cAMP-stimulated termination of vitellogenesis in Hyalophora cecropia: formation of a diffusion barrier and the loss of patency.

Activation of cAMP-dependent protein kinase (PKA) by cell-permeable analogs of cAMP causes early and mid-vitellogenic follicles of Hyalophora cecropia to terminate vitellogenin uptake [[Wang and Telfer, 1996], Insect Biochem. Mol. Biol. 26, 85-94 (1996)]. The response is shown here to entail the formation of an epithelial diffusion barrier. Follicle cells that have been loosely organized to provide intercellular pathways for the movement of vitellogenin to the oocyte surface transform into a tight epithelium within 1-2h of exposure to PKA activators. The follicle cells can now prevent the escape of Lucifer yellow CH that has been iontophoresed into the space surrounding the oocyte, and the entry of labeled vitellogenin from the medium. As they form this functional equivalent of a tight junction, the follicle cells further reduce the intercellular spaces by enlarging and pressing against each other, and by slowing the secretion of the sulfated glycosaminoglycan matrix that separates them during vitellogenesis. The activation of PKA in early and mid-vitellogenic follicles thus appears to trigger prematurely a set of changes that do not normally occur until the follicle has grown to a length of about 2.0mm.