Thrombin and histamine activate phospholipase C in human endothelial cells via a phorbol ester‐sensitive pathway

The effects of phorbol esters and synthetic diglycerides on thrombin‐ and histamine‐stimulated increases in inositol trisphosphate (IP 3 ) and cytosolic free calcium ([Ca 2+ ] i ) were studied in cultured human umbilical vein endothelial cells (HEC). Thrombin (0.003–3.0 U/ml) and histamine (10 −7 –10 −4 M) induced rapid increases in [Ca 2+ ] i in suspended cells as monitored with the fluorescent calcium indicator fura‐2. In [ 3 H]myoinositol‐labeled cells, both thrombin (3 U/ml)‐ and histamine (10 −4 M)‐induced IP 3 increases (195% ± 6% and 98% ± 4%, respectively) occurred in less than 15 sec and were temporally correlated with [Ca 2+ ] i increases. Brief incubations (5–60 min) with different protein kinase C activators [4‐β‐phorbol 12‐myristate 13‐acetate (1–100 nM), mezerein (100 nM), and sn ‐1,2 dioctanoylglycerol (0.1–10 μM)] attenuated agonist‐induced increases in [Ca 2+ ] i . These compounds also inhibited thrombin‐ and histaminestimulated IP 3 formation, thus suggesting a tight coupling between phospholipase C activation and calcium flux in cultured HEC. Overall, these observations suggest that the pathway linking receptors to phospholipase C stimulation in human endothelial cells is sensitive to protein kinase C activation.