Hypertrophic smooth muscle. III. Increase in number and size of gap junctions.

The smooth muscle cells of the circular musculature of the guinea pig ileum are connected by gap junctions (nexuses) which occupy about 0.21% of the cell surface. When the muscle hypertrophies in the portions of the ileum oral to an experimental stenosis, the muscle cells increase in size and number. Gap junctions become markedly larger than in control muscles and occupy 0.49% of the cell surface. While the cells double their surface area, the number of nexuses per unit surface remains unchanged (47--48 per 1000 microns2). The packing density of intramembrane particles (or pits) in the nexuses of hypertrophic muscle cells is 6700 . microns-2, which is slightly less than in control muscle cells (7200 . microns-2). A characteristic grouping of the particles (or the pits) within the nexus is often observed. Nexuses between two processes originating from the same cell are common. Nexuses do not occur in the longitudinal muscle.