ADH and thirst function in defence with hypernatremia formation
Introduction: Thirst and ADH secretion are two principle mechanisms for controlling body fluid osmolarity. Even an excessive load of salt can effectively be handled by these systems. They can control plasma osmolarity in a vast range of water and sodium consumption. We studied the effect of salt solution consumption on serum sodium level and osmolarity and the action of these two systems.
Method: Seven groups each consisting of 6 male rats weighing 200±20 gr. were chosen. Salt solutions (1,3,5, 7 and 9%) were also provided. The rats were denied tap water and these prepared solutions were given to each group. Our control group used Zanjan potable water and another test group drank twice distilled water. Because the mortality rate in rats consuming higher salt solutions (>3 percent) began to rise on fifth day, the total water consumption, serum sodium level and osmolality were measured on day 5 in all groups.
Result: Water intake in groups using 1 and 3 percent solutions, increased 78 and 36 percent respectively. Their urine output was also considerably increased. The groups maintained on distilled water, 5, 7 and 9% solutions showed respective decreases of 11, 56, 73, and 85% in their water intake. Osmolarity and sodium level significantly increased in groups using 3% and higher solutions. ADH and thirst mechanisms could not control serum sodium level and osmolality when the concentration of salt in distilled water exceeded 1.4%.
Discussion: The use of higher concentration salt solutions could not decrease osmolarity and quench thirst. Because higher salt solutions stimulate the neurological protective mechanisms, which act more powerfully in higher concentrations, the animal shows no tendency to drink water and osmolarity will significantly increase. The animals refusal to drink highly concentrated solutions will keep it alive for some days and if it can get fresh water at this time it will escape death. We could propose that ADH and thirst could not control osmolarity when human or animal uses a large load of salt and increase in osmolarity leads to death in a few days.