Are gap junctions necessary for cell-to-cell coupling of smooth muscle?: an update

Abstract

Earlier, it was questioned whether gap junctions (GJs) were necessary for cell-cell communication in smooth muscle, and GJs were not seen in some smooth muscles. We reexamined this question in the myometrium and in intestinal smooth muscle, in light of current knowledge of the presence and function of GJs. In the uterus, numerous studies show that an increase in GJ number is associated with the onset of delivery and is required for effective parturition. In all cases, this increase in GJ number and the changes in uterine contractility were correlated with increased electrical and metabolic coupling. Evidence for the much smaller, but detectable, degree of electrical coupling in the preterm uterus is explained by the small (but again detectable) number of GJs present. In the intestine, GJs are readily detected in the circular muscle layer but have not been described in the adjacent longitudinal layer. While our immunohistochemical studies failed to detect GJs in the longitudinal layer, this may not be adequate to prove their absence. Therefore, current knowledge of GJ number and function is adequate to explain cell-cell coupling in the uterus. Although it remains uncertain whether GJs are absent from the longitudinal muscle of the intestine, there is no definitive evidence that cell-cell coupling can occur by means other than GJs.

Plasma Membrane Channels Formed by Connexins: Their Regulation and Functions

“…114) where they allow the formation of a muscular syncytium that can synchronize contractile activity. Intestinal smooth muscle cells are electrically coupled (1,21,143,177,212,572), and they show intercellular passage of microinjected dyes (675). In contrast, gap junctions appear to be absent in the longitudinal muscle layer (212), which may explain why the contraction of canine duodenal circular muscle correlates much better with spike potentials than does contraction of longitudinal muscle.…”

Connexins: Key Mediators of Endocrine Function

“…In contrast, connexin channels have been largely implicated in a variety of functions (159,227,345,504,506,509), including embryonic development, morphogenesis, and cell differentiation, as well as in the control of adult cell proliferation and migration, the functioning of muscle cells, hormonal transmission, electrical and mechanical synchronization, resistance to cytotoxic agents, compensation of enzymatic defects, transmission of trophic or deadly molecules, and secretion under both normal and pathological conditions (14, 53-55, 92, 110, 167, 507, 509). Whereas several of these functions have been attributed to connexins solely on the basis of circumstantial, correlative studies, a number of others have been documented after careful interference with selective connexins.…”

Hypothetical Role of Prostaglandins in the Onset of Preterm Labor after Fetal Surgery

“…Human myometrial cells are linked together by wide GAP junctions that allow for intracellular communication and calcium wave propagation within myocyte bundles. Connexin 43 is the most abundant junctional protein, and has been shown to be expressed in the preterm uterus [36] and to increase in density and overall amount as gestation progresses [37] . Young showed a separate mechanism of action potential wave propagation through the uterine fasciculi.…”