The force-frequency relationship (Bowditch staircase / Treppe) in the heart occurs due to increased intracellular availability of which ion?

• A Na⁺, directly increasing contractility via Nav1.5
• B K⁺, increasing resting membrane potential and trigger potential
• C Mg²⁺, co-activating myosin ATPase with ATP
• D Ca²⁺, accumulating via increased Na⁺ influx and NCX (Na⁺/Ca²⁺ exchanger) reverse mode ✓

Explanation

At higher heart rates, each action potential's Na⁺ influx accumulates slightly, raising intracellular Na⁺ concentration. This reduces the electrochemical gradient driving NCX in forward mode (Na⁺ in, Ca²⁺ out), and can switch NCX toward reverse mode (Ca²⁺ in), leading to higher sarcoplasmic reticulum Ca²⁺ loading and greater Ca²⁺ release per contraction. The result is increased contractility with increasing rate — the positive force-frequency (Bowditch/Treppe) staircase effect.

Reference: Guyton & Hall, Textbook of Medical Physiology, 14th ed.

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