Adenosine receptor-mediated accumulation of adenosine 3',5'-monophosphate in guinea pig thyroid tissue

The cAMP content of guinea pig thyroid fragments was increased 2- to 3-fold by adenosine analogs, 5'-N-ethylcarboxamide adenosine (NECA), the most potent, caused half-maximal stimulation of 0.7 microM, whereas N6-phenylisopropyl adenosine was much less potent (half-maximal stimulation at 20 microM). Exogenous adenosine by itself was slightly active, and its activity was increased in the presence of adenosine deaminase or transport inhibitors. Although adenosine analogs are partial agonists compared to TSH or the diterpene cyclase activator forskolin, the fractional rate of cAMP accumulation is greater for NECA than for TSH. The stimulatory effect of NECA was antagonized by theophylline. These effects of adenosine and its analogs on cAMP formation in intact thyroid tissue are characteristic of stimulatory adenosine receptors.