Which type of membrane transport is directly affected by cardiac glycosides?
A. Simple diffusion
B. Facilitated diffusion
C. Primary active transport
D. Secondary active transport
The Correct Answer and Explanation is:
The correct answer is C. Primary active transport.
Explanation:
Cardiac glycosides, such as digoxin and ouabain, are a class of medications that exert their therapeutic effects primarily by inhibiting the sodium-potassium ATPase pump (Na+/K+ ATPase), a key player in primary active transport mechanisms in cells. This pump is responsible for maintaining the electrochemical gradient across the plasma membrane by actively transporting sodium (Na+) ions out of the cell and potassium (K+) ions into the cell against their concentration gradients. For every three sodium ions expelled from the cell, two potassium ions are imported, which is crucial for maintaining cellular homeostasis, electrical excitability, and nutrient transport.
When cardiac glycosides inhibit this pump, several physiological changes occur. The intracellular concentration of sodium increases due to reduced extrusion, leading to a secondary increase in intracellular calcium. This happens because the Na+/Ca2+ exchanger, which usually extrudes calcium in exchange for sodium, is driven to operate less effectively as a consequence of the increased intracellular sodium. As a result, calcium levels rise within the cardiac myocytes (heart muscle cells), enhancing contractility (a phenomenon known as positive inotropy). This action is particularly beneficial in treating heart failure, as it helps improve cardiac output by strengthening the force of heart contractions.
In summary, cardiac glycosides directly affect primary active transport by inhibiting the Na+/K+ ATPase pump, leading to increased intracellular sodium and calcium concentrations, ultimately enhancing cardiac contractility. This mechanism underscores the critical role of primary active transport in regulating cellular ion concentrations and maintaining the overall function of excitable tissues such as the heart.