Effect of dipyridamole and 6-(2-hydroxy-5-nitro)-benzylthioguanosine on low-frequency-stimulated relaxation in the guinea pig taenia coli.

Isolated guinea pig taenia coli responded to electrical field stimulation at 0.3 s-1 with relaxation which was potentiated from 6 to 230% in the presence of 1-1.5 micron dipyridamole. However, no potentiation was seen in the presence of 10 micron 6-(2-hydroxy-5-nitro)-benzylthioguanosine (HNBTG), another more selective inhibitor of adenosine uptake. Both drugs effectively potentiated the relaxant response to low doses of added adenosine. When complete frequency-response curves were recorded in the range 0.1 to 5 s-1, neither dipyridamole nor HNBTG caused any shift of the curves, although a statistically significant increase in relaxation was again seen in the presence of dipyridamole at 0.3 s-1. Thus, the response of taenia coli to transmural stimulation is not modified by all concentrations of inhibitors of adenosine uptake and the limited effect seen with dipyridamole at 0.3 s-1 may be based on another unknown mechanism. No evidence for or against the purinergic nerve hypothesis can be derived from our experiments.