Heating of cargo compartments will depend on the aircraft. 737 classics, the rear hold wasn't heated wihile the forward hold was heated via exhaust air from the equipment cooling vent, this is a fairly common philosophy in the Boeing fleet, the larger aircraft also have temp controlled aft cargos and bulk cargo. This can be controlled from the flight deck for temp sensitive cargos. hope this helps. Don't know how Airbi are heated....
