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The effect of adenosine compounds (ATP, ADP, AMP and adenosine) on membrane potential, current and contractile tension on the bullfrog atrium were studied under voltage clamped and unclamped conditions. The compounds produced immediate positive and late negative inotropic effects in unclamped conditions. The positive inotropic effect and the potency of drugs appeared less marked in the order of ATP, ADP, AMP, and adenosine. Under voltage clamped conditions, only the energy rich compounds, ATP and ADP, produced an enhancement of calcium inward current (Ica) and Ica-dependent phasic tension, while AMP and adenosine elicited a negative inotropic effect. The delayed outward current was initially depressed but later augmented epecially in case of ATP and ADP where Ica was enhanced. All adenosine compounds, however, inhibited the Ica-independent tonic tension. This effect, appearing nonspecific, was ascribable to the action of common structure of these compounds, purine-riboside moiety.

AN ANALYSIS OF THE ACTION OF ATP AND RELATED COMPOUNDS ON MEMBRANE CURRENT AND TENSION COMPONENTS INBULLFROG ATRIAL MUSCLE