Effects of short-term spaceflight and recombinant human growth hormone (rhGH) on bone growth in young rats.

The purpose of this study was to characterize the effects of short-term spaceflight and recombinant human growth hormone (rhGH) on bone growth in rapidly growing male rats. Bone growth was determined by dynamic bone histomorphometry and rhGH was administered using implanted ALZET osmotic pumps. Treatment with rhGH for 5 d had no effect on body weight but a 4-d spaceflight resulted in a small reduction in the rate of weight gain. Neither spaceflight nor rhGH altered bone length or the rate of bone elongation. In contrast, rhGH and spaceflight each resulted in site- and bone-specific increases (periosteal) and decreases (endocortical) in formation of cortical bone. The results demonstrate that: a) inhibition of periosteal bone formation observed during longer spaceflight is not apparent during a 4-d spaceflight; and b) infusion of rhGH stimulates cortical bone formation in weightless as well as in weight bearing animals.