| 12756484 |
Gusev GP, Ivanova TI: Activation of Na+/H+ exchange by protein phosphatase inhibitors in red blood cells of the frog Rana ridibunda. J Comp Physiol B. 2003 Jul;173(5):429-35. Epub 2003 May 20.
The present study was designed to evaluate the role of protein phosphatases in regulation of sodium transport in the marsh frog erythrocytes using 22Na as a tracer. For this purpose the cells were treated with several known inhibitors of protein phosphatases. In standard isotonic medium, exposure of the cells to 10 mmol l (-1) NaF, 20 nmol l (-1) calyculin A or 0.1 mmol l (-1) cantharidin resulted in a significant (1.7-fold) increase in unidirectional ouabain-insensitive Na+ influx. The Na+ influx in frog red cells was progressively activated as the medium osmolality was increased by addition of 100, 200 or 300 mmol l (-1) sucrose to standard isotonic medium. The stimulatory effect of protein phosphatase blockers on Na+ influx was much higher in hypertonic medium containing 100 or 200 mmol l (-1) sucrose than that in isotonic medium. Stimulation of Na+ transport enhanced with increasing concentrations of calyculin A, and half-maximal activation (EC50) was obtained at 16 nmol l (-1). However, Na+ influx induced by strong hypertonic treatment (+300 mmol l (-1) sucrose) was not altered further in the presence of protein phosphatase inhibitors. The changes in Na+ influx evoked by protein phosphatase inhibitors and hypertonic treatment were associated with a rise in the intracellular Na+, but not K+, content. Enhancement in Na+ influx after addition of protein phosphatase blockers to cell suspension in isotonic or hypertonic media was almost completely inhibited by Na+/H+ exchange inhibitors, amiloride and ethyl-isopropyl-amiloride. The basal Na+ influx in frog erythrocytes in isotonic medium was relatively low (1.7 mmol/l cells/h) and not affected by 1 mmol l (-1) amiloride. Thus, the data obtained clearly indicate that Na+/H+ exchanger in the marsh frog red blood cells is under tight regulatory control, in all likelihood via protein phosphatases of types PP-1 and PP-2A. |
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