Vasomotion of mice mesenteric arteries during low oxygen levels

Abstract

Background: Ischemia of intestinal organs is a main cause of complications in surgical intensive care patients. Changes in the tonus of arteries contributing to vascular resistance play an important role in the determination of blood flow and thus oxygen supply of various abdominal organs. It is generally acknowledged that hypoxia itself is able to alter arterial tonus and thus blood flow.

Methods: The present study compared the effects of various degrees of hypoxia on second-order mesenteric arteries from male C57BL/6J mice. After vessel isolation and preparation, we assessed vessel diameter using an arteriograph perfusion chamber. Investigating mechanisms promoting hypoxia-induced vasodilatation, we performed experiments in Ca²⁺-containing and Ca²⁺-free solutions, and furthermore, Ca²⁺-influx was inhibited by NiCl₂, eNOS^-/--, and TASK1^-/--mice were investigated too.

Results: Mild hypoxia 14.4% O₂ induced, in 50% of mesenteric artery segments from wild-type (wt) mice, a vasodilatation; severe hypoxia recruited further segments responding with vasodilatation reaching 80% under anoxia. However, the extension of dilatation of luminal arterial diameter reduced from 1.96% ± 0.55 at 14.4% O₂ to 0.68% ± 0.13 under anoxia. Arteries exposed to hypoxia in Ca²⁺-free solution responded to lower oxygen levels with increasing degree of vasodilatation (0.85% ± 0.19 at 14.4% O₂ vs. 1.53% ± 0.42 at 2.7% O₂). Inhibition of voltage-gated Ca²⁺-influx using NiCl₂ completely diminished hypoxia-induced vasodilatation. Instead, all arterial segments investigated constricted. Furthermore, we did not observe altered hypoxia-induced vasomotion in eNOS^-/-- or TASK1^-/- mice compared to wt animals.

Conclusions: The present study demonstrated that hypoxic vasodilatation in mice mesenteric arteries is mediated by a NO-independent mechanism. In this experimental setting, we found evidence for Ca²⁺-mediated activation of ion channels causing hypoxic vasodilatation.

Keywords: Hypoxia; Mesenteric artery; Mice; Vasomotion.

Fig. 1

Percentage of dilating arterial segments and extension of vasodilation. a Percentage of dilating arterial segments compared to all the examined arteries. More arterial segments respond with vasodilatation when hypoxia becomes more severe. Under anoxia, 80% of all investigated arteries dilated. The dotted line may stress that the frequency of occurrence of dilatation was dependent from hypoxic oxygen concentrations. b Percentage changes of arterial vessel diameter under different hypoxic conditions in relation to the maximum vasodilatation using 0.1 mM nitroprusside sodium. n = number of arteries that responded to hypoxia with vasodilatation. Statistical testing was performed using the maximal evoked diameter using nitroprusside sodium as control. Student paired t test, Wilcoxon rank-sum test for non-parametric data (*p < 0.05)

Fig. 2

Vasomotion of arterial segments in Ca²⁺-free solution. a Percentage of dilating arterial segments in Ca²⁺-free buffer solutions. Under each oxygen concentration, we investigated eight arterial segments each from different animals. In Ca²⁺-free solution, more vessel segments responded with vasodilation compared to Ca²⁺-containing solutions. When reducing oxygen concentrations more arteries responded with vasodilatation. The black line represents the experiments in Ca²⁺-free solution and the grey line represents the controls in Ca²⁺-containing buffer. Controls (n = number of arteries responding with vasodilatation from a total number of eight). b Percentage change of luminal diameter in Ca²⁺-free buffer solution is represented in relation to the maximal dilatation evoked by 0.1 mM nitroprusside sodium. n = number of arterial segments that responded with hypoxia-induced vasodilatation. c Percentage change of luminal diameter is represented in relation to the maximal contraction induced by 1 mM Phe. Negative algebraic sign on the Y-axis: constriction of the arterial segment (n = number of arteries from different individual animals; *p < 0.05; Student t test for paired samples)

Fig. 3

Hypoxia-induced vasomotion of arterial segments from eNOS^−/−- and TASK1^−/−-mice. a Percentage of dilating arterial segments of eNOS^−/−-mice in Tyrode’s solution containing Ca²⁺ (Y-axis) as function of the oxygen concentration (X-axis). The black line represents the experiments with eNOS^−/−-mice and the grey line represents the controls from wt-mice (n = number of arteries responding with vasodilatation from a total number of eight). b Percentage change in luminal diameter from eNOS^−/−-mice is represented in relation to the maximal dilatation induced by 0.1 mM nitroprusside sodium (n = arterial segments responding with vasodilation from a total number of eight). c Percentage change in luminal diameter from TASK1^−/−-mice in relation to the maximal reaction (dilatation or contraction). Negative algebraic sign on the Y-axis: constriction of the arterial segment; *p < 0.05; **p < 0.01 (n = number of independent experiments; Student t test for paired samples)

Fig. 4

Comparison of vasomotion at moderate and severe hypoxic conditions. The results are shown as percentage changes in relation to the initial diameter under normoxic conditions. The X-axis represents the investigated groups, wt-co.: wild-type controls, Ca²⁺-free: Ca²⁺-free Tyrode’s solution, NiCl₂: Tyrode’s solution containing 2.5 mM NiCl₂. a Vasomotion under 12.2% oxygen; b vasomotion under 2.7% oxygen; negative algebraic sign on the Y-axis: constriction of the arterial segment; (*p < 0.05; **p < 0.01; Mann–Whitney U test for non-parametric data)