Inhibition of the negative chronotropic action of adenosine by caffeine in the dog

The sinus node artery was perfused in situ in 15 dogs. Injection of adenosine into the sinus node artery induced a negative chronotropic effect at doses of 1, 3 and 10 μg. The negative chronotropic response to adenosine was blocked by 300 μg to of caffeine given into the same artery. On the contrarym, the negative chronotropic response to ACh was not inhibited but rather enhanced by caffeine, so that the atrial fibrillation threshold to ACh was lowered. Neither norepinephrine nor calcium when injected into the sinus node artery inhibited the deceleration response to adenosine.