MODULATION OF THE JUNCTIONAL INTEGRITY BY LOW OR HIGH-CONCENTRATIONS OF CYTOCHALASIN-B AND DIHYDROCYTOCHALASIN-B IS ASSOCIATED WITH DISTINCT CHANGES IN F-ACTIN AND ZO-1

In a study of Necturus gallbladder epithelium Benzel et al. (Benzel et al., 1980) found that low (0.2-1.2 mu M) and higher concentrations (1 .5 mu M and more) of cytochalasin B (CB) caused an increase and decrea se in the transepithelial electrical resistance (TER), respectively. M oreover, there were slight changes in the height and complexicity of t ight junction (TJ) strands, as visualized by freeze-fracture and freez e-etching. To elucidate the mechanisms of these findings, we first dem onstrated that the effect is also present in monolayers of Madin-Darby Canine Kidney strain I (MDCK-I) cells. Thus, a low concentration (0.1 ng/ml) cytochalasin B (CB) strengthened the permeability barrier, as evidenced quantitatively by increases in TER on transepithelial electr ical measurements. Furthermore, indirect immunofluorescence and confoc al microscopy demonstrated that this effect was paralleled with an acc umulation of F-actin and the tight junction marker protein, ZO-1, at t he level of TJ, Equimolar concentrations of dihydrocytochalasin B (dhC B), on the other hand, did not lead to a tightening of the epithelium. Confirming previous studies, there was a general decrease in epitheli al resistance after treatment with high concentrations (1 mu g/ml) of CB and dhCB, which was accompanied by distinct changes in the F-actin network and distribution of ZO-1. We speculate that the divergent effe cts of CB and dhCB on the F-actin and ZO-1 organization might be due t o specific effects on the transport of monosaccharides across the plas ma membrane, or that CB and dhCB in distinct ways involve the turnover of phosphatidylinositols in the membrane, thereby modulating junction al permeability and F-actin structure.