<prompt> Major cation in ECFNaKPO4-2Mg  <response>: Na, K ATPase catalyzes the hydrolysis of ATP to adenosine diphosphate ADP and uses the energy to extrude three Na from the cell to ECF and take two K into the cell for each molecule of ATP hydrolyzed.REF: GANONGS REVIEW OF MEDICAL PHYSIOLOGY 23rd EDITION page no.47. 