We investigated the effect of heat shock on the fibrinolytic potential of human umbilical vein endothelial cells (HUVECs) in culture. When cultured at 43 degrees C, the mRNA for heat shock protein 70 (HSP70) was dramatically induced within 120 min with a maximal induction of more than 90-fold compared with that in HUVECs cultured at 37 degrees C. The level of urokinase-type plasminogen activator (u-PA) receptor (u-PAR) mRNA increased up to 2.2-fold in response to heat shock, which was associated with the increased u-PA binding and cell-surface u-PA activity determined by adding exogenous u-PA to acid-treated HUVECs. The increased u-PAR mRNA returned to normal level when HUVECs were further incubated at 37 degrees C for 180 min, and this decline was not affected in the presence of actinomycin D. Though the secreted antigens for tissue-type plasminogen activator (t-PA) and type 1 plasminogen activator inhibitor (PAI-1) in the conditioned medium (CM) of HUVECs were simultaneously increased at 43 degrees C during this period, the increase in the levels of t-PA (about 26.6-fold at 120 min) was greater than that of PAI-1 (1.8-fold at 120 min). The fibrinolytic activity of CM obtained from HUVECs at 43 degrees C was significantly enhanced up to 3-fold, indicating that heat shock induced hyperfibrinolytic states in HUVECs. The secretion of u-PA into CM was also enhanced by heat shock. These results suggested that human endothelial cells respond to hyperthermia by inducing HSP70 followed by hyperfibrinolytic states with the enhanced expression of u-PAR as well as that of t-PA and u-PA.