Effects of Cyclic Nucleotides on Deoxyribonucleic Acid Synthesis in Hypophysectomized Rat Cartilage: Stimulation of Thymidine Incorporation and Potentiation of the Action of Somatomedin by Analogs of Adenosine 3′,5′-Monophosphate or a Cyclic Nucleotide Phosphodiesterase Inhibitor*

Abstract

The actions of cyclic nucleotides on basal and somatomedin-stimulated thymidine incorporation into DNA by costal cartilage from hypophysectomized rats were investigated. Three analogs of cAMP (dibutyryl, 8-bromo, and 8-dimethylamino derivatives, which are alternate activators of cAMP-dependent protein kinase and resistant to degradation by cAMP phosphodiesterase but represent a wide difference in potency as phosphodiesterase inhibitors) in range of concentrations from about 10(-5) to 3 X 10(-4) M enhanced basal and somatomedin-stimulated thymidine incorporation. Each cAMP analog at optimal concentration produced combined effects with a suboptimal concentration of somatomedin which were additive or greater. cAMP itself, 5'-AMP, adenosine, 8-Br-5'-AMP, 8-Br-AMPT, and cGMP at concentrations from 10(-7)--10(-3) M or dibutyryl cGMP at concentrations from 10(-10)--10(-3) M did not reproduce the effects of the cAMP analogs. A phosphodiesterase inhibitor (1-methyl-3-isobutylxanthine) at concentrations of 100 or 500 microM also potentiated the effects of somatomedin. At 100- or 500-microM concentrations, the phosphodiesterase inhibitor increased cartilage levels of cAMP and cGMP. These results suggest a role for cAMP in DNA synthesis in rat cartilage. However, they fail to support the hypothesis that all effects of somatomedin on that process are mediated by cAMP, since stimulation of thymidine incorporation by the hormone can be demonstrated in cartilage maximally stimulated by analogs of cAMP.