Differences in endothelial function and vascular reactivity between Scottish and Arabic populations

 SMJ 2003 48(3): 85-87

Khalid E Elherik, Faisel Khan, Jill JF Belch

Vascular Diseases Research Unit, University Department of Medicine, Ninewells Hospital and Medical School, Dundee, DD1 9SY, UK.

Correspondence to:      Dr. Faisel Khan , University Department of Medicine , Ninewells Hospital and Medical School , Dundee, DD1 9SY

E.mail: f.khan@dundee.ac.uk

Abstract

Background: The endothelium is a major organ that plays a central role in the regulation of blood flow, resists thrombus formation and maintains blood fluidity. Environmental, dietary, but also genetic factors might affect endothelial cell function and vascular activity.

Methods and Results: Endothelium-dependent and -independent vascular responses were evaluated in 42 male subjects from two different populations: 30 Scottish and 12 Arabic (transient visitors to UK) (20 - 41 years old). Endothelium-dependent and -independent vascular responses were measured in the forearm skin using laser Doppler imaging after iontophoresis of increasing doses of acetylcholine and sodium nitroprusside, respectively. Endothelium-dependent responses were significantly greater in Arabic subjects compared with responses in Scottish subjects (mean ACh ratio 4.9 ± 1.2 v 3.8 ± 1.5, respectively, P = 0.013, ANOVA). Although there was no significant difference between the groups in endothelium-independent vascular responses (mean SNP ratio 3.4 ± 1.2 v 2.8 ± 1.2, respectively, P = 0.26, ANOVA), there was a trend however towards a higher peak SNP response in the Arabic group (5.3 ± 1.5 v 4.1 ± 1.9, respectively, P = 0.019).

Conclusions: These results suggest that a significant difference exists in the endothelial function and vascular reactivity between these populations. Although the reasons for these differences are not clear, they may be related to differences in life styles, tradition, food intake, environmental and genetic factors.

Key words: Endothelium, Vascular function, iontophoresis, laser Doppler imaging, Scottish, Arabic.

Introduction

Environmental, dietary, but also genetic factors may affect endothelial cell function. Scottish, Mediterranean, and other populations have different dietary traditions.¹ The typical Mediterranean/Arabic diets are characterised by a high proportion of monounsaturated fatty acids, in particular olive oil, and a low intake of saturated fatty acids with high intakes of fruits, vegetables, and cereals. Ballmer showed that a Mediterranean diet has a strong secondary preventive effect after myocardial infarction.² Bolton-Smith et al. used a semi-quantitative food-frequency questionnaire to survey the diet of 10,359 Scottish men and women aged 40-59 and found that industrially hydrogenated trans fatty acids made up nearly 58% of the total intake for men and 61% for women.³ It has been reported that diets high in fat can adversely affect endothelial function.⁴ Thus on the basis of different life style, tradition, food intake, environmental and genetic factors, it is possible that endothelial cell function and vascular reactivity might differ between Scottish and Mediterranean/Arabic populations. This possibility was investigated by measuring endothelium-dependent and independent vascular responses in a Scottish and Arabic population.

Methods

The study took place in the University Department of Medicine at Ninewells Hospital and Medical School, Dundee, Scotland. Thirty Scottish males and 12 Arabic males were enrolled after ethical permission from the local ethics committee had been obtained. Written consent was obtained from each subject. Subject characteristics are shown in Table 1. The Arabic subjects consisted of students and doctors studying/working in Dundee for less than 12 months, but who had maintained an Arabic diet: 3 were from Libya, 5 from Iraq and 4 from Palestine, Egypt, Oman, and Kuwait, respectively. All subjects were non smokers and had no serious past medical history and were on no regular medication. No alcohol was consumed for at least 3 days prior to the study.

Table 1. Characteristics of subjects. There were no significant differences between the groups. Values are means ± SD.

Measurement of endothelial function and vascular reactivity

Experiments were performed in a quiet, temperature controlled laboratory set at 22-23°C. Acetylcholine (ACh, Sigma Chemical, St Louis, MO, USA), and sodium nitroprusside (SNP, David Bull laboratories, Warwick, UK) were made up to a 1% solution in deionized, sterile water. SNP solutions were covered with aluminium foil to avoid light exposure. Subjects were seated comfortably with the arms supported at heart level and had a 30 minute equilibration period. The skin of the volar surface of the forearm was cleaned gently with one-sided tape to remove dead skin and cleaned once again with alcohol and finally with sterile water. A direct iontophoresis electrode chamber was attached approximately midway between the wrist and elbow. Solutions (approximately 2.5 ml) were added to the chamber and were delivered transdermally by iontophoresis. The indifferent electrode was a Velcro strap soaked in deionized, sterile water and placed around the subject’s wrist to complete the circuit. The leads from the electrodes were connected to a battery-powered iontophoresis controller MIC1, (Moor Instruments, Axminster, UK), which provided a direct current for the delivery of solutions. ACh was iontophoresed for 10 seconds using an anodal current of 0.1 mA., giving a charge of 1milliCoulomb (mC). Cutaneous vascular responses were measured for 100 seconds. To increase the dose, ACh was iontophoresed for 20, 40, and 80 seconds (i.e. to give 2, 4 and 8 mC charges, respectively) with skin perfusion recorded for 100 seconds between each dose. Iontophoresis of SNP was carried out using a cathodal current to achieve similar charges as those for ACh (i.e. 1-8 mC). Skin perfusion recorded for 240 seconds between each dose.

Skin perfusion was measured at the volar surface of the forearm using the laser Doppler perfusion imager. Such a machine scans a low power laser beam in a raster pattern over skin surface. Moving blood in the microvasculature causes a Doppler shift which is processed to build up a colour-coded image of blood flow. The scanning configuration was set to cover an area of 80cm² using 256 x 256 pixels. The scan speed was 4msec/pixel. The scanner head was set at 50cm from the surface of the forearm. The scans were analysed using the Moor Instruments LDI software package. Blood flow was quantified in arbitrary perfusion units.

Statistical analysis

Blood flow values in arbitrary perfusion units (± SD) are expressed as the ratio of skin perfusion after each dose of ACh and SNP divided by the baseline measurement. Data from the two groups were analysed and examined for normality using the SPSS statistical package (version 10). As the data were not normally distributed they was log₁₀ transformed to achieve normality before statistical analyses were performed. Differences in dose-responses to ACh and SNP between the two groups were compared using two way analysis of variance (ANOVA) followed by modified post-hoc t-tests. Equality of means for each parameter in the two groups was examined by using the unpaired t-test. The null hypothesis was rejected at p<0.05.

Results

There was a significant difference between the two groups in ACh responses (P = 0.013) (Figure 1). Post-hoc tests showed significant differences at the 2 highest doses (P=0.006 and P=0.001, respectively). At these two doses, vascular responses in the Arabic population were greater by 32.8% and 42.3%, respectively.

Over the 4 doses, there were no significant differences between the two groups for SNP responses (P = 0.26) (Figure 2). However, there was a trend towards a higher mean SNP response (averaged over the 4 doses) in the Arabic group (P = 0. 09) (Figure 3). Additionally, the peak SNP response was significantly higher in the Arabic group (P=0.019) (Figure 3).

Discussion

Significant differences in endothelial function (ACh responses) and responses to the nitric oxide donor (SNP) were noticed between the two groups, with higher vascular responses in the Arabic group. Although the precise reason for these differences was not determined in this study, these variations in vascular responses underline the possible influence of dietary, environmental, genetic, or other unknown factors on the behaviour of the blood vessels in individuals from different parts of the world.

Most of the subjects in the Arabic group could be considered as Mediterranean. Although we did not actually record detailed information regarding dietary intake, the Mediterranean diet is assumed to be similar across all Mediterranean countries. The Scottish diet has less n-3 fatty acids than the Mediterranean diet. Deficiencies of the n-6 fatty acid, linoleic acid, have been associated with increased risk of coronary artery disease.⁵ Throughout Europe, levels of linoleic acid are inversely related to the gradient of coronary heart disease⁶ and in Scotland, men with coronary heart disease have lower than expected levels of adipose linoleic acid.⁵  Mediterranean diets are characterised by a high portion of monounsaturated fatty acids and a low intake of saturated fatty acids with a high intakes of fruits, vegetables and cereals (in the form of bread). The high portion of monounsaturated fatty acids in olive oil causes a decrease in LDL cholesterol and an increase in HDL cholesterol. In addition, antioxidant vitamins such as vitamin E, C and beta-carotene and phenolic substances such as the flavonoids, decrease the oxidation of LDL cholesterol and thus atherogenicity. The efficacy of the Mediterranean diet has been explored in the secondary prevention of coronary artery disease, in that supplements of n-3 fatty acids may be preventive.⁷ Lasheras et al showed that a Mediterranean dietary pattern was associated with a significant reduction in overall mortality in elderly subjects and was shown to favourably affect life expectancy.⁸ This pattern is also thought to reduce the risk of cancer, in addition to being cardioprotective.

Clearly, a difference in diet is not the only factor that might be responsible for the greater vascular responses in the Arabic subjects. Blood pressure and body mass index were not contributory factors. We did not measure lipid profiles and it is possible that differences in this might influence vascular responses.

The current study has shown a significant variation in endothelial function and vascular reactivity between the two different groups. This indicates the possibility of contributions from geographical, genetic and dietary variations between these two groups.

Acknowledgements

Khalid Elherik was funded by the Ministry of High Education (Libya). Jill Belch holds a Leverhulme Fellowship. The authors gratefully acknowledge Ministry of Agriculture Fishers and Food, UK (MAFF) and Chest, Heart and Stroke, Scotland for financial support and assistance. TENOVUS provided funds for the laser Doppler Imager.

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