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

The relations between the filling state of intracellular calcium store s that are regulated by the endoplasmic Ca2+-ATPase and trans plasma m embrane sodium and calcium influx were investigated. The effects of sp ecific inhibition of endoplasmic Ca2+-ATPase by thapsigargin, cyclopia zonic acid, and 2,5 di-(tert-butyl)-1,4-benzohydroquinone (BHQ) on cyt osolic 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 isophtha late 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 aboli shed in the absence of external sodium, indicating that thapsigargin i nduced a trans plasma membrane sodium influx. In the absence of extern al calcium the thapsigargin-induced [Ca2+](i) increase was significant ly reduced, whereas the thapsigargin induced [Na+](i) increase remaine d the same. This finding indicates that the filling state of intracell ular calcium pools rather than the elevation of [Ca2+](i) per se regul ates the plasma membrane permeability for sodium in lymphocytes. The i nhibition of the tyrosine kinase by genistein inhibited the thapsigarg in-induced increases of both [Na+](i) and [Ca2+](i) in lymphocytes. Th e present study shows that the filling state of intracellular thapsiga rgin-sensitive calcium pools regulates trans plasma membrane sodium an d calcium influx via a tyrosine kinase-dependent pathway.