Ang II has been shown to directly stimulate the activity of the Na,K-ATPase in the proximal tubule. The first contribution of the work presented here was to examine the mechanisms for this rapid stimulation. Using a method to quickly measure Na,K-ATPase activity in a manner that can clearly separate direct stimulation from the secondary effects of increasing [Na]i, this data demonstrates direct stimulation of Na,K-ATPase activity at rate-limiting concentrations of [Na]i in within and sustained for 15 minutes or less. Further exploration of the mechanism by which this rapid stimulation occurs, two distinct possibilities were considered; changes in V max and changes in the kinetic properties of the transport mechanism or K m . Evidence presented here demonstrates that the phosphorylation status of the α-subunit of the Na,K-ATPase changes in a complex manner under the influence of angiotensin II and that these changes appear to be involved in both of these mechanisms. These data show that mutating any of the three known serine phosphorylation sites alters the ability of angiotensin II to mobilize more sodium pump to the plasma membrane, in otherwords the V max . Likewise, mutating any of these phosphorylation sites alters the affinity of the α-subunit for ouabain and digoxin, important cardiotonic steroids shown to be endogenous to mammals. The implications of these findings include the possibility that the ubiquitously expressed Na,K-ATPase could actually be a pharmacological target for the treatment of hypertension and/or congestive heart failure.
