A 63-Year-Old Man Was Admitted to the Hospital 1 Hour

A 63-year-old man was admitted to the hospital 1 hour after experiencing substernal chest pain.He was weak and diaphoretic,and his heart was beating rapidly at 90 beats/minute as a consequence of a coronary artery occlusion that impaired blood flow to the left ventricle.From an ECG,it was determined that the tachycardia originated in the SA node.Before therapy could be started,the man became much weaker; his arterial pulse rate was about 45 beats/minute,and yet the ECG revealed an atrial rate of 90 beats/minute.A cardiac pacemaker was inserted because the AV node had been damaged by ischemia.With the implanted pacemaker set at 75 beats/minute,he felt somewhat better,and drug therapy was initiated. The mechanism by which the SA node generated impulses at a rapid rate during the early stages of the coronary artery occlusion involves an increase in:

A) The slope of the action potential upstroke (phase 0) of the automatic cells.
B) The slope of the slow diastolic depolarization of the automatic cells.
C) Firing threshold (less negative potential) of the automatic cells.
D) Negativity (hyperpolarization) of the initial portion of the slow diastolic depolarization.
E) Action potential amplitude of the automatic cells.