Complementarity of particles and pits in freeze-fractured hepatic and cardiac gap junctions

Particles and pits of freeze-fractured gap junctions are considered as complementary structures despite the frequent observations of more regular and closer spacings of pits, ascribed to plastic deformation of particle arrays. Recently, however, the noncomplementarity of pits and particles in Purkinje fibers has been reported. To ascertain the relationship between both structures, gap junctions from fixed, cryoprotected liver and myocardium were investigated using spacing and density measurements and complementary replicas. In hepatocyte gap junctions, the center-to-center distances (mean +/- SD) among pits, 9.57 +/- 1.49 nm, and particles, 9.70 +/- 1.77 nm, are not significantly different. Density determinations yielded a slightly higher value for the pits, (11,510 +/- 830)/microns 2, than for the particles, (11,230 +/- 950)/microns 2. In the myocardium, the spacing of the regularly arrayed pits, 9.55 +/- 1.33 nm, barely exceeds the value of 9.44 +/- 1.62 nm for the particles, which show some clustering. However, the packing density for the pits, (10,090 +/- 740)/microns 2, appears a little higher than that of the particles, (9,890 +/- 920)/microns 2. As density and spacing measurements provided no decisive answers, the positions of individual pits and particles of complementary junctional faces were recorded on transparent sheets and compared. In this fashion, a one-to-one correspondence between particles and pits could be established, while small discrepancies may be attributed to plastic deformation. Moreover, the co-linearity of pits and particles may be suggested by the observation of a platinum grain in the center of many pits.