such as aldosterone), and small amounts of sex hormones (such as adrenal androgens). Glucocorticoids are involved in regulating metabolism and immune responses, while mineralocorticoids help regulate electrolyte and fluid balance in the body.