Osmotic regulation of intestinal epithelial Na(+)-K(+)-Cl- cotransport: role of Cl- and F-actin

Stimulation of Na+-K+-2Cl- cotransport by arsenite in ferret erythrocytes

1. Na+-K+-2Cl- cotransport activity was measured in ferret erythrocytes as the bumetanide-sensitive uptake of 86Rb. 2. The Na+-K+-2Cl- cotransport rate was stimulated by treating erythrocytes with sodium arsenite but not by sodium arsenate (up to 1 mM). Stimulation took an hour to develop fully. Arsenite had no effect on bumetanide-resistant 86Rb uptake. 3. In cells stored for 3 days or less, cotransport stimulation by arsenite could be described by assuming arsenite either acts at a single site (EC50, 60+/-14 microM, mean +/- S.E.M., n = 3) or that it acts at both high- (EC50, 35+/-9 microM, mean +/- S.E.M., n = 3) and low- (EC50 >2 mM) affinity sites. 4. Stimulation by 1 mM arsenite was greatest on the day of cell collection (rate about 3 times that of the control), even exceeding that produced by 20 nM calyculin A, and declined during cell storage. Addition of calyculin A to arsenite-stimulated cells resulted in further stimulation of Na+-K+-2Cl- cotransport, suggesting that arsenite and calyculin act synergistically. This was most apparent in stored cells. 5. Stimulation by 1 mM arsenite was not affected by treating cells with the mitogen-activated protein kinase inhibitors SB203580 (20 microM) and PD98059 (50 microM), but was both prevented and reversed by the kinase inhibitors staurosporine (2 microM), 4-amino-5-(4-methylphenyl)-7-(t-butyl)pyrazolo[3,4-d]pyrimidine (PP1, 50 microM) and genistein (0.3 mM), and with a combination of 10 microM A23187 and 2 mM EDTA (to reduce intracellular Mg2+ concentration). Only treatment with EDTA and A23187 prevented stimulation by the combination of 1 mM arsenite and 20 nM calyculin, whereas no treatment was able to fully reverse this stimulation once elicited. 6. Our data are consistent with arsenite stimulating (perhaps indirectly) a kinase that phosphorylates and activates the Na+-K+-2Cl- cotransporter.

Regulation of Na+-K+-2Cl- cotransport by protein phosphorylation in ferret erythrocytes

1. Na+-K+-2Cl- cotransport in ferret erythrocytes was measured as the bumetanide-sensitive uptake of 86Rb. 2. The resting cotransport rate was high but could be increased threefold by treating erythrocytes with calyculin A, a potent inhibitor of serine/threonine phosphatases. Twenty nanomolar was sufficient to maximally and rapidly (within 4 min) stimulate transport. 3. The effects of several kinase inhibitors were tested. High concentrations of K-252a, K-252b, calphostin C and hypericin caused less than 20 % inhibition. Staurosporine (IC50, 0.06 microM) and 4-amino-5-(4-methylphenyl)-7-(t-butyl)pyrazolo[3,4-d]pyrimidine (PP1; IC50, 2.5 microM) were more potent but still only partially (40-50 %) inhibited transport, an effect mimicked by reducing ionized intracellular Mg2+ concentration to submicromolar levels. Genistein may inhibit all transport at a sufficiently high dose (IC50, 0.36 mM) perhaps by directly inhibiting the transporter. 4. Staurosporine, PP1 and the removal of Mg2+ all prevented subsequent stimulation by calyculin A, and all inhibited calyculin-stimulated transport by 20-30 %. The effects of staurosporine, PP1 and Mg2+ removal were not additive. 5. The phosphatase that dephosphorylates the cotransporter is probably Mg2+ (or possibly Ca2+ or Mn2+) sensitive and not the target for calyculin A. The data suggest that this phosphatase is inhibited by phosphorylation, and that it is the regulation of this process which is affected by calyculin A and the kinase inhibitors tested here. Phosphorylation of the phosphatase is probably regulated by members of the Src family of tyrosine kinases.