THROMBIN-INDUCED MITOGENESIS IN CULTURED AORTIC SMOOTH-MUSCLE CELLS REQUIRES PROLONGED THROMBIN EXPOSURE

Thrombin has been implicated in vascular smooth muscle cell (VSMC) pro liferation after vessel injury. Its proliferative effects, which are m ediated via proteolytic activation of a receptor similar or identical to the cloned thrombin receptor (TR), have markedly delayed kinetics. The present study demonstrates that, despite rapid thrombin receptor a ctivation and similar time to S phase entry compared with classic poly peptide growth factors, prolonged thrombin exposure is required to pro mote maximal VSMC mitogenesis. Flow cytometric analysis of thrombin-st imulated cells revealed that thrombin induced a progressive increase i n the growth fraction over 3 days in culture, an effect that was block ed by hirudin even late after thrombin addition. Northern blot hybridi zation after thrombin stimulation demonstrated that thrombin upregulat es TR mRNA expression within 6 h. These findings indicate that VSMC pr oliferate in response to prolonged thrombin exposure and suggest that the mitogenic delay may involve not only the thrombin-dependent synthe sis and activation of newly made TR but also the progressive thrombin- dependent recruitment of cells into the growth fraction.