Chronic Nicotine Modifies Skeletal Muscle Na,K-ATPase Activity through Its Interaction with the Nicotinic Acetylcholine Receptor and Phospholemman

Our previous finding that the muscle nicotinic acetylcholine receptor (nAChR) and the Na,K-ATPase interact as a regulatory complex to modulate Na,K-ATPase activity suggested that chronic, circulating nicotine may alter this interaction, with long-term changes in the membrane potential. To test this hypothesis, we chronically exposed rats to nicotine delivered orally for 21-31 days. Chronic nicotine produced a steady membrane depolarization of similar to 3 mV in the diaphragm muscle, which resulted from a net change in electrogenic transport by the Na,K-ATPase alpha 2 and alpha 1 isoforms. Electrogenic transport by the alpha 2 isoform increased (+1.8 mV) while the activity of the alpha 1 isoform decreased (24.4 mV). Protein expression of Na,K-ATPase alpha 1 or alpha 2 isoforms and the nAChR did not change; however, the content of alpha 2 subunit in the plasma membrane decreased by 25%, indicating that its stimulated electrogenic transport is due to an increase in specific activity. The physical association between the nAChR, the Na,K-ATPase alpha 1 or alpha 2 subunits, and the regulatory subunit of the Na, K-ATPase, phospholemman (PLM), measured by co-immuno precipitation, was stable and unchanged. Chronic nicotine treatment activated PKC alpha/beta 2 and PKC delta and was accompanied by parallel increases in PLM phosphorylation at Ser(63) and Ser(68). Collectively, these results demonstrate that nicotine at chronic doses, acting through the nAChR-Na,K-ATPase complex, is able to modulate Na,K-ATPase activity in an isoform-specific manner and that the regulatory range includes both stimulation and inhibition of enzyme activity. Cholinergic modulation of Na,K-ATPase activity is achieved, in part, through activation of PKC and phosphorylation of PLM.