Objective      To determine the effect of X-ray radiation and microgravity on bone metabolism.        Methods      Male Wistar rats were divided into a control group

an X-ray radiation group (X-ray group)

a hindlimb suspension group (HLS group)

and a hindlimb suspension and X-ray radiation group (HLS+X-ray group). The X-ray radiation dose was 4 Gy

and the hindlimb suspension was adopted to simulate microgravity environment. The bone mineral density

trabecular number

trabecular separation

trabecular thickness

bone volume/trabecular volume

and structural model index were detected

and the changes of bone alkaline phosphatase (ALP)

tartrate-resistant acid phosphatase (TRAP)

osteocalcin (OCN)

and bone regulatory genes (Rankl

Runx2 and Nfatcl) were analyzed.       Results      The hindlimb suspension and X-ray radiation caused significant reduction (P0.05) in bone mineral density

trabecular number

trabecular thickness

and bone volume/trabecular volume in the HLS group

X-ray group and HLS+X-ray group. The bone loss was the most severe in the HLS+X-ray group (P0.05). The expression of Rankl

Nfatcl and TRAP were significantly increased (P0.05) in the HLS group

X-ray group and HLS+X-ray group

while the expression of ALP and OCN was significantly decreased (P0.05). Moreover

Runx2 was not changed in the HLS group and X-ray group

but was down-regulated in the HLS+X-ray group (P0.05).       Conclusion      The combination of hindlimb suspension and X-ray radiation can more severe bone loss than alone.