What is the effect of increasing extracellular calcium on cardiac contractility?

• A. Decreases contractility
• B. Increases heart rate but not contractility
• C. Increases contractility
• D. Causes no change in contractility

Answer: (C)

Increasing extracellular calcium enhances the amount of calcium that enters cardiac muscle cells during the action potential through L-type calcium channels. That extra entry triggers more calcium release from the sarcoplasmic reticulum, increasing the intracellular calcium transient. With more calcium available, calcium binds to troponin C on the thin filaments, promoting more cross-bridge cycling between actin and myosin, and thus stronger, more forceful contractions. In other words, this is a positive inotropic effect: contractility goes up. (Heart rate is a separate parameter and is not the primary effect of small increases in extracellular calcium.)