Sodium and Proton effects On Inward Proton Transport through Na/K Pumps (LB848)

The Na/K pump (NKA) exports 3Na in exchange for 2K, across animal plasma membranes. Ion binding sites I&II can bind Na or K but site‐III exclusively binds Na in a voltage (V)‐dependent fashion. Without external Na or K the NKA imports H generating a current (I H ). To crack I H relationship to ion interaction at their sites we investigated the characteristics of I H in ventricular myocytes and in Xenopus oocytes expressing ouabain‐resistant pumps. Lowering pH enlarged I H in both systems. In oocytes at pH 蠄5.6 a rapid increase in I H was followed by a slower inhibition (80% at pH 5). H‐activation was V‐dependent whilst H‐inhibition was V‐independent. In myocytes at pH 6 Na o stimulated I H at [Na] 蠄5 mM and inhibited at higher [Na]. In oocytes, only V‐independent Na o ‐dependent inhibition was observed at pH 6 or 7.6 (K 0.5 ~7 mM), but at pH 5 Na induced a nearly identical biphasic response than in myocytes at pH 6. Affinity for K o was reduced (~10 fold) by pH 5 compared to pH 7.6. Our results suggest that H+ leak through site‐III, that binding of 2Na or 2H to sites‐I&II inhibit permeation and that NKA with mixed Na/H occupancy of sites‐I&II are also permeable to H. Supported by NSF MCB‐1243842. Grant Funding Source : NSF MCB‐1243842