We have combined freeze-fracture and electrophysiological methods in a study of It has been suggested that the direct diffusion of small molecules between the interiors of adjacent cells could be an important factor in cellular homeostasis and even the regulation of growth and differentiation (1-3). This exchange of molecules is believed to occur through gap junctions (4-7), which are specializations of two closely apposed membranes (8), each containing an aggregate of intramembranous particles (9, 10). The particles are presumed to bridge the narrowed extracellular space of a gap junction amd may provide an intercytoplasmic pathway for diffusing molecules (6, 10).While many ultrastructural and physiological properties of gap junctions have been elucidated (1,11,12)

Cells in isolated rat islets of Langerhans were microinjected with Lucifer Yellow CH in a medium containing 16.7 millimolar glucose. Dye was rapidly transferred from the injected cell to neighboring islet cells without specificity with regard to the immunocytochemical identity of either the donor or the recipient cells. The transfer of dye between the islet cells (types A, B, and D) demonstrates homologous and heterologous cell coupling in a system where the normal proportions and relationships of the cell types are maintained.

The transformed or normal phenotype of cultured normal rat kidney cells infected with a temperature-sensitive mutant of avian sarcoma virus is conditional on the temperature at which the cells are grown . Using dye injection techniques, we show that junction-mediated dye transfer is also temperature-sensitive . The extent and rate of transfer between infected cells grown at the transformation-permissive temperature (35°C) is significantly reduced when compared to infected cells grown at the nonpermissive temperature (40.5°C) or uninfected cells grown at either temperature . Infected cells subjected to reciprocal temperature shifts express rapid and reversible alterations of dye transfer capacities, with responses evident by 15 min and completed by 60 min for temperature shifts in either direction . These results suggest that altered junctional capacities may be fundamental to the expression of the ASV-induced, transformed phenotype .

Cell junctions between Novikoff hepatoma cells (N1S1-67) growing as small clumps or chains in suspension culture have been studied with ultrastructural, electrophysiological, and dye-injection techniques. Cells within clumps are commonly electrically coupled and can exchange dyes with a molecular weight of 332 to 500. Gap junctions and intermediate junctions are present, whereas true tight junctions and desmosomes are absent or very rare. This system should provide a useful model for studying the properties of "communicating" junctions.

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