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The aim of this study was to assess whether the expression of E-cadherin at the surface of rat β-cells is regulated by insulin secretagogues and correlates with insulin secretion. When cultured under standard conditions, virtually all β-cells expressed E-cadherin observed by immunofluorescence, but heterogeneous staining was observed. Using fluorescence-activated cell sorting (FACS), two β-cell sub-populations were sorted: one that was poorly labeled (‘ECad-low’) and another that was highly labeled (‘ECad-high’). After 1-h stimulation with 16.7 mM glucose, insulin secretion (reverse hemolytic plaque assay) from individual ECad-high β-cells was higher than that from ECad-low β-cells. Ca2+-dependent β-cell aggregation was increased at 16.7 mM glucose when compared with 2.8 mM glucose. E-cadherin at the surface of β-cells was increased after 18 h at 11.1 and 22.2 mM glucose when compared with 2.8 mM glucose, with the greatest increase at 22.2 mM glucose + 0.5 mM isobutylmethylxanthine (IBMX). While no labeling was detected on freshly trypsinized cells, the proportion of stained cells increased in a time-dependent manner during culture for 1, 3, and 24 h. This recovery was faster when cells were incubated at 16.7 vs 2.8 mM glucose. Cycloheximide inhibited expression of E-cadherin at 2.8 mM glucose, but not at 16.7 mM, while depolymerization of actin by either cytochasin B or latrunculin B increased surface E-cadherin at low glucose. In conclusion, these results show that expression of E-cadherin at the surface of islet β-cells is controlled by secretagogues including glucose, correlates with insulin secretion, and can serve as a surface marker of β-cell function.

The kidney medulla is physiologically exposed to variations in extracellular osmolality. In response to hypertonic cell shrinkage, cells of the rat kidney medullary thick ascending limb of Henle's loop undergo p38 kinase-dependent regulatory volume increase (RVI). In the present study, we investigated the role of actin cytoskeleton reorganization in this process. Addition of hyperosmotic NaCl or sucrose, which activates MAP kinases and reduces cellular volume, induced a sustained actin polymerization occurring after 10 min and concurrently with RVI. In contrast, hyperosmotic urea, which does not modify MAP kinase activity and cellular volume, did not induce sustained actin polymerization. Fluorescence microscopy revealed that hyperosmotic NaCl and sucrose, but not urea, induced the redistribution of F-actin from a dense cortical ring to a diffuse network of actin bundles. Stabilization of actin filaments by jasplakinolide and inhibition of the generation of new actin filaments by swinholide A prevented RVI, whereas depolymerization of actin filaments by latrunculin B attenuated cell shrinkage and enhanced RVI. These actin-interfering drugs did not alter extracellular regulated kinase and p38 kinase activation under hypertonic conditions. Similar to swinholide A, inhibiting p38 kinase with SB-203580 abolished sustained actin polymerization, actin redistribution, and decreased RVI efficacy. We therefore propose that in rat kidney the medullary thick ascending limb of Henle's loop exposed to extracellular hypertonicity, p38 kinase activation induces depolymerization of the F-actin cortical ring and polymerization of a dense diffuse F-actin network that both contribute to increase RVI efficacy.