Which of the following statements about aldosterone is NOT correct?
1.Aldosterone is stimulated by decreased plasma sodium levels and increased plasma potassium levels.
2.Aldosterone increases the number of passive sodium channels in the luminal membrane of the distal tubule and collecting duct, thus aiding sodium reabsorption.
3.Aldosterone increases sodium reabsorption by increasing the number of Na+-K+ ATPase pumps in the luminal membrane of the proximal tubule.
4.Aldosterone is produced in the adrenal cortex.
The Correct Answer and Explanation is:
The statement that is NOT correct is:
3. Aldosterone increases sodium reabsorption by increasing the number of Na+-K+ ATPase pumps in the luminal membrane of the proximal tubule.
Explanation:
Aldosterone is a steroid hormone produced in the adrenal cortex, primarily regulating sodium and potassium levels in the body. It plays a crucial role in maintaining blood pressure, fluid balance, and electrolyte homeostasis. Let’s break down each statement to understand why statement 3 is incorrect.
- Stimulated by Decreased Plasma Sodium and Increased Plasma Potassium Levels: This statement is true. Aldosterone secretion is indeed stimulated by low sodium levels (hyponatremia) and high potassium levels (hyperkalemia). The renin-angiotensin-aldosterone system (RAAS) activates when the kidneys detect low blood flow or low sodium, leading to the release of renin, which ultimately stimulates aldosterone release.
- Increases the Number of Passive Sodium Channels: This statement is also true. Aldosterone acts primarily on the distal convoluted tubule and collecting duct of the nephron. It increases the expression of epithelial sodium channels (ENaC) in the luminal membrane, enhancing sodium reabsorption from the tubular fluid into the bloodstream.
- Increases the Number of Na+-K+ ATPase Pumps in the Proximal Tubule: This statement is not correct. While aldosterone does indeed increase the number of Na+-K+ ATPase pumps, it mainly affects the distal tubule and collecting duct, not the proximal tubule. The proximal tubule already reabsorbs the majority of filtered sodium (around 65-70%) through different mechanisms, and aldosterone’s primary action is in the later segments of the nephron.
- Produced in the Adrenal Cortex: This statement is true. Aldosterone is synthesized and secreted by the zona glomerulosa of the adrenal cortex, confirming its role as a crucial hormone in regulating sodium and potassium balance.
In summary, while aldosterone has significant effects on sodium reabsorption, its action is primarily on the distal tubule and collecting duct, rather than increasing Na+-K+ ATPase pumps in the proximal tubule.