INHIBITION OF NA(+) K+ ATPASE UNDER HYPERTONIC CONDITIONS IN RAT MAST-CELLS/

Ionic fluxes that contribute to changes in membrane potential and vari ations of pHi (intracellular pH) are not well known in mast cells, alt hough they can be important in the stimulus-secretion coupling. Cellul ar volume regulation implies changes in the concentration of intracell ular ions, such as sodium and potassium and volume changes can be impo sed varying the tonicity of the medium. We studied the physiology of s odium and examined the effect of ouabain on [Na-22] entry in mast cell s in isotonic and hypertonic media. We also recorded changes in membra ne potential and pHi using the fluorescent dyes bis-oxonol (Bis-(1,3-d iethylthiobarbituric acid) trimethine oxonol) and BCECF (2',7'-bis(car boxyethyl)-5(6)-carboxyfluorescein acetoxymethyl ester) in hypertonic conditions. The results show that [Na-22] influx increases four fold i n hypertonic solutions and it is mediated mainly by an amiloride-sensi tive Na+/H+ exchanger. This transporter is involved in the shrinkage-a ctivated cellular alkalinization and the pHi recovery is accelerated b y inhibition of the Na+/K+ ATPase with ouabain in the absence of extra cellular calcium. Under hypertonic conditions Na-22 influx is apparent ly not increased by ouabain, while the Na+/K+ ATPase inhibitor clearly increases [Na-22] uptake and also induces membrane depolarization in isotonic conditions. All together, these findings suggest that Na+/KATPase is partially inhibited in hypertonic conditions.