The combined effects of teleostean GH and human IGF-I in the regulation of teleost branchial cartilage growth were examined. Ceratobranchial cartilages were dissected from the common carp (Cyprinus carpio) and maintained in a defined culture medium supplemented with recombinant striped bass GH (sbGH), human IGF-I (hIGF-I) or both, and the uptake of [35S]sulphate by cartilage explants was measured. sbGH alone in the culture medium did not exhibit a significant stimulatory effect on the uptake of [35S]sulphate when compared with the controls. However, with hIGF-I in the culture medium, as low as 1 ng/ml, the stimulatory effect of sbGH was apparent and dose-dependent. The synergism of sbGH and hIGF-I was observed at the concentrations of 1 and 10 ng IGF/I/ml tested. At a constant hIGF-I concentration (10 ng/ml), a maximum stimulatory effect was detected with 3 μg recombinant sbGH/ml, at which point a 2·3-fold increase in sulphation activity was obtained when compared with the treatment with hIGF-I alone. A similar dose-dependent stimulatory effect was observed when native common carp and bonito GHs were tested using this assay system. These results suggest that the synergistic effect of sbGH and hIGF-I on sulphation activity is a common biological function of teleost GH, thus ruling out the possibility of experimental artifacts resulting from utilization of heterologous GH. Furthermore, experiments were conducted to test whether the synergism between sbGH and hIGF-I occurred between sbGH and insulin. Results showed that although bovine insulin was less effective than hIGF-I on the stimulation of [35S]sulphate uptake, the synergistic effects of sbGH with hIGF-I and sbGH with insulin were similar at the concentrations tested. The concept of a 'dual effector' mechanism for GH action in teleosts is supported by these findings.
Journal of Endocrinology (1995) 147, 67–73