Abstract

Introduction

Materials & Methods

Results

Discussion & Conclusion

References

Discussion
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We have demonstrated (1) hemolysate produced concentration-dependent contractions in rabbit basilar artery and pre-incubation with MAPK kinase inhibitor PD PD-98059 abolished the contraction to hemolysate. (2) PD 98059 relaxed significantly sustained contraction of rabbit basilar arteries induced by hemolysate. (3) Hemolysate enhanced MAPK phosphorylation in rabbit basilar artery. The initial effect on MAPK was observed at 1 min and the peak response obtained at 5 min after exposure to hemolysate. The effect of hemolysate on MAPK lasted up to 2 hours. (4) PD PD-98059 abolished the effect of hemolysate on MAPK phosphorylation.Mitogen-activated protein kinase (MAPK)

Mammalian cells respond to external stimuli by activation of a variety of signal transduction pathways and result in proliferation, hypertrophy, differentiation or apoptosis. The cellular kinase cascades that mediate these responses to the stimuli are collectively called the mitogen-activated protein kinase (MAPK) cascades.(8) Three MAPK cascades have been characterized in mammalian cells including vascular smooth muscle cells.(1) The extracellular signal-regulated kinase (ERK) cascade is the most established and characterized pathways responding to most growth factors such as EGF, PDGF, endothelin, vasopressin or angiotensin II.(4,8) The ERK cascade is critical to the mitogenic response to cellular differentiation and hypertrophy. (2) The other two MAPK cascades are activated by cellular stress. One of these cascades is called the P54 p54 MAPK or the SAPK/JNK (Stress-activated protein kinase/c-jun N-terminal kinase) pathway. This pathway is activated by oxidant stress, reperfusion of ischemic tissue, cell stretch, shear stress or exposure to inflammatory cytokines or vascular peptides such as endothelin.(12,25) The third MAPK or the second stress-response MAPK cascade is p38. p38 is activated by similar stimuli that activate the SAPK/JNK pathway but transduce growth inhibitory or apoptotic signals.(8,12,25) All three MAPK activate transcription factors and regulate gene expression for biological responses. MAPK is activated by growth factors such as EGF or PDGF. These growth factor receptor agonists cause dimerization of the receptors and stimulate their tyrosine kinase activities. This allows the transautophosphorylation of specific tyrosine residues in the intracellular domains of these receptors.(26) The phosphotyrosine and its neighboring C-terminal residues are recognized by the Src-homology 2 (SH2) domains in the adapter protein Grbs. Formation of the receptor-Grb2-mSOS complex promotes activation of Ras and Raf-1. (8,26) In some cases, activation of Ras needs a second adapter protein such as Shc family. Shc binds to phosphotyrosine sequences in activated receptors and then phosphorylates a tyrosine residue in its own central domain. This phosphorylation site is recognized by Grb2 SH2 domains. (26) An increasing number of G-protein coupled receptor (GPCR) agonists have been shown to increase tyrosine phosphorylation of Shc and association of Shc with Grb2/mSOS. These agonists include Angiotensin II, endothelin, thrombin, bradykinin and carbachol. (8,26)

Besides regulation of cell growth and differentiation, MAPK is involved in the regulation of smooth muscle contraction by phosphorylating thin filament associated proteins such as caldesmon.(5) Two isoforms 42 and 44 kDa are the most well studied MAPK and they are activated by dual phosphorylations of the threonine and tyrosine residues.(11) Both 42 and 44 kDa were shown presented in vascular smooth-muscle cells.(27) Membrane depolarization and agonist activation increased MAPK activity in swine carotid artery.(13) MAPK activity was enhanced within 0.5-1 minutes, reached maximal level within 2 minutes and maintained high level of activity for at least 30 minutes.(7,13) We have observed similar results in rabbit basilar artery that MAPK activation was observed within 1 minutes and reached the peak level by 5 minutes. The prolonged activity of MAPK for up to 2 hours after exposure to hemolysate indicates MAPK may play an important role in cerebral vasospasm, a prolonged vasoconstriction. PKC activation has been demonstrated to be involved in cerebral vasospasm.(20,21,24) Activation of PKC may lead to the activation of MAPK.(14,16) PKC inhibitor staurosporine abolished vasopressin-induced MAPK activity in rat aortic smooth muscle cells.(16) In addition, activation of MAPK may regulate smooth muscle contractility by phosphorylating thin filament associated proteins such as caldesmon. Caldesmon has been shown to be involved in prolonged vasoconstriction such as in cerebral vasospasm.(9) Activation of PKC and elevation of intracellular Ca2+ may activate protein tyrosine kinases. Tyrosine kinase such as Pyk2 has significant homology with the focal adhesion kinase, pp125FAK. Hemolysate activated pp125FAK in cultured endothelial and smooth muscle cells.(19) Genistein and tyrphostin, two tyrosine kinase inhibitors, reduced hemolysate-induced Ca2+ elevation in cultured endothelial cells (19) and hemolysate-induced contraction of rabbit basilar artery.(15) Activation of tyrosine kinase may stimulate Ras and Raf-1 and lead to MAPK activation.(10) PD-98059 inhibits MEK activation by Raf-1 and thus inhibits MAPK.(3) Indeed, pre-incubation of tissues with PD-98059 in this study abolished MAPK phosphorylation and the contractile response of rabbit basilar artery to hemolysate. However, when tissues were pre-contracted with hemolysate (or when MAPK was activated), PD-98059 produced only partial relaxation.

Cerebral vasospasm is probably a process of prolonged vascular contraction and vascular wall proliferation. Etiological factors and their signal transduction pathways may need to cover both contraction and proliferation of cerebral arteries. MAPK is involved in cell proliferation and in smooth muscle contraction. Hemolysate may produce contraction in rabbit basilar artery by activation of MAPK. This study raises a possibility that MAPK may be involved in the signal transduction in cerebral vasospasm and inhibition of MAPK may open a new avenue in the management of cerebral vasospasm.

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