MODIFICATION BY EXTERNAL CALCIUM AND MAGNESIUM IONS OF THE POTENTIATIVE ACTION OF 5'-AMP ON THE ATP-INDUCED CONTRACTION OF ISOLATED GUINEA-PIG ILEUM

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In the present study, it was confirmed that the contractile response of isolated guinea-pig ileum to ATP and acetylcholine was enhanced by lowering Mg++ in the medium and reduced by low Call. However, differing from acetylcholine, the contractile response to ATP at low concentration of 3×10-6 to 3, ×10-5 M was not reduced by lowering Ca++ from 1.8 to 0.1 mM in Mg++-free medium. On the other hand, the potentiative action of 5'-AMP on the ATP-induced contraction observed in normal Tyrode solution disappeared at 0.1 mM Ca++ in Mg++-free medium. It was also observed that not only the inhibitory effect of 5'-AMP and adenosine on the acetylcholine-induced contraction, but also that of adenosine on the ATP-induced contraction was augmented by lowering Ca++ in the medium. From these findings, it is suggested that the disappearance of the potentiative action of 5'-AMP may be due to the inhibitory action of the adenosine moiety in the molecule augmented by low Ca++.
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