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

Abstract

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 μM dipyridamole. However, no potentiation was seen in the presence of 10 μM 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.