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

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Abstract
Na+-K+-2Cl− cotransport activity was measured in ferret erythrocytes as the bumetanide-sensitive uptake of 86Rb. 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. 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 μM, mean ± s.e.m., n = 3) or that it acts at both high- (EC50, 35 ± 9 μM, mean ± s.e.m., n = 3) and low- (EC50 > 2 mM) affinity sites. 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. Stimulation by 1 mM arsenite was not affected by treating cells with the mitogen-activated protein kinase inhibitors SB203580 (20 μM) and PD98059 (50 μM), but was both prevented and reversed by the kinase inhibitors staurosporine (2 μM), 4-amino-5-(4-methylphenyl)-7-(t-butyl)pyrazolo[3,4-d]pyrimidine (PP1, 50 μM) and genistein (0.3 mM), and with a combination of 10 μM 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. Our data are consistent with arsenite stimulating (perhaps indirectly) a kinase that phosphorylates and activates the Na+-K+-2Cl− cotransporter.

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