What are negative inotropic agents?

• A. Agents that decrease contractility by reducing available calcium in the sarcoplasm.
• B. Agents that increase contractility by increasing calcium in the sarcoplasm.
• C. Agents that decrease heart rate directly.
• D. Agents that increase afterload to boost stroke volume.

Answer: (A)

Negative inotropic agents are substances that weaken the heart’s pumping strength. In cardiac muscle, contraction is driven by calcium entering the cell and being available in the sarcoplasm to trigger cross-bridge cycling between actin and myosin. When an agent reduces the amount of calcium in the sarcoplasm, fewer cross-bridges form, so the force of contraction decreases. That’s why agents that decrease contractility by lowering intracellular calcium are the negative inotropes.

By contrast, increasing intracellular calcium would enhance contractility (positive inotropy). Changing heart rate is a chronotropic effect, not an inotropic one, and increasing afterload affects the pressure the heart must work against rather than directly altering the strength of contraction.