CYCLIC NUCLEOTIDE-INDUCED TERMINATION OF VITELLOGENIN UPTAKE BY HYALOPHORA-CECROPIA FOLLICLES

Endocytosis of vitellogenin by isolated follicles of Hyalophora cecrop ia terminated after membrane-permeable analogs of cAMP or cGMP were ad ded to the culture medium, Depending on the concentration of the analo g, a lag period of 30 min to 3h preceded termination, Forskolin and IB MX both stimulated a rise in endogenous cAMP, and this also induced te rmination, as did pharmacological activation of the cyclic nucleotide- dependent protein kinases PKA and PKG. Inhibitors of PKA or PKG protec ted follicles from the corresponding cyclic nucleotide effect, When cA MP or cGMP was added to homogenates of vitellogenic follicles, a 32 kD a polypeptide was phosphorylated; inhibition of PKA, prevented phospho rylation of this protein. The rate of vitellogenin uptake did not acce lerate significantly when PKA or PKG was inhibited in culture, which s uggests that these kinases are normally inactive or operating below th reshold during the several days of vitellogenesis. They seem thus not to be involved in the steady-state modulation of protein upake. A more likely function of this control pathway in follicle development would be to trigger the termination of vitellogenesis, which normally occur s spontaneously in follicles of this species as they reach a length of 2 mm.