What is the effect of digitalis on cytosolic calcium levels?

Digitalis, commonly used in the treatment of heart failure and certain arrhythmias, primarily acts by inhibiting the Na+/K+ ATPase pump on cardiac myocytes. This inhibition leads to an increase in intracellular sodium levels. When sodium accumulates, it reduces the gradient for the Na+/Ca2+ exchanger, which normally exports calcium out of the cell in exchange for sodium. Consequently, as sodium levels rise, calcium is retained inside the cell.

This mechanism results in an increase in cytosolic calcium levels. The elevated calcium in the cytosol enhances the contractility of the heart muscle (a positive inotropic effect), improving cardiac output. Additionally, the increased calcium may also lead to a greater release of calcium from the sarcoplasmic reticulum during cardiac muscle contraction, further amplifying the effects on contractility.

In summary, digitalis increases cytosolic calcium levels primarily through the inhibition of the Na+/K+ ATPase, resulting in less calcium extrusion and more calcium retention within cardiac myocytes, which aligns with the correct answer.