In extreme case of fluid loss, adrenal gland does not respond to agiontensin II. So aldosterone won't be released. ADH will be released in response to low blood volume. This increases water permeability in the collecting duct and distal part of the distal convoluted tubule. Increasing water reabsorption with not so much of sodium causing the dilutional effect.
In hyponatremia, extracellular is less positive so K leaves intracellular space. This makes intracellular compartment to become more negative and hyperpolarized. Please correct if I make any mistakes.