Gap junction ultrastructure in three states of conductance

Summary Junctional conductance between the epidermal cells of the beetle Tenebrio molitor is raised after exposure to the hormone 20-hydroxyecdysone and lowered reversibly by exposure to chlorpromazine. Gap Junctional particle size, density and arrangement associated with these conductance changes were studied. We found no significant difference in particle density in gap junctions of control (2456±471 particles/m², mean ±S.D.) and hormone-treated epidermis (2490±315); however, a significant increase in packing density occurred in chlorpromazine-uncoupled epidermis (3133±665). The particles are randomly arranged in all three states of conductance. Particle size measurements show that the E-face gap junctional particles are heterogeneous with a mean diameter ±S.D. of 15.2±2.0 nm. No significant difference in particle size between controls and experimentals was detected. Although glutaraldehyde irreversibly uncoupled these cells, the absence of glutaraldehyde fixation but presence of glycerol induced marked alterations in the appearance of the gap junctions such that quantification was no longer possible. From this particle-packing data and our previous thin-section data, we estimate that there are 90000 gap junctional particles per cell (within junctional plaques). The conductance of a single gap junctional channel (assuming one population) changes from 94 pS to 213 pS after hormone treatment.