Sulfate uptake and somatomedin levels in the "little' (lit/lit) mouse

The mutant "little' (lit/lit) mouse is deficient in growth hormone and has correspondingly low levels of serum somatomedin. Injection of these mice with human or bovine growth hormone significantly raises serum somatomedin levels within 6 h. In vivo uptake of radioactive sulfate by costal cartilage in lit/lit mice is similar to that of normal mice, which is unexpected in view of the low levels of circulating somatomedin. If costal cartilage from normal and lit/lit mice are preincubated in medium 199 in vitro before transfer to fresh medium containing radioactive sulfate and serum, there is not consistent difference in uptake of sulfate, demonstrating similar endogenous cartilage activity. In contrast, omission of the preincubation step reveals a lower uptake of sulfate in vitro by cartilage from lit/lit mice as compared with normal mice. Cartilage removed from lit/lit mice 24 h after injection with growth hormone, however, takes up greater amounts of sulfate than cartilage from untreated normal mice. Taken together, these data suggest that the level of circulating somatomedin is not a dominant factor in controlling the uptake of radioactive sulfate into costal cartilage of mice in vivo. This cells into question the relationship between somatomedin levels, in vivo rates of sulfate incorporation and rates of growth.