FILLING STATE OF INTRACELLULAR CA2+ POOLS TRIGGERS TRANS PLASMA-MEMBRANE NA+ AND CA2+ INFLUX BY A TYROSINE KINASE-DEPENDENT PATHWAY

The relations between the filling state of intracellular calcium stores that are regulated by the endoplasmic Ca2+-ATPase and trans plasma membrane sodium and calcium influx were investigated. The effects of specific inhibition of endoplasmic Ca2+-ATPase by thapsigargin, cyclopiazonic acid, and 2,5 di-(tert-butyl)-1,4-benzohydroquinone (BHQ) on cytosolic free sodium concentration ([Na+](i)) and cytosolic free calcium concentration ([Ca2+](i)) were evaluated in lymphocytes from healthy subjects using the fluorescent dyes sodium-binding benzofuran isophthalate and fura2. The specific inhibition of endoplasmic Ca2+-ATPase by thapsigargin, cyclopia tonic acid, or BHQ increased lymphocytic [Na+](i) and [Ca2+](i). The thapsigargin-induced [Na+](i) increase was abolished in the absence of external sodium, indicating that thapsigargin induced a trans plasma membrane sodium influx. In the absence of external calcium the thapsigargin-induced [Ca2+](i) increase was significantly reduced, whereas the thapsigargin induced [Na+](i) increase remained the same. This finding indicates that the filling state of intracellular calcium pools rather than the elevation of [Ca2+](i) per se regulates the plasma membrane permeability for sodium in lymphocytes. The inhibition of the tyrosine kinase by genistein inhibited the thapsigargin-induced increases of both [Na+](i) and [Ca2+](i) in lymphocytes. The present study shows that the filling state of intracellular thapsigargin-sensitive calcium pools regulates trans plasma membrane sodium and calcium influx via a tyrosine kinase-dependent pathway.