Inhibition of Ca2+-induced relaxation by oxidized tungsten wires and paratungstate.

Recent studies of rat mesenteric arteries using a wire myograph detected decreased Ca2+ and acetylcholine-induced relaxation responses. Preliminary experiments indicated the reduced responses were associated with the tungsten wire used in the myograph system. Compared with earlier observations, arteries mounted on aged 28-microm tungsten wire showed decreased maximal Ca2+-induced relaxation responses of arteries precontracted with phenylephrine (91.9 +/- 1.5 versus 54.8 +/- 4.5%, p < 0.001) and reduced sensitivity to Ca2+ (ED50 = 1.65 +/- 0.07 versus 4.58 +/- 0.16 mM, p < 0.001). Similar shifts were seen for acetylcholine. When the surface of the wire was cleaned by abrasion with fine sandpaper, both the ED50 for Ca2+ and maximal relaxation significantly improved. An enhanced sensitivity to Ca2+ was also seen when arteries were mounted on newly purchased 14-microm tungsten or 14-microm 24K gold wire with the rank order: 14-microm gold > 14-microm tungsten >> 28-microm aged tungsten wire. Laser Raman spectral analysis of the aged 28-microm tungsten wire showed that the surface was in an oxidized state that shared spectral characteristics with the paratungstate [W12O42](-12) anion. The effect of the paratungstate anion on arterial relaxation was therefore tested. Paratungstate, but not the structurally dissimilar tungstate and metatungstate anions, significantly reduced the sensitivity and magnitude of relaxation induced by Ca2+ and to a lesser extent, relaxation induced by acetylcholine. To learn whether paratungstate inhibits relaxation through the generation of oxygen radicals, the effect of the superoxide dismutase mimetic 4-hydroxy-2,2,6,6-tetramethylpiperidine 1-oxyl (1 mM) was assessed and found to have no effect. Since Ca2+-induced relaxation is inhibited by iberiotoxin, the effect of paratungstate on K+ channel activity was assessed. Paratungstate had no effect on currents through large conductance, Ca2+-activated K+ channels in whole-cell recordings from vascular smooth muscle cells, ruling out an action at the BK(Ca) channel. We conclude that: 1) surface oxidation of tungsten wire commonly used in wire myography significantly and adversely affects vascular responses to vasodilator compounds, 2) the effect is likely mediated by the paratungstate anion, and 3) the effects of the anion are not associated with free radical generation or K+ channel inhibition.