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Comp Biochem Physiol B Biochem Mol Biol. 2006 Sep;145(1):60-7. Epub 2006 Jun 21.

Activation of sodium transport in rat erythrocytes by inhibition of protein phosphatases 1 and 2A.

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  • 1Laboratory of Comparative Biochemistry of Inorganic Ions, Sechenov Institute of Evolutionary Physiology and Biochemistry, Russian Academy of Sciences, 44 Thorez pr., 194223, St. Petersburg, Russia.


Four structurally different protein phosphatases (PPs) inhibitors - fluoride, calyculin A, okadaic acid and cantharidin--were tested for their ability to modulate unidirectional Na(+) influx in rat red blood cells. Erythrocytes were incubated at 37 degrees C in isotonic and hypertonic media containing 1 mM ouabain and (22)Na in the absence or presence of PP inhibitors. Exposure of the cells to 20 mM fluoride or 50 nM calyculin A for 1 h under isosmotic conditions caused a significant stimulation of Na(+) influx, whereas addition of 200 microM cantharidin or 100 nM okadaic acid had no effect. After 2 h of treatment, however, all these PPs blockers significantly enhanced Na(+) transport in rat erythrocytes. Selective inhibitors of PP-1 and PP-2A types, calyculin A, cantharidin and okadaic acid, produced similar ( approximately 1.2-1.4-fold) stimulatory effects on Na(+) influx in the cells. Activation of Na(+) influx was unchanged with increasing calyculin A concentration from 50 to 200 nM. No additive stimulation of Na(+) influx was observed when the cells were treated with combination of 20 mM fluoride and 50 nM calyculin A. Na(+) influx induced by PPs blockers was inhibited by 1 mM amiloride and 200 muM bumetanide approximately in the equal extent, indicating the involvement of Na(+)/H(+) exchange and Na-K-2Cl cotransport in sodium transport through rat erythrocytes membrane. Activation of Na(+) transport in the cells induced by calyculin A and fluoride was associated with increase of intracellular Na(+) content. Shrinkage of the rat erythrocytes resulted in 2-fold activation of Na(+) influx. All tested PPs inhibitors additionally activated the Na(+) influx by 70-100% above basal shrinkage-induced level. Amiloride and bumetanide have diminished both the shrinkage-induced and PPs-inhibitors-induced Na(+) influxes. Thus, our observations clearly indicate that activities of Na(+)/H(+) exchanger and Na-K-2Cl cotransporter in rat erythrocytes are regulated by protein phosphatases and stimulated when protein dephosphorylation is inhibited.

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