Is the nexus necessary for cell-to-cell coupling of smooth muscle?

Formation of gap junctions by treatment in vitro with potassium conductance blockers

Gap junctions were regularly seen in thin sections of canine tracheal smooth muscle incubated in vitro. Their number was increased in tissued exposed in vitro to either of two potassium conductance blockers, tetraethylammonium (TEA) and 4-aminopyridine (4-AP), and at the same time the muscles became mechanically active, with spontaneous contractions. The presence of gap junctions in this smooth muscle may provide one basis for cell-to-cell coupling, and their increase after TEA- and 4-AP-treatment could account for a decreased junctional resistance between cells, contributing to a longer space constant. However, an increase in gap junctions was not sufficient to change the behavior of trachealis smooth muscle from multiunit to single-unit type. Gap junctions in increased numbers persisted after washout of 4-AP, which caused inhibition of spontaneous contractions, and despite inhibition of the contractile effects of 4-AP by atropine. The rapid induction of gap junction formation was not dependent on de novo synthesis of protein. The fact that the number of gap junctions can be increased by chemical agents has important implications for control of their formation and provides a tool for analysis fo their role in cell-to-cell coupling.

Gap junctions: their presence and necessity in myometrium during parturition

Thin sections of longitudinal and circular muscle of myometrium obtained from rats during pregnancy, at term, during delivery, and postpartum were quantitatively examined in the electron microscope. Gap junctions (low resistance pathways) were only present during or immediately prior to delivery and immediately postpartum. The absence of gap junctions during gestation may be necessary for maintenance of pregnancy, while their occurrence during parturition may lead to effective termination of pregnancy.

Electron microscopy of the connective tissues between longitudinal and circular muscle of small intestine of cat

Electrotonic spread of applied potentials was observed between longitudinal and circular muscle layers of the small intestine with no rectification. Nexal junctions were demonstrated between muscle fibers of each layer. Connective tissue cells bridged between the two muscle layers. These showed structural characteristics of fibrocytes and of interstitial cells. Some nexuses were seen between connective tissue cells and between these cells and muscle fibers of each layer but in most junctions the membranes were 10-18 nm apart. Since connective tissue can serve for electrical conduction between cultured heart cells and since electrical properties of intestinal muscle permit transmission with low degrees of coupling, it is suggested that interstitial cells and fibrocytes may electrically couple longitudinal and circular muscle layers of cat intestine.

Effects of denervation and reserpine on nexuses in the rat vas deferens

Permanganate-fixed vasa deferentia from rats were examined for the presence of nexal-like contacts by electron microscopy. There was a significantly greater incidence of nexuses (2X) in chronically denervated tissues (5-7 days) but not in tissues from reserpine-pretreated animals (1.0 mg/kg/day for 5-7 days). The results suggest that an increase in nexal regions may not be a general feature of postjunctional supersensitivity but rather may contribute to other denervation-induced changes in contractile response.