Determination of cyclic 3', 5'-AMP accumulation in resting and stimulated thyroid slices: Heterogeneity of the ATP precursor pool

Abstract

cAMP accumulation in dog thyroid slices has been estimated by two methods: from the radioactivity of cAMP in tissues prelabeled with 3H-adenine, 3H-adenosine or 32P-phosphate, and by the protein binding assay of Gilman et al. In the first method, the pitfalls in measuring cAMP accumulation in radioactivity per weight of tissue as a conversion ratio from its precursors ( 3H-cAMP/ 3H of the TCA soluble pool, or 3H-cAMP/ 3H-ATP), or from the specific radioactivity of the precursors used have been demonstrated. It is shown that the measurement of labeled cAMP accumulation may provide reliable estimates of relative increases in concentration but not of the absolute concentrations of cAMP in the slices. The specific radioactivity of cAMP was higher than the specific radioactivity of ATP in slices prelabeled with 3H-adenine and 3H-adenosine which suggests the existence of a specific precursor ATP pool involving less than 50% of tissue ATP. TSH, even when it enhances by a factor of 20 the accumulation of cAMP, does not decrease cell ATP levels which shows that the drain of ATP by this synthesis is easily compensated by the energy providing metabolic pathways of the tissue.