Selective amplification of luteinizing hormone by adenosine in rat luteal cells

Adenosine amplification of LH-stimulated cAMP accumulation in rat luteal cells is rapid and dependent on mitochondrial ATP production. The objective of the present studies was to determine if this effect of adenosine is specific for LH and to gain information on the mechanism of the ATP-dependent amplification of LH action in rat luteal cells. Adenosine significantly amplified maximum cAMP accumulation in response to LH, isoproterenol, forskolin, and cholera toxin. However, amplification of this response by adenosine was significantly greater for LH than for the other agonists. The relative order of amplification by adenosine was LH greater than isoproterenol greater than forskolin greater than cholera toxin; the relative magnitudes of amplification by adenosine were 1, 0.6, 0.2, and 0.2, respectively. Neither LH, isoproterenol, forskolin, nor cholera toxin had any effect on cellular levels of ATP, and adenosine produced a similar rate of increase and maximal levels of ATP in the presence of all agonists. Ionomycin, a calcium ionophore, inhibited LH- and cholera toxin-stimulated cAMP accumulation and produced a dose-dependent depletion of ATP. Adenosine reversed the inhibitory effect of ionomycin on LH-stimulated cAMP accumulation and cellular levels of ATP. However, adenosine did not reverse the inhibitory effect of ionomycin on cholera toxin-stimulated cAMP accumulation, although its effects on cellular ATP levels were identical to those on LH. Thus, the selective amplification of LH by adenosine is not merely a substrate effect on adenylate cyclase activity. The nature of adenylate cyclase activation by cholera toxin and forskolin and the weak amplification by adenosine of these agonists compared to that of LH indicate that the site of the ATP-dependent action of adenosine appears to be before or on the G-protein of adenylate cyclase. We suggest that adenosine, by an ATP-dependent process, either increases the availability of functional LH receptors or increases coupling between the LH receptor and adenylate cyclase.