Adenosine and dipyridamole: actions and interactions on the contractile response of guinea-pig ileum to high frequency electrical field stimulation.

The action of adenosine on the myenteric plexus-longitudinal muscle strips from guinea-pig ileum to high frequency electrical field stimulation (10 Hz) was investigated. Electrically induced contractions were reduced markedly by tetrodotoxin (0.2 microM) and atropine (1 microM), and partially by noradrenaline (3 microM) and morphine (3 microM). Adenosine, adenosine 5'-monophosphate (AMP) and adenosine triphosphate (ATP) produced a concentration-dependent inhibition of the high frequency contractions over the range of 0.1-100 microM, the most potent being adenosine. The concentration-response curve for adenosine was significantly shifted to the left by dipyridamole (10 nM), while dipyridamole at higher concentrations (30 nM-10 microM), depressed the contraction markedly by itself. Dipyridamole decreased [3H]-adenosine uptake into strips of ileum in a concentration-dependent manner. There was a significant correlationship between the reduction of adenosine uptake and the inhibition of the contraction induced by dipyridamole (r = 0.970). In strips desensitized to adenosine or treated with adenosine deaminase, the inhibitory effect of dipyridamole was significantly reduced. The present investigation revealed that adenosine depressed responses of guinea-pig ileum to high frequency electrical stimulation and suggested that the inhibitory effect of dipyridamole may be closely associated with the behaviour of endogenous adenosine or related compounds.