Ouabain (2.5 x 10(-5) M) inhibited preferentially the tonic response to 40 mM K+ medium (containing enough Na+) without affecting the phasic in taenia coli. When 11 mM lactate was added to the medium (pH 6.5) in the presence of ouabain, the tonic phase to 40 mM K+ recovered markedly. 2. Ouabain (2.5 x 10(-5) M) did not affect the tonic tension in 152 mM K+ medium (Na+ 0 mM). However, ouabain inhibited the recovered tension by the addition of 50 mM Na+ in the 152 mM K+ medium. But ouabain failed to inhibit the marked recovered tension by the addition of 11 mM lactate which utilized, even in the absence of external Na+, in 152 mM K+ medium. 3. Ouabain partly inhibited the shortening to 40 mM K+ (containing enough Na+) at light load; however, it inhibited markedly the shortening at heavy load. 4. There is a possibility that ouabain inhibits active transport of glucose depending on external Na+ in taenia coli of smooth muscle. Ouabain could not inhibit the tension by lactate which utilized under conditions of independent on Na+. Furthermore, it is suggested that ouabain inhibits the contraction which depends on aerobic metabolism; however, it has only a slight effect on contraction which depends on aerobic metabolism; however, it has only a slight effect on contraction which was not so dependent on aerobic metabolism.
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