Modulation of the inhibitory action of ATP on acetylcholine-activated current by protein phosphorylation in rat sympathetic neurons.

Modulation by protein phosphorylation of the relation between acetylcholine (ACh)-activated current (IACh) and adenosine triphosphate-(ATP)-activated current (IATP) was investigated with the whole-cell voltage-clamp technique in rat sympathetic neurons. During simultaneous activation by 100 microM ATP of an inward current, the current evoked by 100 microM ACh was reduced to 60-70% of that in the absence of ATP. Effects of compounds that are known to modulate protein phosphorylation were tested by including them in the intracellular solution. The reduction of IACh by ATP was not observed when K252a (1 microM), a non-selective protein kinase inhibitor, adenosine 5'-O-(3-thiotriphosphate) (ATP[gamma S], 1 mM) or alpha, beta-methylene ATP (1 mM) were included in the intracellular solution. Activators of protein kinases, adenosine 3',5'-cyclic monophosphate (cAMP, 100 microM), guanosine 3',5'-cyclic monophosphate (cGMP, 100 microM), phorbol 12-myristate 13-acetate (PMA, 1 microM), also abolished the reduction by ATP of IACh. The effects of okadaic acid, a protein phosphatase inhibitor, were paradoxical: okadaic acid (2 microM) itself abolished the reduction by ATP of IACh but it "antagonized" the abolishment by cAMP or cGMP of the reduction of IACh. Okadaic acid did not affect the disappearance of the reduction of IACh by ATP in the presence of intracellular PMA. The results suggest that the interaction between IACh and IATP is regulated by protein phosphorylation/dephosphorylation. Possible mechanisms underlying the effects of these modulators of protein phosphorylation are discussed.