In a digoxin-treated heart, increased extracellular calcium will lead to what?

In a digoxin-treated heart, increased extracellular calcium will lead to what?

• A. Increased SR calcium loading and irritability
• B. Decreased SR calcium loading
• C. No change
• D. Decreased contractility

Answer: (A)

This question probes how digoxin’s effect on calcium handling interacts with higher outside calcium to alter SR calcium load and electrical stability. Digoxin inhibits the Na+/K+ ATPase, raising intracellular Na+, which reduces the driving force for the Na+/Ca2+ exchanger to remove Ca2+ from the cell. That causes more Ca2+ to accumulate in the cytosol and to be taken up into the sarcoplasmic reticulum, increasing SR Ca2+ loading. Adding more extracellular calcium provides even more Ca2+ entry during each heartbeat via L-type channels, further boosting cytosolic and SR Ca2+ content in a digoxin-treated heart. When the SR is highly loaded with Ca2+, it is prone to spontaneous release during diastole, creating an inward current through NCX and triggering delayed afterdepolarizations, i.e., electrical irritability. So the combination leads to increased SR calcium loading and greater electrical irritability.

This question probes how digoxin’s effect on calcium handling interacts with higher outside calcium to alter SR calcium load and electrical stability. Digoxin inhibits the Na+/K+ ATPase, raising intracellular Na+, which reduces the driving force for the Na+/Ca2+ exchanger to remove Ca2+ from the cell. That causes more Ca2+ to accumulate in the cytosol and to be taken up into the sarcoplasmic reticulum, increasing SR Ca2+ loading. Adding more extracellular calcium provides even more Ca2+ entry during each heartbeat via L-type channels, further boosting cytosolic and SR Ca2+ content in a digoxin-treated heart. When the SR is highly loaded with Ca2+, it is prone to spontaneous release during diastole, creating an inward current through NCX and triggering delayed afterdepolarizations, i.e., electrical irritability. So the combination leads to increased SR calcium loading and greater electrical irritability.