Regulation of M-type potassium current by intracellular nucleotide phosphates

Abstract

The effects of intracellular application of various concentrations of adenine nucleoside phosphates and nucleotide analogs on the M-type K current (I(M)) of single neurons isolated from sympathetic ganglia were studied. With 1 mM MgATP intracellularly I(M) 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 I(M) in the absence of ATP. When ATP and AMP-PNP were combined in the pipette, however, the maintenance of I(M) was prolonged. A series of nucleotides and analogs have been combined with ATP to test for their ability to maintain I(M) and to alter calcineurin phosphatase activity. There was a positive correlation between the ability of a nucleotide to prevent the rundown of I(M) and its ability to inhibit calcineurin phosphatase activity. These findings show that the amplitude of I(M) 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 I(M) 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 I(M) by phosphorylation events that maintain I(M) and dephosphorylation events that lead to current rundown.