The action of an ACE inhibitor interrupts the renin-angiotensin-aldosterone mechanism, thereby producing which of the following?
A. Reduced renal blood flow.
B. Reduced sodium and water excretion.
C. Increased peripheral vascular resistance.
D. Increased sodium excretion and potassium reabsorption.
Correct Answer: D. Increased sodium excretion and potassium reabsorption
The inhibition of aldosterone increases sodium excretion and reduces potassium excretion. Aldosterone is a steroid hormone that causes an increase in sodium reabsorption and potassium excretion at the distal tubule and collecting duct of the nephron. Aldosterone works by stimulating the insertion of luminal Na channels and basolateral Na-K ATPase proteins. The net effect is an increased level of sodium reabsorption.
Option A: ACE inhibitors increase renal blood flow. A common use for ACE inhibitors or ARBs is in the management of hypertension. In these cases, blocking or decreasing levels of angiotensin II will lead to a reduction in blood pressure. They achieve this goal by decreasing sodium and water reabsorption, leading to a reduction in blood volume, and decreasing arteriolar tone.
Option B: Excretion of sodium and water is enhanced by ACE inhibitors. Angiotensin II acts on the brain. Here, it has three effects. First, it binds to the hypothalamus, stimulating thirst and increased water intake. Second, it stimulates the release of antidiuretic hormone (ADH) by the posterior pituitary. ADH, or vasopressin, acts to increase water reabsorption in the kidney by inserting aquaporin channels at the collecting duct. Finally, angiotensin II decreases the sensitivity of the baroreceptor reflex.
Option C: Peripheral vascular resistance is decreased by the vasodilation effect of ACE inhibitors. The effect of angiotensin II on vasoconstriction takes place in systemic arterioles. Here, angiotensin II binds to G protein-coupled receptors, leading to a secondary messenger cascade that results in potent arteriolar vasoconstriction. This acts to increase total peripheral resistance, causing an increase in blood pressure.