Adenosine is a purine analogue. Acts by stimulating outward K+ channel of AV nodal tissue, more specifically in the posteriorly located slow pathway in the vicinity of coronary sinus.
Another action of adenosine is inhibition of cAMP , which is similar to beta blocking action may also help in terminating the tachycardia.
Adenosine : A 10 second cardiac miracle
(It may be apt to recall at this juncture , Aminophylline is used in sinus node dysfunction or AV block to increase heart rate )
A good one from Medscape http://www.medscape.com/viewarticle/585287_2