Cyclic Nucleotide Hydrolysis in the Thyroid Gland

Phosphodiesterase activities of horse (and dog) thyroid soluble fraction were compared with either cyclic AMP (adenosine 3′: 3′‐monophosphate) or cyclic GMP (guanosine 3′: 5′‐monophosphate) as substrate. Optimal activity for cyclic AMP hydrolysis was observed at pH 8, and at pH 7.6 for cyclic GMP. Increasing concentrations of ethyleneglycol bis(2‐aminoethyl)‐ N , N ‐tetraacetic acid inhibited both phosphodiesterase activities; in the presence of exogenous Ca 2+ , this effect was shifted to higher concentrations of the chelator. In a dialysed supernatant preparation, Ca 2+ had no significant stimulatory effect, but both Mg 2+ and Mn 2+ increased cyclic nucleotides breakdown. Mn 2+ promoted the hydrolysis of cyclic AMP more effectively than that of cyclic GMP. For both substrates, substrate velocity curves exhibited a two‐slope pattern in a Hofstee plot. Cyclic GMP stimulated cyclic AMP hydrolysis, both nucleotides being at micromolar concentrations. Conversely, at no concentration had cyclic AMP any stimulatory effect on cyclic GMP hydrolysis. 1‐Methyl‐3‐isobutylxanthine and theophylline blocked the activation by cyclic GMP of cyclic AMP hydrolysis, whereas Ro 20‐1724 (4‐(3‐butoxy‐4‐methoxybenzyl)‐2‐imidazolidinone), a non‐methylxanthine inhibitor of phosphodiesterases, did not alter this effect. In dog thyroid slices, carbamoylcholine, which promotes an accumulation of cyclic GMP, inhibits the thyrotropin‐induced increase in cyclic AMP. This inhibitory effect of carbamoylcholine was blocked by theophylline and 1‐methyl‐3‐iso‐butylxanthine, but not by Ro 20–1724. It is suggested that the cholinergic inhibitory effect on cyclic AMP accumulation is mediated by cyclic GMP, through a direct activation of phosphodiesterase activity.