INTRACELLULAR ATP MODIFIES THE VOLTAGE-DEPENDENCE OF THE FAST TRANSIENT OUTWARD K-STAGNALIS NEURONS( CURRENT IN LYMNAEA)

1. The action of intracellular ATP on the fast transient outward K+ cu rrent (A-current) was studied in dialysed voltage-clamped Lymnaea stag nalis neurones. 2. When introduced intracellularly in millimolar conce ntrations ATP caused a shift of the steady-state inactivation curve al ong the voltage axis in the direction of positive potentials and decre ased A-current at all test voltages. 3. Intracellular treatment with a n inhibitor of ATP synthesis, sodium arsenate, led to the opposite cha nges. The action of arsenate was not reversed upon its removal. After wash-out of arsenate ATP restored the initial voltage dependence. 4. A ddition of Mg2+ to the solution weakened the action of ATP in proporti on to the Mg2+:ATP concentration ratio. On the other hand, in neurones pretreated with arsenate, Mg2+ did not affect the ATP action. 5. When a mixture of glycolytic substrates was applied after arsenate wash-ou t the activation and inactivation curves shifted towards positive volt ages. A substrate of oxidative phosphorylation was ineffective in the same conditions. 6. Non-hydrolysable analogues of ATP, adenosine-5'-O- gamma-thiotriphosphate and adenylyl imidodiphosphate, did not mimic th e ATP action. This means that the ATP effect is mediated by some enzym atic process(es). 7. Elevation of total cytosolic Ca2+ concentration a s 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 Ca2+ transport. 8. Treatment of the neurones with alkaline phosphatase evoked a shift of the inactivation voltage dependence towards hyperpo larizing potentials and increased the A-current amplitudes at all test voltages. 9. The data indicate that a change in intracellular ATP con centration modulates the A-current voltage dependence. The effect of A TP is probably the result of phosphorylation of a channel protein or s ome associated proteins, but lowering of free Mg2+ concentration canno t be excluded. The possible physiological significance of the phenomen on is discussed.