Serotonin activates the mitogen-activated protein kinase pathway in vascular smooth muscle: use of the mitogen-activated protein kinase kinase inhibitor PD098059.

Serotonin (5-hydroxytryptamine, 5-HT) promotes changes in vascular smooth muscle contractility and is a vascular smooth muscle mitogen. The hypothesis that 5-HT-induced arterial contraction is partially dependent on tyrosine kinase activation was tested in this study. Specifically, we examined the role of the tyrosine kinase mitogen-activated protein kinase kinase in 5-HT-induced vascular contraction by using a novel inhibitor of mitogen-activated protein kinase kinase PD098059. Helical strips of rat aorta, mesenteric and tail artery denuded of endothelium were mounted in tissue baths for measurement of isometric contractile force. 5-HT-induced contraction in all arteries was mediated by 5-HT2A receptors as ketanserin (3-30 nM) was a competitive antagonist in all arteries (pKB = 8.58-8.96). Genistein (5 x 10(-6) M) and tyrphostinB42 (3 x 10(-5) M), two unrelated tyrosine kinase inhibitors, shifted 5-HT-induced contraction toward the right in all arteries (approximately eight-fold). By contrast, daidzein (5 x 10(-6) M), the inactive isomer of genistein, did not reduce contraction to 5-HT in any artery. These findings suggest that 5-HT-induced activation of tyrosine kinase(s) may be a signal transduction pathway used by vascular 5-HT2A receptors. PD098059 (1 x 10(-5) M) reduced contraction to 5-HT in all arteries, indicating that 1) the tyrosine kinase(s) inhibited by genistein and tyrphostin42 probably include mitogen-activated protein kinase kinase and 2) activation of MEK is important for 5-HT-induced contraction. Western analyses of aortic strips contracted to 5-HT (10(-5) M) indicates that tyrosyl-phosphorylation of the mitogen activated protein kinases Erk-1 and Erk-2 was increased (approximately 200% and 15% of basal levels, respectively) and PD098059 (1 x 10(-5) M) significantly reduced 5-HT-stimulated tyrosyl-phosphorylation of Erk-1. Thus, these data support the use of the mitogen-activated protein kinase pathway by 5-HT in causing arterial contraction and demonstrates the usefulness of specific inhibition of the mitogen-activated protein kinase pathway.