Interaction between Caffeine and Adenosine on the Membrane Current and Tension Component in the Bullfrog Atrial Muscle
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Antagonistic action of caffeine and adenosine were studied at the membrane current and tension component levels in myocardium of the bullfrog atrium, mainly under voltage clamp using the double sucrose-gap method.<BR>Caffeine (1-5 mM) produced marked augmentation of the phasic and tonic tensions as well as slow inward current (<I>I</I><SUB>s</SUB>). At higher concentrations it elicited an increase of the delayed outward current (<I>I</I><SUB>x</SUB>). The augmentation of Is was principally due to increase of the limiting conductance (<I>g</I><SUB>s</SUB>), while the activation and inactivation variables (<I>d∞, f∞</I>) were not changed significantly. The dose-tension response curve for caffeine, which appeared sigmoidal, was notably lowered in the presence of adenosine (1-3 mM), indicating a competitive type of inhibition. Adenosine, in the presence of caffeine, exerted a narcotic-like action by inhibiting all of the membrane currents (<I>I<SUB>Naf</SUB>, I<SUB>s</SUB>, I<SUB>x</SUB></I> and background inward current, <I>I</I><SUB>b</SUB>) and tension components (<I>I</I><SUB>ca</SUB>-dependent and independent tensions). The inhibition of <I>I</I><SUB>s</SUB> was due to a decrease of gs and no appreciable change was observed in <I>d</I>∞ and <I>f</I>∞. These results suggest that adenosine has a strong stabilizing action on the myocardium, especially when the heart muscle activity is accelerated by increased cyclic AMP.
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日本生理学会 | 論文
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