Reduction of pituitary AVP and OT contents in rats following spaceflight.

Background: Responses of pituitary concentrations of vasopressin (AVP) and oxytocin (OT) during spaceflight have been variable, possibly due to differences in flight conditions or in age and strain of flight animals. Methods: We reviewed findings of three spaceflights of varying flight and recovery durations in which rats of different ages and strains were used. Male rats ranging in weight from 248-396 g were flown in space for 7-14 d. Flight animals were then compared with vivarium controls and synchronous controls. Parallel ground-based studies (hypergravity and simulated hypogravity) were conducted. Results: Pituitary content of AVP was significantly (p ≤ 0.05) decreased by spaceflight (6.3 + 0.3 μg . mg -1 protein in flight vs. 8.3 ± 0.5 μg . mg -1 protein in vivarium). OT content was also reduced during spaceflight (4.3 ± 0.2 μg . mg -1 protein in flight vs. 6.1 ± 0.3 μg . mg -1 protein in vivarium). Vivarium and synchronous control rats showed no difference in pituitary contents. Flight duration or recovery times did not appear to influence pituitary hormone contents. Strain of rat had an effect on content but not on responses to spaceflight. Age of animals confounded the response to spaceflight: pituitary contents of AVP and OT were not altered in young animals (≤60 d old). Hindlimb suspended animals showed no difference in AVP but OT content was decreased. Ground-based exposure to hypergravity (2 G) did not alter content of AVP or OT in young animals. Conclusions: Decreases in pituitary content of AVP and OT with spaceflight may be due to a variety of factors unique to the microgravity environment. Differences between studies may be due in part to differences in size and age of rats used.