Adenine-Derived Purines Increase Adenosine Triphosphate (ATP) Levels in the Luteal Cell: Evidence that Cell Levels of ATP May Limit the Stimulation of Adenosine 3′5′-Monophosphate Accumulation by Luteinizing Hormone*

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Abstract
To assess if amplification of LH-stimulated cAMP accumulation and steroidogenesis by adenine-derived purines may be due to an increase in cell ATP levels, dispersed and enriched rat luteal cells were incubated with the purines under short term culture conditions. Adenosine produced an increase of 2-fold or more in the cell content of ATP; basal levels were about 160 ± 25 pmol/105 cells. An increase in ATP was een as early as 5 min, and the effect was maximal within 15 min of incubation. Incubation alone had little effect on the steady state concentration of ATP, no detectable ATP was seen in the incubation media, and the cells degraded extracellular ATP at a rate of 1 nmol/min · 105 cells. Concentrations of LH up to 500 ng/ml and of prostaglandin F2α at concentrations up to 1 μM had no effect on cell ATP levels in the presence or absence of adenosine. Cell ATP levels were increased by ADP, AMP, adenosine, inosine, adenine, and hypoxanthine, and these effects were paralleled by an increase in cAMP accumulation in response to LH. Guanosine and xanthine were inactive. At high concentrations of purines (50–100 μM), adenosine produced the greatest increase in ATP and cAMP levels, followed by AMP, ADP, inosine, adenine, and hypoxanthine. At low concentrations of the purines (2.5–10 μM), adenine produced the greatest increase in ATP and cAMP levels, followed by adenosine, inosine, and hypoxanthine. Adenosine deaminase reduced the responses of ADP, AMP, and adenosine to levels similar to that of inosine. Deoxycoformycin (an inhibitor of adenosine deaminase) increased the responses of adenosine, ADP, and AMP, but had no effect on the response of inosine. It was concluded that the response to the nucleotides was due to their conversion to adenosine. Dipyridamole, an inhibitor of adenosine transport, showed a dose-dependent inhibition of the increase in ATP and cAMP levels produced by adenosine, whereas little effect on adenine was seen. Based on these studies, it is suggested that purine amplification of LH-stimulated cAMP accumulation is due to an increase in the steady state concentration of ATP in the luteal cell. This increase in ATP may arise from direct conversion of the purines after cellular uptake and thereby increase ATP availability for adenylate cyclase, or ATP may serve other functions which enhance cAMP accumulation and amplify the response to LH.