Adenosine-induced slow ionic currents in the Xenopus oocyte

Adenosine and its 5′-phosphorylated congeners evoke specific membrane-mediated responses in excitable tissues^1,2. Available data suggest that inhibition of the target cell occurs due to hyperpolarization ^3,4, and in some preparations a compound effect of ATP (excitation and inhibition) has been found^5,6. However, the ionic mechanism of the purinergic-mediated response has not been studied by standard intracellular voltage-clamping techniques. Recently, we have discovered purinergic receptors in the Xenopus oocyte, a well defined giant cell amenable to rigorous electrophysiological⁷ and biochemical⁸ studies. We report here that in these cells, adenosine-induced slow membrane responses consisted of an early depolarizing (D) transient current carried by Cl^- ions, followed by a steady hyperpolarizing (H) current involving K⁺ ions. The relative potency sequence for the D current was ATP≃ ADP > AMP ≃ adenosine; this order was reversed for the H current.