Review Question - QID 109637

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Myocardial Action Potential Ventricular Action Potential Phase 2, plateau QID: 109637 (M1.CV.17.4867)

QID 109637 (Type "109637" in App Search)

An 85-year-old male presents to the emergency room complaining of chest pain. He reports an intense pressure over his sternum and difficulty breathing. His past medical history is notable for two prior myocardial infarctions, hyperlipidemia, and hypertension. He takes aspirin, metoprolol, atorvastatin, lisinopril, and hydrochlorothiazide. The patient drinks socially and does not smoke. His temperature is 99°F (37.2°C), blood pressure is 150/85 mmHg, pulse is 130/min, and respirations are 24/min. An electrocardiogram demonstrates ST elevations in leads II, III, and aVF. Despite appropriate care, the patient expires. An autopsy is performed that demonstrates an area of ischemia in the cardiac region supplied by the right coronary artery. A small sample of cells from healthy tissue near the site of injury is isolated and is subjected to additional testing. A current is run across the cells resulting in an action potential as seen in Figure A. The ion channel responsible for maintaining the plateau in Figure A is also responsible for which of the following?

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Phase 0 of the pacemaker action potential

56%

243/432

Phase 3 of the pacemaker action potential

11%

46/432

Phase 4 of the pacemaker action potential

37/432

Phase 1 of the ventricular action potential

21/432

Phase 3 of the ventricular action potential

13%

55/432

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The diagram in Figure A represents a ventricular action potential, with the plateau (phase 2) occurring due to the opening of L-type calcium channels. L-type calcium channels are also responsible for phase 0 of the pacemaker action potential.

Cardiac action potentials differ depending on whether the tissue is derived from myocardial or pacemaker cells. Ventricular action potentials are seen in the cardiac atria, ventricles, bundle of His, and Purkinje fibers, and are characterized by 5 distinct phases. After phase 0 (upstroke) and phase 1 (initial repolarization), phase 2 (plateau) occurs. This phase is characterized by a long period of stable, depolarized cell membrane as the inward calcium current via L-type calcium channels balances the outward potassium current. L-type calcium channels are also responsible for phase 0 (upstroke) of the pacemaker action potential. In this phase, calcium current via T-type calcium channels slowly depolarizes the cell membrane until a threshold is reached, allowing L-type calcium channels to open and activate the upstroke.

For description of Figure A, see above. Illustration A demonstrates the characteristic appearance of a ventricular action potential. Note the rapid upstroke (phase 0), initial repolarization (phase 1), plateau (phase 2), repolarization (phase 3), and resting membrane potential (phase 4).

Incorrect Answers:
Answer 2: Phase 3 of the pacemaker action potential is the repolarization phase after phase 0 (upstroke). In this phase, voltage-gated potassium channels open, leading to increased outward potassium current and repolarization.

Answer 3: Phase 4 of the pacemaker action potential is the diastolic depolarization phase after phase 3 (repolarization). In this phase, hyperpolarization-activated, cyclic nucleotide-gated channels generate a “funny” current leading to slow depolarization that activates T-type calcium channels for continued slow depolarization.

Answer 4: Phase 1 of the ventricular action potential is the initial repolarization phase following phase 0 (upstroke). In this phase, slow delayed rectifier potassium channels open to increase outward potassium current and repolarize the myocyte cell membrane.

Answer 5: Phase 3 of the ventricular action potential is the complete repolarization phase following phase 2 (plateau). In this phase, slow delayed rectifier potassium channels open completely and L-type calcium channels close, repolarizing the myocyte cell membrane.

Bullet Summary:
L-type voltage-gated calcium channels are responsible for phase 2 (plateau) of the ventricular action potential and phase 0 (upstroke) of the pacemaker action potential.

ILLUSTRATIONS:

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