Question

Epithelial cells of the lower ileum express a \(\mathrm{Cl}^{-} / \mathrm{HCO}_{3}^{-}\) exchange coded for by the DRA gene. These cells A. mediate an clectrogenic exchange of 2 luminal \(\mathrm{Na}^{+}\) for 1 cytosolic \(\mathrm{H}^{+}\) B. absorb \(\mathrm{Cl}^{-}\) into the cell in exchange for \(\mathrm{HCO}_{3}^{-}\) moving into the lumen. C. prevent a metabolic acidosis due to loss of HCl. D. mediate \(\mathrm{Na}^{+}\) movement out of the cell as \(\mathrm{Cl}^{-}\) moves into the cell. E. mediate \(\mathrm{Na}^{+}\) movement into the cell via a \(\mathrm{Na}^{+}\) channel.

Short answer

Answer: The primary function of the epithelial cells of the lower ileum and the role of the DRA gene is to absorb Cl- into the cell in exchange for HCO3- moving into the lumen.

Step-by-step solution

Understand the role of the epithelial cells and DRA gene

The epithelial cells of the lower ileum express a \(\mathrm{Cl}^{-} / \mathrm{HCO}_{3}^{-}\) exchange coded by the DRA gene. This is a crucial piece of information to decide which of the given statements is correct.

Review each statement and compare it with the given information

Let's go through each of the five statements and compare them with the information we have: A. The given information does not mention anything about an exchange of 2 luminal \(\mathrm{Na}^{+}\) for 1 cytosolic \(\mathrm{H}^{+}\), so A is not the correct answer. B. This statement does talk about an exchange of \(\mathrm{Cl}^{-}\) and \(\mathrm{HCO}_{3}^{-}\), which matches the information provided. It is possible that this is the correct answer, so let's keep it in mind for now and continue reviewing the other options. C. The given information does not discuss the relationship between metabolic acidosis and the loss of HCl, so C is not the correct answer. D. From the given information, we know that the exchange involves \(\mathrm{Cl}^{-}\) and \(\mathrm{HCO}_{3}^{-}\), not \(\mathrm{Na}^{+}\) and \(\mathrm{Cl}^{-}\). Therefore, D is not the correct answer. E. The given information does not mention anything about \(\mathrm{Na}^{+}\) channels or \(\mathrm{Na}^{+}\) movement into the cell, so E is not the correct answer.

Choose the correct answer based on the analysis

After reviewing all the statements, it is clear that statement B is the only one that aligns with the given information about the epithelial cells of the lower ileum and the DRA gene coding the \(\mathrm{Cl}^{-} / \mathrm{HCO}_{3}^{-}\) exchange. So, the correct answer is B: Absorb \(\mathrm{Cl}^{-}\) into the cell in exchange for \(\mathrm{HCO}_{3}^{-}\) moving into the lumen.

Key concepts

DRA Gene

The Down-Regulated in Adenoma (DRA) gene plays a vital role in maintaining the body's electrolyte and acid-base balance. It encodes a protein that facilitates the exchange of chloride ions (Cl^-) and bicarbonate ions (HCO₃^-) across epithelial cells in the ileum.

This gene's product is essential for proper digestive system function, as it helps regulate the pH level in the ileum by balancing the absorption and secretion of chloride and bicarbonate ions. Defects in the DRA gene can lead to conditions such as congenital chloridorrhea, where there's excessive loss of chloride in stools, leading to dehydration and metabolic alkalosis.

Epithelial Cells

Epithelial cells are a type of cell that lines the surfaces of your body, including the digestive tract. They are responsible for protecting underlying tissues, absorption, secretion, and sensation. When discussing the lower ileum, these cells are specially adapted to transport electrolytes from the digestive system into the bloodstream.

These cells exhibit a property known as polarity, with distinct structural and functional differences between their exposed and attached sides. This polarity is essential for epithelial cells' selective transport functions, such as the chloride/bicarbonate exchange mediated by the DRA gene.

Ileum Function

The ileum is the final and longest segment of the small intestine. Its primary function is to absorb nutrients and water that have not been absorbed by the jejunum. However, its role extends to crucial absorptive duties for vitamin B12 and the reabsorption of bile acids which are necessary for the digestion of fats.

The ileum also plays a significant role in maintaining electrolyte balance through the mechanism exemplified by the DRA gene's activity. The chloride/bicarbonate exchange at this site is essential for acid-base homeostasis, as it compensates for the acids and bases that are ingested or produced during metabolism.

Acid-Base Balance

Acid-base balance is a homeostatic process in the body that maintains the pH level of blood and other bodily fluids within a narrow range. The kidneys and the respiratory system typically regulate this balance, but the gastrointestinal system, particularly the ileum, contributes to this process through the secretion and absorption of ions.

The chloride/bicarbonate exchange mentioned earlier is one of several mechanisms that support the body's efforts to maintain a stable pH. The balance is crucial for normal cellular function as extreme acidosis or alkalosis can be harmful or even fatal.

Electrolyte Transport

Electrolytes—minerals in your body that carry an electric charge—are vital for many bodily functions, including muscle contractions, nerve impulses, and maintaining fluid balance. The transport of electrolytes across the membranes of cells is typically facilitated by proteins like the one encoded by the DRA gene.

These transport mechanisms are particularly important in the intestines, where nutrients and minerals like sodium, chloride, and bicarbonate are absorbed or secreted. Efficient electrolyte transport is crucial for hydration, muscle function, and neural activity, making epithelial cells' function in electrolyte transport essential for overall health.