The Bowditch staircase (Treppe) phenomenon in cardiac muscle describes an increase in contractile force with increasing heart rate. The cellular mechanism underlying this phenomenon is:
- A Increased calcium influx per unit time due to more frequent action potentials exceeding calcium efflux capacity ✓
- B Increased sensitivity of troponin C to calcium at higher frequencies
- C Greater cross-bridge cycling efficiency at shorter sarcomere lengths
- D Beta-adrenergic receptor activation by endogenously released catecholamines
Correct answer: A. Increased calcium influx per unit time due to more frequent action potentials exceeding calcium efflux capacity
Explanation
The Bowditch staircase (positive force-frequency relationship or Treppe) occurs because at higher heart rates, calcium enters the cell via L-type channels with each action potential more rapidly than the Na+/Ca2+ exchanger and SERCA pump can remove it, leading to progressive calcium accumulation in the sarcoplasmic reticulum. This raises releasable calcium stores, increasing contractile force with each subsequent beat. This is an intrinsic cardiac property distinct from sympathetic stimulation, though both ultimately elevate intracellular calcium.
Reference: Guyton & Hall, Textbook of Medical Physiology, 14th ed.
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