| Abstract: | The activity of a plasma membrane cAMP-phosphodiesterase in cultured ovarian granulosa cells was regulated by follicle-stimulating hormone (FSH) and the gonadotropin-releasing hormone (GnRH) agonist [D-Ala6]des-Gly10-GnRH N-ethylamide (GnRHa). Degradation of cAMP was similar in cultures treated with FSH alone or FSH plus GnRHa when the labeled cyclic nucleotide was added from 24 to 42 h of culture. However, at 48 h and subsequent times of incubation, cAMP phosphodiesterase activity was significantly higher in cells incubated with FSH plus GnRHa. Phosphodiesterase activity was progressively increased by GnRHa concentrations between 10(-13) and 10(-10) M, and was maximally stimulated by 10(-9) M GnRHa. In comparison with control cells, FSH lowered the Vmax of cAMP catabolism by the high (1 microM cAMP substrate) and the low (50 microM) affinity phosphodiesterase, while GnRHa raised enzyme activity toward control levels. These actions of FSH and GnRHa were specific for a plasma membrane phosphodiesterase that was accessible to extracellular cAMP, since extracellular substrate was hydrolyzed, no intracellular uptake of [3H]cAMP was observed, and only a small fraction (10%) of cAMP was catabolized in the incubation medium in the absence of cells. Further, the actions of FSH and GnRHa on the membrane enzyme were the opposite of those observed when total phosphodiesterase activity was measured in cellular sonicates. Hormonal changes in phosphodiesterase activity were not due to leakage of the enzyme from damaged cells since a constant percentage of cAMP hydrolysis in the medium was observed during culture. Analysis of cAMP catabolites in granulosa cells indicated that the phosphodiesterase reaction product, 5'-AMP, was rapidly converted to adenosine by a plasma membrane 5'-nucleotidase, independent of the cellular hormonal status. These results indicate that the opposing actions of FSH and GnRHa upon granulosa cell differentiation include modulation of cAMP degradation at the plasma membrane level. |
