AJP - Lung Cellular and Molecular Physiology, Vol 269, Issue 2 144-L150, Copyright © 1995 by American Physiological Society
Ionomycin and PDBU increase MDCK monolayer permeability independently of myosin light chain phosphorylation
D. M. Shasby, J. M. Kamath, A. B. Moy and S. S. Shasby
Department of Internal Medicine, University of Iowa College of Medicine, Iowa City, USA.
It has been hypothesized that modulation of epithelial paracellular
permeability may be mediated by initiation of contraction of a band of
actin and myosin located at the tight junction. Phosphorylation of myosin
light chain (MLC) is an important determinant of actomyosin contraction. We
asked if ionomycin (iono) and phorbol 12,13-dibutyrate (PDBU), which
increase paracellular permeability of Madin-Darby canine kidney (MDCK) cell
monolayers, increased MLC phosphorylation in MDCK cells. MDCK cell MLC was
constitutively phosphorylated by myosin light chain kinase (MLCK), and
after PDBU and iono > 99% of MLC continued to be phosphorylated by MLCK.
Neither iono or PDBU, nor the combination of iono and PDBU, increased MLC
phosphorylation. In contrast, the phosphatase inhibitor okadaic acid did
increase MLC phosphorylation. Adenosine 3',5'-cyclic monophosphate (cAMP)
and forskolin decreased MLC phosphorylation in control MDCK cells and in
cells exposed to iono and PDBU. In contrast, cAMP and forskolin did not
blunt the decrease in transepithelial resistance caused by iono and PDBU.
Iono and PDBU increase MDCK monolayer permeability independently of an
increase in MLC phosphorylation.
