Relaxant effects of ATP and adenosine on canine large and small coronary arteries in vitro.

Rings from greyhound large (3-4 mm internal diameter) and small (less than 200 microns internal diameter) coronary arteries were mounted in water-jacketed tissue baths or a Mulvany myograph, respectively. Relaxations to ATP and adenosine were determined in arteries precontracted with the thromboxane A2 mimetic U46619 or with K+. ATP was a much more effective relaxant of large arteries than adenosine. ATP relaxed large coronaries by an endothelium-dependent mechanism whereas relaxations to adenosine were not endothelium-dependent. In contrast, adenosine appeared to be slightly more effective than ATP in relaxing small coronary arteries. Moreover, adenosine was more effective in relaxing K+-contracted small coronaries than K+-contracted large coronary arteries. These results suggest that ATP may be a more significant relaxant of large coronaries than adenosine but that adenosine may be a more significant relaxant than ATP in small coronary arteries.