Bradykinin is a nine amino acid peptide of the kinin family believed to play a role in pain mediation and in the regulation of blood pressure, fluid balance and smooth muscle contraction. Here we demonstrate that bradykinin is also a potent mitogen for a mutant Ha-ras-transfected cell line, Rat 13. The Rat 13 cells display two binding sites for bradykinin: a moderate number (52,000) of high affinity sites (Kd = 4.9 nM) co-exist with a much smaller number (1100) of very high-affinity sites (Kd = 2.7 pM). Ligand binding stimulates mitogenesis through the lower affinity receptors, which are classified as B2. These receptors are down-regulated in response to ligand. In contrast, Rat 1 cells (the cell line from which Rat 13 was derived) have only 4000 receptors per cell in total and respond weakly to bradykinin as a mitogen. Thus, expression of a mutant ras protein in Rat 13 cells increases their expression of the bradykinin receptor and their sensitivity to ligand stimulation of mitogenesis. Additional binding studies demonstrate that human A431 epithelial cells and Swiss mouse 3T3 fibroblasts also possess high-affinity sites for bradykinin.

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