Early central venous pressure changes in the rat during two different levels of head-down suspension.

The head-down suspension (i.e. antiorthostatic hypokinesia) rat is used to simulate the circulatory effects of weightlessness. However, little is known about the early cardiovascular adaptive response to simulated weightlessness and the effects of varying degrees of head-down suspension. Therefore, the purpose of this study was to characterize the early central venous pressure changes to two different levels of head-down suspension in the rat. Unanesthetized, unrestrained, female Sprague-Dawley rats (wt: 298 +/- 18 gms, mean +/- S.D.) with chronically implanted central venous pressure catheters were subjected to either a 45 degrees C head-down tilt angle (Group A, N = 8) or a 20 degrees head-down tilt angle (Group B, N = 8) using the tail-traction technique of head-down suspension. Central venous pressure increased significantly (p less than 0.05) in both groups of rats during head-down suspension with the absolute pressure level significantly (p less than 0.05) higher in Group A rats compared to Group B rats during the first 8 h of the study. At 24 h, the rats in Group B appeared to adapt earlier to head-down suspension because central venous pressure decreased to the previous baseline level whereas it remained increased in Group A rats. We conclude that the level of head-down suspension significantly affects the early central venous pressure response and subsequent cardiovascular adaption to simulated weightlessness in rats.