REFILLING STATE OF INTERNAL CA2+ STORES IS NOT THE ONLY INTRACELLULAR SIGNAL STIMULATING CA2+ INFLUX IN HUMAN ENDOTHELIAL-CELLS

To further analyse the role of the refilling state of internal Ca2+ pools in the stimulation of Ca2+ influx in human endothelial cells, we investigated the combined effect of thapsigargin (TG) and histamine on cytosolic Ca2+ concentration ([Ca2+](i)) and inositol polyphosphate production. At normal extracellular Ca2+ levels, TG induced a progressive and sustained elevation in [Ca2+](i) which was dose-dependently prevented by pretreatment with 1-10 mu M histamine. Similarly, pretreatment with 0.1 and 1 mu M TG suppressed histamine-induced Ca2+ transients partially and totally, respectively. TG pretreatment did not alter the inositol triphosphate (IP3) level liberated by histamine, but modified IP3 metabolism by decreasing inositol biphosphate (IP2) and increasing inositol monophosphate (IP1) contents. In the absence of Ca2+ influx, 1 mu M TG only induced a small transient increase in [Ca2+](i) whereas the Ca2+ mobilization evoked by 10 mu M histamine was unchanged. In both cases, the absence of any additional effect of either TG, histamine or 2 mu M ionomycin indicated the complete depletion of Ca2+ stores. The re-establishment of the transmembrane Ca2+ gradient induced a transient rise in [Ca2+](i). Its amplitude differed between histamine- and TG-treated cells. It was imposed by cell pretreatment and was selectively affected by changes in the membrane potential. At 5 mM external K+, the transient rise in [Ca2+](i) was more marked in histamine- than in TG-stimulated cells; this difference was suppressed by TG pretreatment. The presence of 130 mM external K+ increased Ca2+ entry in TG-treated cells but reduced it in histamine-stimulated cells. These results indicate that the refilling state of internal Ca2+ stores does not constitute the single regulator of Ca2+ influx. TG and histamine seem to activate Ca2+ influx through distinct but interdependent pathways regulated by membrane potential.