EVIDENCE FOR THE EXISTENCE OF INOSITOL (1,4,5)-TRISPHOSPHATE-SENSITIVE AND RYANODINE-SENSITIVE POOLS IN BOVINE ENDOTHELIAL-CELLS - CA2+ RELEASES IN CELLS WITH DIFFERENT BASAL LEVEL OF INTRACELLULAR CA2+

In single bovine aortic endothelial (BAE) cells pre-loaded with Fura-2 , Ca2+ transients in a Ca2+-free medium have been revealed, which evid ently reflects Ca2+ release from intracellular stores. In cells with d ifferent levels of resting basal cytoplasmic Ca2+ ([Ca2+](i)) from abo ut 50 to 110 nM, a biphasic dependence of the Ca2+ transients on resti ng [Ca2+](i) was shown and spontaneous Ca2+ oscillations were observed . At a [Ca2+](i) level over 110 nM, a pronounced rise in Ca2+ transien ts occurred and only single transients were observed. Ryanodine (10 mu M) produced a transient [Ca2+](i) elevation, suggesting the presence of ryanodine receptors in intracellular store membranes. The results i mply that both inositol 1,4,5-trisphosphate- sensitive Ca2+ release (I ICR) and Ca2+-sensitive Ca2+ release (CICR) take place in BAE; cells. Only IICR seems to be sufficient for generating baseline Ca2+ oscillat ions in BAE cells, whereas the ATP-induced (5-100 mu M) Ca2+ response involves the CICR set in motion by an oscillatory IICR of high Frequen cy. The completion of both the spontaneous and ATP-induced Ca2+ transi ents was associated with a [Ca2+](i) decrease to a level below the ini tial resting [Ca2+](i) (undershoot), Its depth biphasically depended o n the resting [Ca2+](i) from 50 to 110 nM, suggesting that the lack of a Ca2+ leak from inositol 1,4,5-trisphosphate-sensitive stores is res ponsible for the undershoot in this range. The Ca2+ leak is concluded to play a key role in the initiation and termination of regenerative I ICR both in spontaneous oscillations and in ATP-induced transients.