Thrombin-induced mitogenesis in cultured rat aortic smooth muscle cells requires prolonged thrombin exposure. Bachhuber, B. G., I. J. Sarembock, L. W. Gimple, C. A. McNamara, and G. K. Owens. Department of Medicine and Physiology, University of Virginia, Charlottesville, VA 22908.
APStracts 2:0007C, 1995.
Thrombin has been implicated in vascular smooth muscle cell (VSMC) proliferation following vessel injury. Its proliferative effects, which are mediated via proteolytic activation of a receptor similar or identical to the cloned thrombin receptor (TR), have markedly delayed kinetics. The present studies demonstrate that despite rapid thrombin receptor activation, and similar time to S-phase entry as compared to classic polypeptide growth factors, prolonged thrombin exposure is required to promote maximal VSMC mitogenesis. Flow cytometric analysis of thrombin stimulated cells revealed that thrombin induced a progressive increase in the growth fraction over 3 days in culture; an effect that was blocked by hirudin even late after thrombin addition. Northern blot hybridization after thrombin stimulation demonstrated that thrombin upregulates TR mRNA expression within 6 hours. These findings indicate that VSMC proliferate in response to prolonged thrombin exposure and suggest that the mitogenic delay may involve not only the thrombin dependent synthesis and activation of newly made TR, but also the progressive thrombin-dependent recruitment of cells into the growth fraction. 

Received 15 June 1994; accepted in final form 8 November 1994
APS Manuscript Number C0341-4.
Article publication pending Am. J. Physiol. (Cell Physiology).
ISSN 1080-4757 Copyright 1994 The American Physiological Society.
Published in APStracts on 27 February 1995.