Hyponatremia has no effect on the RMP (resting membrane potential) because
The cell membrane is impermeable to Na+
ECF Na+ concentration is high
Intracellular K+ concentration is high
Equilibrium potential for Na+ is +60 mV
Resting membrane potential depends on the concentration of potassium ion and not on sodium ion. (REF TEXTBOOK OF MEDICAL PHYSIOLOGY GEETHA N 2 EDITION, 23)
General physiology