The effects of caffeine (0.2–10 mM) on the electrical and mechanical activities of guinea pig taenia coli were investigated with the double sucrose-gap method. Caffeine evoked a small tension with a latency of 20–30 sec, then phasic contraction developed and finally relaxation. The initial tension development also appeared in the Na-free solution without any marked changes in the membrane potential and membrane resistance. The phasic contraction disappeared in the Na-free solution. The relaxation in the presence of caffeine was accompanied by depolarization block of the spike generation. The minimum concentration of Ca ion needed to evoke the tension development by the caffeine was 10-7 M. Caffeine also potentiated the twitch tension below a concentration of 5 mM either in the Na-free solution or at low temperature (5°C). NO3- and Br- showed a similar response to caffeine on the potentiation of the twitch tension at low temperature
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