The escape of cyclic AMP from dog thyroid slices exposed to positive and negative regulators.

Incubation of dog thyroid slices with 1 mU/ml TSH resulted in enhanced intracellular and extracellular cAMP accumulation. In the absence of TSH, the intra- and extracellular cAMP concentrations remained at a constant low level. The release of cAMP from TSH-stimulated slices was inhibited by 10 microM PGA1, 1 mM probenecid or 1 mM IBMX, which are known inhibitors of cAMP escape in several tissues. Negative controls of intracellular cAMP levels are exerted in the dog thyroid by 10 microM carbamylcholine (shown to activate a Ca++- calmodulin dependent phosphodiesterase), 100 microM norepinephrine and 100 microM iodide (both inhibiting adenylate cyclase activity). The purpose of the present study was to demonstrate that these three agents do not enhance cAMP escape. The results presented here show that these agents decrease both intracellular accumulation and escape in parallel. Moreover, the escape constants obtained by numerical simulation were not greater in the presence of inhibiting concentrations of carbamylcholine, norepinephrine or iodide. Thus the inhibition by these agents of cAMP accumulation in TSH-stimulated dog thyroid slices cannot be explained by a stimulation of cAMP escape from these cells.