Mechanisms of hypoxic vasodilatation of isolated rat mesenteric arteries: a comparison with metabolic inhibition

Myocardial Blood Flow Control by Oxygen Sensing Vascular Kvβ Proteins

[Figure: see text].

Vasomotion of mice mesenteric arteries during low oxygen levels

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.

Electronic supplementary material

The online version of this article (10.1186/s40001-018-0335-8) contains supplementary material, which is available to authorized users.

Intrinsic vascular dopamine - a key modulator of hypoxia-induced vasodilatation in splanchnic vessels

Dopamine not only is a precursor of the catecholamines noradrenaline and adrenaline but also serves as an independent neurotransmitter and paracrine hormone. It plays an important role in the pathogenesis of hypertension and is a potent vasodilator in many mammalian systemic arteries, strongly suggesting an endogenous source of dopamine in the vascular wall. Here we demonstrated dopamine, noradrenaline and adrenaline in rat aorta and superior mesenteric arteries (SMA) by radioimmunoassay. Chemical sympathectomy with 6-hydroxydopamine showed a significant reduction of noradrenaline and adrenaline, while dopamine levels remained unaffected. Isolated endothelial cells were able to synthesize and release dopamine upon cAMP stimulation. Consistent with these data, mRNAs coding for catecholamine synthesizing enzymes, i.e. tyrosine hydroxylase (TH), aromatic l-amino acid decarboxylase, and dopamine-β-hydroxylase were detected by RT-PCR in cultured endothelial cells from SMA. TH protein was detected by immunohistochemisty and Western blot. Exposure of endothelial cells to hypoxia (1% O2) increased TH mRNA. Vascular smooth muscle cells partially expressed catecholaminergic traits. A physiological role of endogenous vascular dopamine was shown in SMA, where D1 dopamine receptor blockade abrogated hypoxic vasodilatation. Experiments on SMA with endothelial denudation revealed a significant contribution of the endothelium, although subendothelial dopamine release dominated. From these results we conclude that endothelial cells and cells of the underlying vascular wall synthesize and release dopamine in an oxygen-regulated manner. In the splanchnic vasculature, this intrinsic non-neuronal dopamine is the dominating vasodilator released upon lowering of oxygen tension.

Contractile responses of isolated rat mesenteric arteries to acute episodes of severe hypoxia and subsequent reoxygenation

This study further investigates the mechanisms responsible for the effects of acute and severe hypoxia, and subsequent reoxygenation, on the contractility of isolated rat mesenteric arteries. In noradrenaline (NA)-contracted arteries, hypoxia caused a relaxation to near baseline levels. Reoxygenation resulted in an immediate transient contraction before tension returned more slowly to prehypoxia levels. Similar responses to hypoxia were observed in tissues precontracted by addition of KCl (60 mM) or U46619 (10 microM); however, the transient contraction upon reoxygenation was absent (KCl) or reduced (U46619). Responses to hypoxia were independent of changes in intracellular calcium ([Ca2+]i), while those to reoxygenation were accompanied by corresponding changes in [Ca2+]i and were completely abolished by ryanodine. In NA-contracted tissues, all responses were unaffected by endothelial removal or by inhibitors of nitric oxide synthase and cyclooxygenase. The K+ channel blockers triethylamine (TEA), glibenclamide, and 4-aminopyridine (4-AP) had no effect on the responses to hypoxia. The transient contractile response to reoxygenation was, however, significantly reduced in the presence of 4-AP. The response to reoxygenation, but not that to hypoxia, was inhibited by the antioxidant dithiothreitol (DTT) and the NAD(P)H-oxidase inhibitor diphenyliodonium (DPI). These data suggest that hypoxic vasodilation occurs independently of reductions in [Ca2+]i. Alternatively, transient contractions on reoxygenation are dependent upon the generation of reactive oxygen species and the release of stored Ca2+.

Origin of ATP for Ca2+-induced contraction in the guinea-pig femoral artery

Previously, we have described differences between the rat proximal colon and femoral artery with respect to the role of ATP newly synthesized by creatine kinase. In the present study the role of newly synthesized ATP was studied in the guinea-pig femoral artery to examine species differences. In the alpha-toxin-permeabilized preparation of the guinea-pig femoral artery, the rapid Ca(2+)-induced contraction was suppressed when creatine kinase activity was inhibited. The contraction was restored completely by treatment with NaN(3), an inhibitor of ecto-ATPase, the enzyme that breaks down exogenous ATP. Thus, ATP newly synthesized by creatine kinase may have no role in contraction of the guinea-pig femoral artery. This is in marked contrast to the rat femoral artery, in which Ca(2+)-induced contractions are almost completely inhibited by inhibition of creatine kinase activity but only partly restored by NaN(3). To characterize the difference between the guinea-pig and rat tissue, the origin of ATP required for contraction was determined in intact preparations. Monoiodoacetic acid, an inhibitor of glycolysis, inhibited the high K(+)-induced contraction in the guinea-pig femoral artery more potently than in the rat tissue. In contrast, an inhibitor of mitochondrial respiration, carbonylcyanide p-(trifluoromethoxy)phenylhydrazone (FCCP), inhibited contraction in femoral arteries from rats, but not from guinea-pigs. These results suggest that contraction in the rat femoral artery is dependent largely on oxidative phosphorylation, while contraction in the guinea-pig tissue is dependent only on glycolysis. Because oxidative phosphorylation generates ATP and phosphocreatine, while glycolysis generates only ATP, the strong dependence of the contraction of the rat femoral artery on the oxidative phosphorylation is consistent with its dependence on ATP newly synthesized by creatine kinase from ADP and phosphocreatine, as previously shown. Thus, it is proposed that ATP, newly synthesized by creatine kinase, in addition to ATP generated by oxidative phosphorylation, is utilized for contraction in the rat femoral artery, while glycolysis produces sufficient ATP for contraction in the guinea-pig femoral artery.

Continuous mixed venous oxygen saturation, not mean blood pressure, is associated with early bupivacaine cardiotoxicity in dogs

PURPOSE

To investigate changes of continuous mixed venous oxygen saturation (cSvO(2)) and mean arterial blood pressure (MBP) in dogs with bupivacaine-induced cardiac depression.

METHODS

Bupivacaine was infused into pentobarbital-anesthetized mongrel dogs (n = 8) at a rate of 0.5 mg.kg(-1).min(-1) until the MBP was 40 mmHg or less (end of bupivacaine infusion; BIE). The infusion time was divided into the early period, first 30 min of bupivacaine infusion and the late period, which was from 30 min of bupivacaine infusion until BIE. cSvO(2) was monitored using a fibreoptic pulmonary artery catheter, and MBP and cardiac output (CO) were measured every ten minutes after the initiation of bupivacaine infusion. Arterial blood gas, serum electrolyte and bupivacaine concentration were measured simultaneously. The relationships between CO and cSvO(2), and of CO vs MBP were compared by regression analysis in the early and late periods.

RESULTS

The Pearson's correlation coefficients between CO and cSvO(2) were 0.782 (P = 2.1 x 10(-7)) in the early period and 0.824 (P = 1.3 x 10(-6)) in the late period. The correlation coefficients between CO and MBP were 0.019 (P = 0.921) in the early period and 0.799 (P = 4.8 x 10(-6)) in the late period.

CONCLUSIONS

cSvO(2), but not MBP, is associated with CO changes in bupivacaine-induced cardiac depression during the early period of bupivacaine intoxication. Decrease of MBP with low cSvO(2) observed during the late period might imply severe cardiac depression induced by bupivacaine infusion.

Hypoxia, energy state and pulmonary vasomotor tone

Vasomotor responses to hypoxia constitute a fundamental adaptation to a commonly encountered stress. It has long been suspected that changes in cellular energetics may modulate both hypoxic systemic artery vasodilatation (HSV) and hypoxic pulmonary artery vasoconstriction (HPV). Although limitation of energy has been shown to underlie hypoxic relaxation in some smooth muscles, the response to hypoxia in vascular smooth muscle does not appear to be a simple function of energy stores, but instead may involve perturbations of ATP or energy delivery to mechanisms controlling muscle force, and/or changes associated with anaerobic metabolism. Recent work in pulmonary vascular smooth muscle has demonstrated that energy stores are maintained during hypoxic pulmonary vasoconstriction, and that this is dependent on glucose availability and up-regulation of glycolysis. There is increasing evidence that glycolysis is preferentially coupled to a variety of membrane associated ATP dependent processes, including the Na(+) pump, Ca(2+)-ATPase, and possibly some protein kinases. These and other mechanisms may influence excitation-contraction coupling in both systemic and pulmonary arteries by effects on intracellular Ca(2+) and/or Ca(2+) sensitivity. Hypoxia has also been postulated to have major effects on other cytosolic second messenger systems including phosphatidylinositol pathways, cell redox state and mitochondrial reactive oxygen species production. This review examines the relationship between energy state, anaerobic respiration and hypoxic vasomotor tone, with a particular emphasis on hypoxic pulmonary vasoconstriction.

Hypoxia, metabolic inhibition, and isolated rat mesenteric tone: influence of arterial diameter

The present study examined and compared the effects of severe hypoxia (induced by either substitution of O(2) in the gassing medium with N(2) or by addition of the O(2) scavenger sodium dithionite) and metabolic inhibition (induced by addition of sodium cyanide) on the tone of isolated rat mesenteric resistance vessels. The influence of vessel diameter and the endothelium on responses to these maneuvers was investigated. Hypoxia (due to both substitution of O(2) with N(2) and by addition of 2 mM sodium dithionite) caused near maximal relaxation of all tissues studied. Addition of 10 mM dithionite, however, produced a significantly smaller response. Two mM cyanide also relaxed mesenteric arteries. In small vessels a near maximal relaxation to cyanide was observed, however, in larger vessels the relaxation to metabolic inhibition was significantly less than that observed to hypoxia. Increasing the concentration of cyanide had no further effect on responses. All responses were found to be endothelium-independent. Thus, as the effects of hypoxia and cyanide are not always similar, care must be taken when extrapolating the effects of metabolic inhibition to those of hypoxia. The results of this study suggest that, in large mesenteric vessels at least, an "O(2) sensing step," in addition to effects of metabolism, may be involved in the hypoxic response.