Why are Na+, K+, and Cl– the only ions considered in the GK equation when calculating resting membrane potential (Vm)? Why are Na+, K+, and Cl– the only ions considered in the GK equation when calculating resting membrane potential (Vm)? These ions are found in the highest concentrations inside the cell. These are the ions to which cell membranes are permeable at rest. These ions bind to receptors that increase the charge on the membrane. Only these ions have enough charge to influence Vm.