Tissue-specific granularity of gap junction cytoplasmic surfaces revealed by rapid-freeze, deep-etch replicas

Previous rapid-freeze, deep-etch replica studies have revealed the differences between heart and liver gap junctions; cytoplasmic surfaces of in situ and phenylmethylsulfonyl fluoride (PMSF)-unproteolyzed isolated cardiac gap junctions (MW 47 kD) have a particulate substructure, which is absent both in the proteolyzed heart junctions (MW 29 kD) and in the liver junctions isolated with PMSF (MW 28 kD). The present deep-etch replica studies of gap junction cytoplasmic surface (CS) membranes in several tissues of rats and mice were performed to examine whether or not this difference between liver and heart is typical of variations in gap junction proteins from tissue to tissue. In surface mucous cells of the stomach, intestinal epithelial cells, and kidney tubule cells, these epithelial gap junctions always showed smooth cytoplasmic surfaces, similar to the liver gap junctions. In contrast, in the atrial myocardium, aortic endothelium, and the ciliary process, cytoplasmic surface membranes of the gap junctions consistently revealed particulate patterns. Close examinations disclosed that those granular structures were not merely attached to the memvrane surface, but they also protruded from the membrane interior as an integral component of gap junction particles. Furthermore, in the pregnant rat uterus at term, cytoplasmic surface membranes of myometrial smooth muscle gap junctions were particulate, but those of endometrial epithelium were smooth. The present observation suggest that tissue specificity exists in cytoplasmic surface structures of gap junctions between the "true" epithelial and the nonepithelial tissues: the nonepithelial gap junctions contain the additional cytoplasmic surface domain that is absent in the gap junctions of "true" epithelial origin.