Synergism between the contractile effect of epidermal growth factor and that of des‐Arg9‐bradykinin or of α.‐thrombin in rabbit aortic rings

Abstract

1 Rabbit aortic rings were used to test the possible contractile effects of growth factors and their interaction with other stimuli. A rapid potentiation of kinin-induced contraction by epidermal growth factor (EGF) has been previously observed in this preparation. 2 EGF (5–1500 ng ml⁻¹) and the isoform BB of platelet-derived growth factor (PDGF-BB; 1–126 ng ml⁻¹) exerted modest but sustained contractile effects in rabbit aortic rings. 3 EGF pretreatment (100 ng ml⁻¹) potentiated the contractile responses to des-Arg⁹-bradykinin (des-Arg⁹-BK), an agonist of the B₁ receptors for kinin found in this preparation, and to human α.-thrombin but not to several other contractile stimuli. The interaction appeared also relatively selective for the growth factor, because PDGF-BB pretreatment potentiated neither des-Arg⁹-BK nor α.-thrombin-induced contraction. 4 EGF, applied on a contraction plateau induced by des-Arg⁹-BK or α.-thrombin, exerted a synergistic contractile effect, with a time course and a half-maximal concentration for EGF-induced contraction similar to the ones recorded in resting tissues (between 67 and 220 ng ml⁻¹, depending on the series of experiments). 5 The direct or synergistic contractile effects of EGF were not modified by the removal of the endothelium or by treatment with indomethacin. However, the tyrosine kinase inhibitors, erbstatin or genistein, inhibited the synergistic effect of EGF with des-Arg⁹-BK. The small direct contractile effect of EGF was significantly reduced by genistein. The synergistic effect of EGF with α.-thrombin was comparatively more resistant to the tested tyrosine kinase inhibitors. 6 An inhibitor of the catalytic activity of α.-thrombin, d-Phe-Pro-Arg-CH₂Cl, prevented the contractile effect of α.-thrombin in the aortic rings. In this system, a tetradecapeptide derived from a recently cloned α.-thrombin receptor was a contractile stimulus at and above 10 μm. Consistent with the hypothesis that this peptide could behave as an α.-thrombin receptor agonist, its contractile effect was potentiated by EGF pretreatment. Pharmacological evidence was provided to show that the receptors for α.-thrombin were distinct from the B₁ receptors for kinins. Together, these findings suggest that a model of a cleavable receptor recently elaborated to account for α.-thrombin effects on human platelets is valid in blood-free vascular smooth muscle preparations such as the rabbit isolated aorta. 7 The synergism between EGF and kinin- or α.-thrombin-induced contractions constitutes a novel mode of myotropic action for growth factors. The synergism is probably dependent on the tyrosine kinase activity of receptors for EGF. These combinations of stimuli could occur in various types of vascular disease and account for abnormal vascular reactivity often associated with atheroma lesions or vascular wound healing.