Increased vascular thromboxane generation impairs dilation of skeletal muscle arterioles of obese Zucker rats with reduced oxygen tension

Abstract

This study determined if altered vascular prostacyclin (PGI(2)) and/or thromboxane A(2) (TxA(2)) production with reduced Po(2) contributes to impaired hypoxic dilation of skeletal muscle resistance arterioles of obese Zucker rats (OZRs) versus lean Zucker rats (LZRs). Mechanical responses were assessed in isolated gracilis muscle arterioles following reductions in Po(2) under control conditions and following pharmacological interventions inhibiting arachidonic acid metabolism and nitric oxide synthase and alleviating elevated vascular oxidant stress. The production of arachidonic acid metabolites was assessed using pooled arteries from OZRs and LZRs in response to reduced Po(2). Hypoxic dilation, endothelium-dependent in both strains, was attenuated in OZRs versus LZRs. Nitric oxide synthase inhibition had no significant impact on hypoxic dilation in either strain. Cyclooxygenase inhibition dramatically reduced hypoxic dilation in LZRs and abolished responses in OZRs. Treatment of arterioles from OZRs with polyethylene glycol-superoxide dismutase improved hypoxic dilation, and this improvement was entirely cyclooxygenase dependent. Vascular PGI(2) production with reduced Po(2) was similar between strains, although TxA(2) production was increased in OZRs, a difference that was attenuated by treatment of vessels from OZRs with polyethylene glycol-superoxide dismutase. Both blockade of PGH(2)/TxA(2) receptors and inhibition of thromboxane synthase increased hypoxic dilation in OZR arterioles. These results suggest that a contributing mechanism underlying impaired hypoxic dilation of skeletal muscle arterioles of OZRs may be an increased vascular production of TxA(2), which competes against the vasodilator influences of PGI(2). These results also suggest that the elevated vascular oxidant stress inherent in metabolic syndrome may contribute to the increased vascular TxA(2) production and may blunt vascular sensitivity to PGI(2).

Fig. 1.

Dilator reactivity of isolated skeletal muscle resistance arterioles from lean Zucker rats (LZRs) and obese Zucker rats (OZRs) in response to acute reductions in Po₂. Data, presented as means ± SE, are shown for arterioles under control conditions and following removal of the vascular endothelium using air bolus perfusion (A); inhibition of nitric oxide synthase (NOS) with N^G-nitro-l-arginine methyl ester (l-NAME) and/or inhibition of cyclooxygenase (COX) with indomethacin (Indo; B); and treatment with l-NAME and/or Indo following incubation of the arteriole with the antioxidant polyethylene glycol-superoxide dismutase (PEG-SOD; C). Please see the text for complete details. *P < 0.05 vs. control responses in that strain; †P < 0.05 vs. responses in LZR controls; ‡P < 0.05 vs. responses in OZRs + PEG-SOD.

Fig. 2.

Vascular production of 6-keto-PGF_1α (A; as an estimate of PGI₂) and 11-dehydro-thromboxane B₂ [11-dehydro-TxB₂; B; as an estimate of thromboxane A₂ (TxA₂)] by pooled arteries of LZRs and OZRs in response to an acute reduction in Po₂. Data, presented as means ± SE, are shown for arteries under control conditions and after pharmacological inhibition of COX with Indo, and, for 11-dehydro-TxB₂, thromboxane synthase with carboxyheptyl imidazole (CHI). *P < 0.05 vs. control (21% O₂) in that strain; †P < 0.05 vs. 0% O₂ in that strain; ‡P < 0.05 vs. responses in LZRs (0% O₂).

Fig. 3.

Vascular production of 6-keto-PGF_1α (A; as an estimate of PGI₂) and 11-dehydro-TxB₂ (B; as an estimate of TxA₂) by pooled arteries from OZRs in response to an acute reduction in Po₂. Data, presented as means ± SE, are shown for arteries under control conditions, following treatment of arteries with the antioxidant PEG-SOD, and following pharmacological inhibition of COX with Indo. *P < 0.05 vs. responses under control (21% O₂) conditions; †P < 0.05 vs. responses under 0% O₂; ‡P < 0.05 vs. responses determined under 0% O₂ conditions + PEG-SOD.

Fig. 4.

Dilator reactivity of isolated skeletal muscle resistance arterioles of LZRs and OZRs in response to acute reductions in Po₂. Data (means ± SE) are presented for each strain under control conditions and following pharmacological inhibition of the PGH₂/TxA₂ receptor with SQ-29548 or thromboxane synthase with CHI. *P < 0.05 vs. LZR controls; †P < 0.05 vs. OZR controls.

Fig. 5.

Vascular reactivity of isolated skeletal muscle resistance arterioles of LZRs and OZRs (means ± SE) in response to increasing concentrations of thromboxane under control conditions and following treatment of vessels with PEG-SOD (A) or SQ-29548 (B) as well as increasing concentrations of prostacyclin under control conditions and following treatment with PEG-SOD (C). The term “max” represents the maximum bound (the maximum change) in vessel diameter in response to increasing concentrations of either thromboxane or prostacyclin, which was estimated from the logistic regression equation in materials and methods. *P < 0.05 vs. control responses within that strain; †P < 0.05 vs. responses in LZRs under control conditions.