Abstract

In order to explore the mechanism of action of vanadyl sulfate (VOSO₄), previously described as an antidiabetic and antihypertensive agent, we have investigated the role of calcium and tyrosine phosphorylation in the contractile responses of rat aorta or skinned rabbit mesenteric artery rings. VOSO₄ induced a concentration-dependent contraction of aorta (pD₂ = 3.2), which was potentiated by endothelium removal (pD₂ = 4.2). After a first exposure to VOSO₄, no change in responsiveness was observed even though high vanadium concentrations had accumulated in the aortic tissue (≈4 × 10⁻³M). VOSO₄ induced, in calcium-free medium, a significant response that, relative to contractions measured in Krebs-Henseleit buffer, was higher (36%) than norepinephrine (16%)-, arginine-vasopressin (8%)- or KCl (5%)-induced responses. 8-(N,N-diethylamino)octyl 3,4,5-trimethoxybenzoate hydrochloride (TMB-8), an intracellular calcium release inhibitor, did not modify VOSO₄-induced response either in the presence or in the absence of ambient calcium. On skinned preparations, VOSO₄antagonized Ca⁺⁺-induced contraction. The tyrosine kinase inhibitors tyrphostin 23 (T₂₃) and tyrphostin 47 (T₄₇) potentiated by 4- and 14-fold, respectively, the activity of VOSO₄, in contrast to the lack of effect of T₄₇ on pervanadate-induced contraction. When phosphotyrosine content was revealed by Western blotting, VOSO₄ had no effect alone, but in the presence of T₄₇, it dramatically increased the phosphotyrosine content. This result contrasts again with PV-induced tyrosine phosphorylation, which was blocked by T₄₇. These data suggest that the signaling events involved in vascular effects of VOSO₄, although they depend little on calcium mobilization, are related to tyrosine phosphorylation, likewise through a pathway different from that of pervanadate.