Endothelin Antagonism Uncovers Insulin-Mediated Vasorelaxation In Vitro and In Vivo

<jats:p> The endothelial actions of insulin remain an area of intense research because they relate to both insulin sensitivity and vascular tone. Physiological doses of insulin evoke endothelium-dependent vasorelaxation in humans; however, this remains a pharmacological phenomenon in rat aortas. Because insulin may stimulate the divergent production of both nitric oxide and endothelin-1, we hypothesized that the lack of insulin-induced vasorelaxation at low/subthreshold concentrations may be due to the concurrent production of endothelin-1, which in turn serves to inhibit nitric oxide-dependent, insulin-mediated dilation. To investigate this, we studied the effects of subthreshold concentrations of insulin (100 mU/L) on norepinephrine-induced contraction in rat aortas following short-term and long-term endothelin blockade. In addition, the effects of tetrahydrobiopterin inhibition (with diaminohydroxyprimidine) on norepinephrine-induced contraction in the presence of insulin and endothelin receptor blockade were investigated. Subthreshold concentrations of insulin failed to evoke vasorelaxation in rat aortas. Strikingly, short-term endothelin A/B receptor blockade with bosentan (10 <jats:sup>−2</jats:sup> mmol/L) uncovered insulin-mediated dilation; the percent maximum contraction and sensitivity of aortas to norepinephrine were attenuated (% maximum relaxation: bosentan+insulin 74±4%* versus bosentan 92±3%, insulin 107±5% <jats:italic>P</jats:italic> <0.002; pD <jats:sub>2</jats:sub> values: bosentan+insulin 6.87±0.14* versus bosentan 7.40±0.15, insulin 7.63±0.11, * <jats:italic>P</jats:italic> <0.002). This effect was mediated through endothelin A receptors because bosentan and BQ-123 (10 <jats:sup>−2</jats:sup> mmol/L) attenuated norepinephrine-induced contraction to a similar degree. In addition, insulin evoked vasorelaxation in aortas isolated from rats after long-term bosentan treatment (100 mg · kg <jats:sup>−1</jats:sup> · d <jats:sup>−1</jats:sup> , 3 weeks). The component of insulin-mediated vasorelaxation uncovered by endothelin receptor blockade was tetrahydrobiopterin-dependent because it was reversed by diaminohydroxyprimidine. These data demonstrate, for the first time, the functional interaction between insulin, endothelin-1, and tetrahydrobiopterin in modulating vascular tone in rat aortas in vitro and in vivo. </jats:p>