Effect of acute and repeated immobilization stress on food and water intake, urine output and vasopressin changes in rats.

In several experiments the rats were subjected to immobilization stress (IMO) for 150 min daily for 7 to 38 days and after each IMO they were placed into individual metabolism cages. A considerable decrease of food and water intake was found even after the first IMO and lasted up to the 7th IMO. This resulted in a decrease of body weight and of diurnal urine output, while the osmolality of urine increased. However, the expected corresponding increase in urinary excretion of vasopressin (AVP) did not occur. In contrast, AVP excretion after 6th and 7th IMO decreased by about 50 per cent. In some experiments the urine was collected during the period of IMO. It was found that the total urine output sharply increased and its osmolality decreased. Moreover, the excretion of AVP decreased and the excretion of calcium and Na :K ratio in urine were elevated. After repeated IMO (i. e. after 7th, 16th and 38th IMO) all mentioned changes were still more expressed. Thus, the urine output increased more than two fold. However, the administration of dDAVP (synthetic analogue of AVP) prior to IMO resulted either in the normalization of urine output or anuria, respectively, depending on the dose administered. When 30 min portions of urine excreted during 150 min IMO were collected, it was found that during the first period, when the level of AVP in plasma was increased, the urine output was almost zero. During the second period of IMO the level of AVP in plasma decreased even bellow the control values which was accompanied by water diuresis. Similar biphasic changes were found even previously. It is suggested that the increased AVP release during the first period might be due to the physical stress stimuli (manual squeezing of animals, pain etc.), while the inhibition of AVP release during the second period may be caused by a simultaneous activation of other endocrine mechanisms (endogenous opioids, steroids, catecholamines, atrial natriuretic peptides etc.).