Cyclic nucleotide-induced termination of vitellogenin uptake by Hyalophora cecropia follicles

Endocytosis of vitellogenin by isolated follicles of Hyalophora cecropia terminated after membrane-permeable analogs of cAMP or cGMP were added to the culture medium. Depending on the concentration of the analog, a lag period of 30 min to 3 h preceded termination. Forskolin and IBMX both stimulated a rise in endogenous cAMP, and this also induced termination, as did pharmacological activation of the cyclic nucleotide-dependent protein kinases PKA and PKG. Inhibitors of PKA or PKG protected follicles from the corresponding cyclic nucleotide effect. When cAMP or cGMP was added to homogenates of vitellogenic follicles, a 32 kDa polypeptide was phosphorylated; inhibition of PKA, prevented phosphorylation of this protein. The rate of vitellogenin uptake did not accelerate significantly when PKA or PKG was inhibited in culture, which suggests that these kinases are normally inactive or operating below threshold during the several days of vitellogenesis. They seem thus not to be involved in the steady-state modulation of protein uptake. A more likely function of this control pathway in follicle development would be to trigger the termination of vitellogenesis, which normally occurs spontaneously in follicles of this species as they reach a length of 2 mm.