Isolated chick sclera shows a circadian rhythm in proteoglycan synthesis perhaps associated with the rhythm in ocular elongation

In the growing chick, ocular elongation is rhythmic, increasing during the day and decreasing at night. Because experimentally induced changes in the rate of ocular elongation are associated with changes in the rate of synthe sis of scleral proteoglycans, we asked whether there is a diurnal rhythm in scleral proteoglycan synthesis, whether the rhythm is endogenous, and whet her scleras from normal eyes differed from those of faster growing form-dep rived eyes. To assess proteoglycan synthesis, we measured the incorporation of labeled sulfate into glycosaminoglycans using two paradigms: (1) punche s of sclera were cultured for either 2 or 10 h at various times of day, and (2) punches were cultured in a perifusion system for up to 80 h, and sampl es of the medium were collected for analysis at 2-h intervals. Synthesis of scleral proteoglycans is higher during the day than during the night. This rhythm persists for at least three cycles in vitro with a period of approx imately 24 h. There are no significant differences between rhythms in scler as from normal and form-deprived eyes. Finally, biochemical analyses show t he labeled molecule to be similar to aggrecan, the cartilage proteoglycan. We conclude that the synthesis of proteoglycans by scleral chondrocytes is circadian, and we speculate that this rhythm may influence the rhythm in oc ular elongation.