Role of an intracellular Ca store in trepibutone (AA-149)-induced relaxation of the depolarized smooth muscle of the guinea-pig taenia coli.

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The depolarized smooth muscle of guinea pig taenia coli suspended in Ca-free, K-rich Krebs solution showed a tension development with the addition of CaCl2 (Ca contracture) and the increased tension returned to the original level by rinsing with the solution. Application of carbachol (CCh) to this preparation caused an increase in tension despite the absence of Ca in the medium (CCh contraction), indicating a Ca release from intracellular stores. Trepibutone, a newly synthesized smooth muscle relaxant, given before CaCl2 prevented the Ca contracture and reduced the increased tension when given after CaCl2. The CCh contraction was enhanced after inhibition of the Ca contracture by trepibutone. D-600, a Ca antagonist, inhibited the Ca contracture and CCh contraction. The inhibitory effect of trepibutone on the Ca contracture was markedly reduced in the presence of CCh, whereas that of D-600 was not affected. These results suggest that trepibutone, unlike D-600, does not have an inhibitory action on Ca influx but rather an accelerating action on the uptake of intracellular Ca into the Ca stores sensitive to CCh.
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