Role of tyrosine phosphatase in the modulation of pulmonary vascular tone

Abstract

In the vascular system, synthesis of the potent vasodilator nitric oxide (NO) is tightly regulated by the constitutively expressed endothelial NO synthase (eNOS). Activity of eNOS is controlled by Ca 2z /calmodulin and various seryl/threonyl protein kinases. Less is known about the importance of phosphorylation and dephosphorylation of tyrosyl residues. Therefore the role of tyrosine phosphatase on the modulation of isolated rat pulmonary artery tone has been assessed. Inhibition of tyrosine phosphatase by sodium orthovanadate (SOV, 1610 -6 M) significantly: 1) increased phenylephrine-induced vasoconstriction and 2) decreased endotheliumdependent relaxation to acetylcholine, but had no effect on endothelium-independent relaxation to the NO donor, sodium nitroprusside. In phenylephrine-precontracted pulmonary arterial rings, SOV (1610 -7 -1610 -5 M) had no effect on vascular tone but significantly relaxed rings which were pretreated with the NO-synthase inhibitor, N v -nitro-L-arginine-methyl ester (L-NAME). SOV-induced relaxation in the presence of L-NAME was, however, abolished by glibenclamide.In conclusion, inhibition of tyrosine phosphatase altered pulmonary vascular tone by increasing vasoconstrictor response to phenylephrine and decreasing endotheliumdependent relaxation to acetylcholine. Furthermore, the tyrosine phosphatase inhibitor, sodium orthovanadate, exhibited original vasodilator properties which were only observed when nitric oxide synthesis was inhibited. Thus a new pathway involving the inhibitory effect of nitric oxide on a glibenclamide-sensitive diffusible relaxing factor, that might play an important role in the control of pulmonary vascular tone is described. Pulmonary vascular tone is modulated by a variety of endothelium-derived relaxing factors including prostacyclin, nitric oxide (NO) [1,2], and the endothelium-derived hyperpolarizing factors (EDHF) [2,3]. NO is one of the most potent vasodilators known to date. Its synthesis is tightly regulated by intracellular free Ca 2z and the Ca 2z / calmodulin complex that, in turn, stimulate the constitutive isoform of endothelial NO synthase (eNOS). Alternatively, phosphorylation on seryl and threonyl residues by various seryl/threonyl kinases is also thought to modulate eNOS activity [4,5].Shear stress is the major physiological factor modulating pulmonary vascular tone in health and disease. The underlying mechanism most likely involves synthesis and release of NO from pulmonary endothelial cells. Recent evidence suggests that endothelial production of NO is increased in response to shear stress through tyrosine phosphorylation via a calcium-independent pathway in the systemic circulation [4,6]. Contradictory studies alternatively suggest that tyrosine phosphorylation of eNOS might also reduce its activity [7]. Whether or not alteration of phosphorylation and/or dephosphorylation of tyrosyl residues of eNOS affects its activity in the pulmonary artery has yet to be investigated.In various vascular beds, EDHF causes relaxatio...