Regulation of cartilage glycosaminoglycan synthesis in the rainbow trout, Oncorhynchus mykiss, by 3,3',5-tri-iodo-L-thyronine and IGF-I.

Abstract

The actions of 3,3',5-tri-iodo-L-thyronine (T3) and recombinant human IGF-I (rhIGF-I) as well as their interaction on cartilage growth in rainbow trout (Oncorhynchus mykiss) were examined. Uptake of 3H-methyl thymidine and 35S-sulfate by isolated branchial cartilage was measured as a marker for chondrocyte proliferation and sulfated glycosaminoglycan synthesis respectively. When T3 (1.0 microgram/g) was injected intraperitoneally, plasma T3 levels reached a transient peak after 1 day and decreased rapidly thereafter. Sulfate and thymidine uptake were not affected by T3 at 1 and 3 days post-injection, but at 6 days post-injection both were significantly higher in T3-injected fish than those in controls. The stimulatory effects of a T3 injection on sulfate and thymidine uptake were dose-dependent over the range of 0.01, 0.1 and 1.0 micrograms/g. In vitro exposure of cartilage to T3 (0.075, 0.75, 7.5, 75 and 750 nM) for 6 days resulted in dose-dependent stimulation of sulfate uptake, with a maximum response at 7.5 nM and higher. T3 exposure (7.5 nM) for 2 or 3 days also increased sulfate uptake, but only slightly. Thymidine uptake was not clearly affected by T3. In vitro addition of rhIGF-I (0.075, 0.75 and 7.5 nM) increased sulfate uptake, but not thymidine uptake, dose-dependently. Compared with T3, rhIGF-I induced a greater maximum level of sulfate uptake: at 7.5 nM rhIGF-I increased the uptake 17-fold whereas T3 increased the uptake fourfold. When T3 (0.075, 0.75 or 7.5 nM) and rhIGF-I (0.1 or 1.0 nM) were added together, stimulative actions of T3 on sulfate uptake were largely additive to those of rhIGF-I. The results indicate that T3 as well as IGF-I are important modulators of sulfated glycosaminoglycan synthesis in rainbow trout cartilage.