We have previously demonstrated that elevating intraluminal pressure from 90 to 140 mm Hg in isolated mesenteric arteries increases the expression of proto-oncogenes. These proto-oncogenes encode nuclear transcription factors that regulate the expression of target genes during various stages of the cell cycle. Thus, pressure-induced proto-oncogene expression may represent a mechanism by which pressure can induce growth and/or proliferation of vascular smooth muscle. The purpose of this study was to determine the intracellular signals that contribute to the pressure-induced increase in c-fos expression. Small mesenteric arteries were isolated from male Wistar rats and transferred to a dual-vessel chamber. The arteries were cannulated and slowly equilibrated to initial conditions (90 mm Hg, 37 degrees C) while being continuously superfused with a HEPES-bicarbonate-buffered Krebs' solution. After the equilibration period, the intraluminal pressure in 1 artery was increased to 140 mm Hg for 1 hour. In experiments designed to determine the intracellular signals involved in the pressure-induced increase in c-fos expression, specific inhibitors were introduced to the superfusate reservoir of both arteries before the pressure increase. The arteries were then fixed in phosphate-buffered formalin and embedded in paraffin blocks. Sections of paraffin-embedded arteries were fixed on slides, and the expression of c-fos was determined by in situ hybridization with the use of (35)S-labeled riboprobes. The pressure-induced expression of c-fos was not inhibited by nitrendipine (10 micromol/L), a calcium-free Krebs' solution containing EGTA (1 to 2 mmol/L), calphostin C (0.1 micromol/L), or cytochalasin D (0.4 micromol/L) but was inhibited by genistein (30 micromol/L). The results suggest that activation of a tyrosine kinase is required for pressure-induced c-fos expression, but the signaling pathway does not require extracellular calcium entry, intact actin filaments, or protein kinase C. As we have shown previously, the expression of c-fos correlated with wall stress.