Abstract
A relatively small dose of verapamil, 1-100 .mu.g, did not influence the developed tension at 0.5 Hz. The tension at a higher freqeuncy of 1, 2 or 3 Hz was markedly depressed. Even a larger dose of verapamil caused much greater suppression at higher frequencies. Manganese chloride at 1-10 mg caused a rather uniform depression of the developed tension at 0.5, 1, 2 and 3 Hz in 3 experiments.

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