A Increased heart rate

- When more action potentials occur per unit time, more Caz+ enters the myocardial cells during the action potential plateaus, more Ca2+ is released from the SR, and greater tension is produced during contraction.

- Examples of the effect of increased heart rate are:

(1) Positive staircase, or Bowditch staircase. Increased heart rate increases the force of contraction in a stepwise fashion as the intracellular [Ca2+] increases cumulatively over several beats.

(2) Post-extrasystolic potentiation. The beat that occurs after an extrasystolic beat has increased force of contraction because "extra" Ca2+ enters the cells during the extrasystole.

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