The reduction of neuronal calcium currents by ATP-[gamma]-S is mediated a G protein and occurs independently of cyclic AMP-dependent protein kinase

Abstract

We studied the effects of ATP-[gamma]-S on the T, N and L calcium current components of nodose ganglion neurons using the whole cell variation of the patch clamp technique. ATP-[gamma]-S can serve as a phosphate donor in kinase-mediated reactions, the donated phosphate group being resistant to the action of phosphateses. We therefore compared the effect of ATP-[gamma]-S to that of the catalytic subunit of the cyclic AMP-dependent protein kinase (AK-C), included in the recording pipette with 5 mM ATP. AK-C (50 [mu]g/ml) had no effect on the T current, and caused a ~30% increase in currents containing the N and L components during a 20-min recording, as compared to a ~45% decrease in control currents. In contrast, in the presence of 2.5 mM ATP-[gamma]-S, T currents declined ~30%, and currents containing the N and L components declined to a greater extent than control currents, about 65%. In addition, the time to peak current was increased from ~ 14 ms to ~40 ms. This effect of ATP-[gamma]-S on calcium currents was similar to that of certain neurotransmitters or GTP-[gamma]-S, an activator of G proteins, except that the effects of ATP-[gamma]-S were delayed 5-7 min relative to GTP-[gamma]-S. The effects of both ATP-[gamma]-S and GTP-[gamma]-S were reduced or abolished in neurons treated with pertussis toxin. We conclude that AK-C regulates neuronal calcium currents, presumably by phosphorylation of channels or associated proteins, and that the ATP-[gamma]-S-induced reduction of calcium currents cannot be due to its serving as a phosphate donor for endogenous AK. Instead, ATP-[gamma]-S reduces calcium currents via a G protein, perhaps by its conversion to GTP-[gamma]-S.