Synthesis of new cyclic nucleotides and their differential stimulatory effects on thyroid function in mice

Various 8-substituted cAMP analogues were converted into the corresponding N6,O2'-dibutyryl, N6-monobutyryl, or O2'-monobutyryl derivatives and the parent compounds as well as the new derivatives were tested for TSH-like stimulation of the thyroid function in mice in vivo by means of the McKenzie bioassay. It was found that a pronounced stimulatory effect on the thyroid secretion can be produced by certain cyclic nucleotides. The most potent compounds, 8H2N-cAMP, 8MeS-cAMP, and 8N3-cAMP at 15-30 mg/kg iv, showed an activity approximately one order of magnitude higher than that of cAMP and comparable to the action of 0.6-1.0 mU of TSH per mouse iv. Radioimmunological determination of thyroxine levels in the serum confirmed these results. Serum T4 levels in mice injected, for instance, with 8MeS-cAMP (30 mg/kg) rose from 4.85 +/- 0.25 to 6.32 +/- 0.41 mug/100 ml in 1 h, the net increase in T4 being comparable to that produced by TSH (1 mU/mouse) under identical experimental conditions. Introduction of one or two butyryl groups (in the N6 and/or O2' position) significantly increased or decreased the biological activity, dependent on the parent compound, but no direct correlation between biological activity and degree of butyrylation was apparent. The effect of these compounds on the thyroid seems to be specific for cyclic nucleotides because 8MeS-5'AMP which resembles 8MeS-cAMP but lacks the cyclic phosphate structure is inactive in the McKenzie bioassay and in the T4 radioimmunoassay.