The effects of alkylated xanthines on cyclic AMP accumulation in dog thyroid slices exposed to carbamylcholine

F Miot, C Erneux, JN Wells and JE Dumont

In dog thyroid slices stimulated by thyrotropin (TSH), activation of muscarinic cholinergic receptors leads to a decrease in cyclic AMP levels. Previous studies have established that carbamylcholine enhances cyclic GMP levels and inhibits cyclic AMP accumulation. Several experimental data have suggested that these effects are mediated by an increase in intracellular Ca2+ levels. The inhibition of cyclic AMP accumulation results in accelerated catabolism. Dog thyroid phosphodiesterase activity is due to a mixture of three enzyme forms: a calmodulin-sensitive form, a cyclic GMP-stimulated form and a cyclic AMP-specific form. This report is concerned with the comparison of the effects of several phosphodiesterase inhibitors on cell-free phosphodiestease activity and on cyclic nucleotide accumulation in intact cells. Alkylated xanthines, 4-(3-butoxy-4-methoxybenzyl)-2- imidazolidinone (Ro 20-1724), and 2-O-propoxyphenyl-8-azapurin-6-one (M & B 22948) were studied as inhibitors of partially purified dog thyroid phosphodieterases and for their ability to alter cyclic AMP and cyclic GMP accumulation in dog thyroid slices that had been stimulated with TSH and/or carbamylcholine. 1-Methyl-3-isobutylxanthine (MIX) and 7- benzyl MIX were the most potent inhibitors of phosphodiesterase activities in the crude soluble and particulate fractions but exhibited no selectivity for inhibiting cyclic AMP or cyclic GMP hydrolysis. In dog thyroid slices stimulated by TSH and in the absence of carbamylcholine, Ro 20-1724 and 1-isoamyl-3-isobutylxanthine (IIX) were the most effective compounds to potentiate the accumulation of cyclic AMP. The rank order of abilities to potentiate cyclic AMP accumulation in dog thyroid slices stimulated by TSH paralleled the rank order of potencies to inhibit the cyclic AMP-specific phosphodiesterase. In the presence of carbamylcholine, the observed decrease in cyclic AMP levels was attenuated by MIX, 8-methoxymethyl MIX (8-MeOMe MIX), 7-benzyl MIX, and M & B 22948, the most potent inhibitors of the calmodulin-sensitive phosphodiesterase. MIX, 8-MeOMe MIX, and 7-benzyl MIX inhibited the cyclic GMP-stimulated phosphodiesterase in the same rank order of potencies as the calmodulin-sensitive enzyme, but M & B 22948 did not significantly inhibit the cyclic GMP-stimulated enzyme activity. IIX and Ro 20-1724 did not alter the carbamylcholine-induced inhibition of cyclic AMP accumulation.(ABSTRACT TRUNCATED AT 400 WORDS)

Volume 25, Issue 2, pp. 261-266, 03/01/1984
Copyright © 1984 by American Society for Pharmacology and Experimental Therapeutics

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