Decrease in Ca2+ sensitivity as a mechanism of hydrogen peroxide-induced relaxation of rabbit aorta.

OBJECTIVE

In vascular strips, hydrogen peroxide (H2O2) relaxes alpha 1-adrenergic agonist-induced but not high-K(+)-induced contractions. The aim of this study was to explore H2O2-induced changes in [Ca2+]i of vascular smooth muscle and to elucidate the mechanisms of action of H2O2.

METHODS

Isolated rabbit aortic strips were isometrically contracted with high-K+ (64.7 mM) or phenylephrine (PE, 0.3 microM). The effects of 300 microM H2O2 on [Ca2+]i of endothelium-denuded vascular smooth muscle and tension were determined simultaneously by the fura-2 method. Changes in [Ca2+]i were expressed as percentages of high-K(+)-induced values measured at the beginning of the experiments. In another series of experiments, the relaxant effect of 300 microM H2O2 was examined in high-K+ (20 mM)-induced contraction in the presence of the protein kinase C activator, phorbol 12,13-dibutyrate (PDBu).

RESULTS

Hydrogen peroxide caused a reversible rise in [Ca2+]i of vascular smooth muscle under both resting conditions and in the precontracted state. During high-K(+)-induced contraction, H2O2 further increased [Ca2+]i by 26.6(s.e.m. 1.7)% accompanied by a small increase in tension of 6.5(1.9)% of high-K(+)-induced tension. By contrast, during PE-induced contraction, although H2O2 caused a comparable additional increase in [Ca2+]i (26.4(4.7)%), muscle tension fell by 28.9(2.2)% of the steady-state PE-induced tension. Hydrogen peroxide had a relaxant effect on augmented high-K(+)-induced contraction in which Ca2+ sensitivity of the contractile apparatus was elevated by PDBu.

CONCLUSIONS

In spite of its effect of increasing [Ca2+]i of vascular smooth muscle, hydrogen peroxide causes relaxation of endothelium-denuded, PE-precontracted rabbit aorta. The mechanism is probably through suppression of agonist-induced augmentation of Ca2+ sensitivity of the contractile apparatus.