Calcium signaling in endothelia: cellular heterogeneity and receptor internalization

The vasoactive factors thrombin, bradykinin (BK), and ATP are released in response to tissue damage and inflammation and act on endothelium to modulate vascular perfusion. We have investigated the second messenger response of endothelium activated by these agonists and, in particular, the mechanism of desensitization to BK. Fura-2 fluorescence ratio imaging of calf pulmonary artery endothelial cells (CPAE) revealed 5- to 10-fold increases on intracellular Ca (Cai) in response to these agents. Maximal doses caused Cai to increase from 52 to 248 nM (thrombin), 556 nM (BK), and 643 nM (ATP). Agonists elicited a rapid (within 30 s) increase of Cai due to release of Ca from intracellular stores followed by a secondary elevation of Cai dependent on entry of external Ca. The temporal characteristics of the Cai responses to all agonists were heterogeneous from cell to cell, and, interestingly, repeated stimulation gave identical signature responses from individual cells, although the amplitude of the Cai response decreased to thrombin and especially bradykinin but not for ATP. This decrease was agonist specific because ATP elicited large increases of Cai after thrombin or BK desensitization. Maximal desensitization was obtained with BK applied for 5-10 min followed by a rest of