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The effects of intracellular application of various concentrations of adenine nucleoside phosphates and nucleotide analogs on the M-type K current (IM) of single neurons isolated from sympathetic ganglia were studied. With 1 mM MgATP intracellularly IM decreased to 25% of its initial level 39 min after the start of whole-cell recording. In the absence of ATP the current decreased more rapidly. Addition of glucose and pyruvate extracellularly was equivalent to adding 1 mM MgATP intracellularly. AMP-PNP, a nonhydrolyzable ATP analog, at a concentration of 1 or 3 mM was unable to maintain IM in the absence of ATP. When ATP and AMP-PNP were combined in the pipette, however, the maintenance of IM was prolonged. A series of nucleotides and analogs have been combined with ATP to test for their ability to maintain IM and to alter calcineurin phosphatase activity. There was a positive correlation between the ability of a nucleotide to prevent the rundown of IM and its ability to inhibit calcineurin phosphatase activity. These findings show that the amplitude of IM is dually regulated by cellular levels of adenine nucleotide diphosphates and triphosphates. A hydrolyzable form of ATP is necessary to maintain the M current. The maintenance of IM is further enhanced by the simultaneous presence of ADP or other adenine nucleotides that alter calcineurin activity, but not by higher concentrations of ATP alone. These results are consistent with regulation of IM by phosphorylation events that maintain IM and dephosphorylation events that lead to current rundown.

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1 The actions of wortmannin, an inhibitor of myosin light chain kinase (MLCK), on M-type potassium current of dissociated bullfrog sympathetic neurones have been examined. 2 The amplitude of M-current was measured by whole cell recordings from cells pretreated with wortmannin (0.01-10gM) or the wortmannin vehicle, dimethylsulphoxide (0.0001-0.1 vol%), for 30 min. Internal (recording pipette) solutions having three different pCa values (6, 7 and 8) were used for the measurements. 3 Irrespective of the pCa, M-current was not detectable when the cells were pretreated with 10 AM wortmannin. Wortmannin, 3 tLM, produced 85-95% inhibition of the M-current. Pretreatment with 10-30 nM wortmannin was without effect on M-current. 4 The M-current inhibition by wortmannin at concentrations of 0.1-1 gM depended on the pCa of the internal solution. Inhibition occurred only when the calcium-rich (pCa = 6) internal solution was used. 5 Pre-treatment of the cells with wortmannin (10 IM) did not affect rapidly-inactivating A-type or delayed rectifier-type potassium currents nor did it alter inwardly rectifying sodium-potassium current (IH)-6 These observations show that M-current inhibition by wortmannin has two pharmacological profiles. One is calcium-dependent and occurs at lower concentrations (0.1-1 LM), and is attributed to inhibition of MLCK by wortmannin. At higher concentrations (3-10 IM), wortmannin has an additional, calciumindependent action, inhibiting the M-current by an unknown mechanism.