Norepinephrine is considered a positive chronotropic agent because it causes
A. an increase in the firing rate of SA node cells.
B. thyroid hormone to have a steadying effect on heart activity.
C. heart cell membrane potentials to become more positive during action potentials.
D. less calcium to enter heart cells, which leads to lower risk of heart attack.
E. more forceful contractions during each heart rate.
Answer: A
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