Stabilizing Effects of Adenosine on the Membrane Currents and Tension Components of the Bullfrog Atrium

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Abstract
The effects of exogenously applied adenosine on the membrane potentials, currents and tension components of the bullfrog atrium were analyzed under voltage clamped and unclamped conditions with the double sucrose-gap method. Adenosine above 10-3 M produced a prolongation of action potential, accompanied by a slight augmentation of contraction followed by a sustained depression. Recovery was slow. Under voltage clamp conditions, adenosine merely produced a negative inotropic effect depressing ICa-dependent and -independent tensions. In the membrane currents, INaf, Is(ICa), Ix and background current (Ib) were all depressed. In the presence of adrenaline [epinephrine], the increased Is, Ix were contraction were inhibited by adenosine in a lower dose (10-5-10-3 M), and the inhibition of Is and ICa-dependent tension as more prominent in the presence than in the absence of adrenaline. Under the effect of ATP, which has a catecholamine-like action, similar selective inhibitions were observed. Adenosine apparently has a sustained stabilizing action on the myocardium to survive, especially in the presence of adrenaline, by depressing the augmented ICa and contraction.