oa Microvascular Dysfunction in Morbid Obesity: Role of Enhanced Thromboxane A2 Sensitivity

Background

The world's obese population is continuing to grow at an alarming rate and Qatar posts one of the highest obese populations in the Middle East relative to its total population. Data from the supreme council for health shows that 70% of Qataris are either overweight or obese1. This development represents an increased risk for notable complications of obesity such as type 2 diabetes, hypertension and other cardiovascular diseases among the population. Although angiogenesis tries to keep up with the need of the expanding fat, the balance is not always achievable resulting in hypoxia and dysregulated secretory functions2. As a result, a more pro-inflammatory milieu is established with adverse impact on local blood vessels. Thus, vascular dysfunctions account for much of the morbidity of obesity with the vascular endothelium often an early target of the changing metabolic status of the obese individual. In this regard, arteries that supply visceral fat tissues are more prone to dysfunction as we previously showed in omental (OM) arteries from morbidly obese Qataris3.

Endothelial dysfunction typically refers to reduced ability of the endothelium to influence the relaxation of the underlying vascular smooth muscle. This is widely believed to be due to reduction in endothelium-derived dilators particularly nitric oxide (NO) 4. Although a number of potential causes of reduction in NO and other endothelium-derived dilator molecules have been investigated over the years including decrease in eNOS expression and increase in reactive oxygen species, the molecular mechanisms underlying endothelial dysfunction in obesity are still not well understood. In a recent report, it was suggested that enhanced cyclooxygenase (COX) enzyme activity which favour increased formation of vasoconstrictor prostanoids might play a major part in the endothelial dysfunction suffered by blood vessels embedded in the visceral adipose tissue depots of individuals with morbid obesity5. COX converts arachidonic acid into endoperoxide (PGH2), an intermediate in prostanoid biosynthetic pathway, which can either act as an endothelium-derived contracting factor (EDCF) or be further transformed into prostacyclin (PGI2), thromboxane A2 (TXA2) or various other prostaglandins6, 7. In healthy endothelium, the balance is in the favour of relaxing prostanoids mainly prostacyclin8. However, this balance can be reversed in diseases such as hypertension, diabetes and atherosclerosis, in the favour of contractile prostanoids such as TXA2, which otherwise is predominantly formed in platelets. TXA2 and EDCF antagonize relaxation through binding to distinct receptors, the TP receptors expressed on vascular smooth muscle. Although circulating levels of TXA2 are increased in obesity9, the link with endothelial dysfunction has not been fully established. The aim of the current study was therefore to determine the sensitivity of the OM and subcutaneous (SC) arteries from morbidly obese Qataris to TXA2 and to relate this to the functional state of their endothelium.

Methods

A total of 12 obese Qatari male (4) and female (8) patients undergoing bariatric surgery for weight reduction at Hamad general hospital were studied. Their physical (anthropometric) and clinical (blood pressure) parameters were measured. Body Mass Index (BMI) was calculated from their heights and weights using the formula: Weight (Kg)/Square of Height (M). Mean arterial blood pressure (MAP) was calculated with the formula: 1/3 pulse pressure (systolic – diastolic pressures) + diastolic pressure. Blood was collected prior to surgery and separated to obtain plasma or serum. Fasting serum insulin, plasma Il-6 and leptin were measured by Enzyme-linked immunosorbent assay (ELISA, R & D Systems). OM and SC adipose tissue samples were collected during surgery into Cellgro medium. Arteries embedded in the adipose tissues were isolated and cut into segments (∼2 mm long) and mounted on wire myographs in normal physiological solution (PSS) containing (in mM), NaCl 112, KCl 5, CaCl2 1.8, MgCl2 1, NaHCO3 25, KH2PO3 0.5, NaH2PO3 0.5 and glucose 10. The segments were pre-tensioned to an equivalent of 100 mmHg and continuously aerated with 95% O2/5% CO2 to pH 7.4 at 37 °C. An equilibration period of at least 1 h was allowed during which 90 mM KCl was added to test for viability and thereafter 10 μM noradrenaline (NA) to optimize tissue responsiveness. Cumulative concentration-response curves were then constructed for NA (10-9 – 10-4.5 M) and the thromboxane analogue U46619 (10-11 – 10-5.5 M) in separate experiments on the same segments.

Results

The patients were 33 ± 3 years old and had BMI of 51 ± 4 Kg.m^− 2, mean arterial pressure (MAP) 88 ± 3 mmHg, fasting serum insulin of 7.04 (2.27–38.67) miU/ml, plasma Il-6 3.73 (2.70–8.70) pg/ml and plasma leptin 51.33 ± 5.81 ng.ml. Maximum NA contractions of both OM and SC arteries from same patient were similar. In contrast, U46619 contractions were significantly greater in OM compared with SC arteries from same patient (p < 0.01). When contractions to both agonists were compared on the same vessel, differences were only recorded in the OM arteries which were more sensitive to U46619 (log EC50 − 7.81 ± 0.08) compared with NA ( − 6.65 ± 0.13, p < 0.001). In addition, the OM artery curve for U46619 was significantly shifted to the left of the curve for NA (p < 0.001) with Emax for U46619 (0.05 ± 0.005 mN/μm) significantly greater than that for NA (0.03 ± 0.006 mN/μm, p < 0.01). There were no differences in both U46619 and NA contractions of the SC arteries. Emax for U46619 in SC arteries was 0.03 ±  0.008 mN/μm and for NA was 0.03 ± 0.005 mN/μm. There were also no differences in the sensitivities of the SC arteries to U46619 and NA. The log EC50 values for these agonists in the SC arteries were − 7.59 ± 0.19 vs − 6.92 ± 0.35 for U46619 and NA respectively.

Conclusion

The data show that OM arteries are hyper responsive to thromboxane A2 compared with SC arteries. In contrast, the response to noradrenaline was muted in the same arteries. We previously showed that in the Qatari obese population, endothelial dysfunction specifically occurs in OM arteries and not in SC arteries3. Since enhanced COX activity has been demonstrated in OM arteries in obesity5, we hypothesized that under this condition, increased production of contractile prostanoids would contribute to vascular dysfunction. The enhanced responsiveness to thromboxane A2 seen in the current study suggests that contractile prostanoids could indeed contribute to the endothelial dysfunction suffered by OM arteries in morbid obesity. The data also suggests that circulating levels of thromboxane A2 would more likely differentially and adversely impact OM arteries in obesity.

References

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