Increasing intracellular calcium in the sarcoplasm has what effect on contractility?

• A. Increases contractility
• B. Decreases contractility
• C. No effect on contractility
• D. Increases heart rate

Answer: (A)

Increasing intracellular calcium in the sarcoplasm increases the contractile force of cardiac muscle. When Ca2+ levels rise during systole, calcium enters from the extracellular space and triggers additional release of Ca2+ from the sarcoplasmic reticulum. The higher cytosolic Ca2+ binds to troponin C on the thin filament, causing a shift that moves tropomyosin away from actin’s myosin-binding sites. With those sites exposed, myosin heads can form cross-bridges with actin and generate more power strokes, producing a stronger contraction. This is known as positive inotropy—the contractile strength increases with greater Ca2+ availability. While heart rate can be influenced by calcium signaling in other cellular contexts (such as autonomic effects on pacemaker cells), the direct effect of increasing sarcoplasmic Ca2+ is on the strength of contraction, not the beat rate.