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

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.