Dogs don't actually lose electrolytes like humans or say horses do. Because they can only sweat through their pads salt and mineral loss is minimal. Panting only loses water. If they've gone over the top and got to the wobbly stage glucose water is good (buy glucose in the baby section of any supermarket for cheap and add 1-2Tbsp to a litre of water). Feeding extra fat (oil, lards, fatty trimmings) helps replace the muscle sugar stores too.