Cell junction and cyclic AMP: II. Modulations of junctional membrane permeability, dependent on serum and cell density.

Junctional molecular transfer (as indexed by the number of cell interfaces transferring fluorescent-labelled molecules) and concentration of endogenous cAMP were determined in mammalian cells in culture at varying serum concentration and cell density. In several cell types, on stepping the serum concentration from 10% (the concentration to which the cells had been adapted) to zero, the junctional transfer rose (reversibly) within 48 hr, as the endogenous cAMP concentration rose. The junctional transfer was inversely related to serum concentration over a range, most steeply so the transfer of large and charged molecules. One cell type showed no junctional change in response to serum; it showed also no endogenous cAMP change. Junctional transfer varied inversely with cell density over the range of 0.7-7 (10(4) cells/cm2) in 3T3 cells. In cultures seeded to various densities, or growing to various densities on their own, junctional transfer fell with rising density, and so did the endogenous cAMP concentration. Upon downstep from high density, junctional transfer was independent of cell density over the aforementioned range, and so was the endogenous cAMP concentration. These results, in conjunction with the effects of exogenous cAMP described in the preceding paper of this series, point to a cAMP-mediated junctional effect; a possible teleonomy for control of membrane junction is discussed.