Final answer:
The correct answer is E. Increased plasma K+ is NOT a positive chronotropic agent.
Step-by-step explanation:
The correct answer is E. Increased plasma K+ is NOT a positive chronotropic agent.
A positive chronotropic agent refers to any factor that increases heart rate. Options A, B, D are all positive chronotropic agents because they increase heart rate. Option C, increased plasma Ca2+, is also a positive chronotropic agent because calcium ions play a role in the excitability of cardiac cells.
However, increased plasma K+ (potassium ions) has the opposite effect. High levels of potassium ions in the blood can lead to a condition known as hyperkalemia, which can cause the heart to beat more slowly and irregularly.