Which substance antagonizes Adenosine?

Theophylline is a methylxanthine drug that acts as an antagonist of adenosine receptors. By binding to these receptors, theophylline can inhibit the actions of adenosine, which usually slows down heart rate and causes vasodilation. This antagonistic effect is beneficial in certain conditions, such as asthma and some heart block scenarios, as it can help raise heart rates or relieve bronchospasms. Theophylline's ability to block adenosine leads to increased heart contractility and improves respiratory function.

Adenosine triphosphate (ATP) is primarily known as an energy currency of the cell and does not directly antagonize adenosine. Amiodarone is an antiarrhythmic medication that primarily works through multiple mechanisms, including sodium channel blockade and affecting potassium channels, but it does not specifically antagonize adenosine. Calcium plays a crucial role in muscle contraction and cardiac function but does not act as an antagonist to adenosine receptors either. Thus, theophylline is the most suitable choice for antagonizing the effects of adenosine.