5. Wrhen a mixture of glycolytic substrates was applied after arsenate wash-out the activation and inactivation curves shifted towards positive voltages. A substrate of oxidative phosphorylation was ineffective in the same conditions. 6. Non-hydrolysable analogues of ATP, adenosine-5'-0-y-thiotriphosphate and adenylyl imidodiphosphate, did not mimic the ATP action. This means that the ATP effect is mediated by some enzymatic process(es).7. Elevation of total cytosolic Ca21 concentration as well as intracellular application of agents increasing intracellular free Ca2' reduced A-current amplitude but failed to alter its voltage dependence. Therefore, ATP action cannot be related to activation of Ca21 transport.8. Treatment of the neurones with alkaline phosphatase evoked a shift of the inactivation voltage dependence towards hyperpolarizing potentials and increased the A-current amplitudes at all test voltages.9. The data indicate that a change in intracellular ATP concentration modulates the A-current voltage dependence. The effect of ATP is probably the result of phosphorylation of a channel protein or some associated proteins, but lowering of free Mg2+ concentration cannot be excluded. The possible physiological significance of the phenomenon is discussed.
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