Vernon M.S. Oh

Contractile and Vasorelaxant Effects of Hydrogen Sulfide and Its Biosynthesis in the Human Internal Mammary Artery

This study aimed to test these hypotheses: cystathionine γ-lyase (CSE) is expressed in a human artery, it generates hydrogen sulfide (H2S), and H2S relaxes a human artery. H2S is produced endogenously in rat arteries from cysteine by CSE. Endogenously produced H2S dilates rat resistance arteries. Although CSE is expressed in rat arteries, its presence in human blood vessels has not been described. In this study, we showed that both CSE mRNA, determined by reverse transcription-polymerase chain reaction, and CSE protein, determined by Western blotting, apparently occur in the human internal mammary artery (internal thoracic artery). Artery homogenates converted cysteine to H2S, and the H2S production was inhibited by dl-propargylglycine, an inhibitor of CSE. We also showed that H2S relaxes phenylephrine-precontracted human internal mammary artery at higher concentrations but produces contraction at low concentrations. The latter contractions are stronger in acetylcholine-prerelaxed arteries, suggesting inhibition of nitric oxide action. The relaxation is partially blocked by glibenclamide, an inhibitor of KATP channels. The present results indicate that CSE protein is expressed in human arteries, that human arteries synthesize H2S, and that higher concentrations of H2S relax human arteries, in part by opening KATP channels. Low concentrations of H2S contract the human internal mammary artery, possibly by reacting with nitric oxide to form an inactive nitrosothiol. The possibility that CSE, and the H2S it generates, together play a physiological role in regulating the diameter of arteries in humans, as has been demonstrated in rats, should be considered.

Haplotypes of the β2-adrenergic receptor gene are associated with essential hypertension in a Singaporean Chinese population

Objective To investigate the relation between the gene encoding the β2-adrenergic receptor (B2AR) and essential hypertension in a Singaporean Chinese cohort. Methods Three single nucleotide polymorphisms (SNPs) were genotyped in 190 cases and 323 controls, and eight haplotypes were determined and tested for association using the likelihood test statistic. Results We observed a significant difference in haplotype frequency distributions between the cases and the controls (P < 0.00001). A logistic regression model fitted to the data supported this finding. Conclusion The results suggest that variants at the B2AR locus may play a role in the pathophysiology of hypertension in this population.

Computer-based testing of the modified essay question: the Singapore experience

Background: The modified essay question (MEQ), featuring an evolving case scenario, tests a candidates problem-solving and reasoning ability, rather than mere factual recall. Although it is traditionally conducted as a pen-and-paper examination, our university has run the MEQ using computer-based testing (CBT) since 2003. Aims: We describe our experience with running the MEQ examination using the IVLE, or integrated virtual learning environment (https://ivle.nus.edu.sg), provide a blueprint for universities intending to conduct computer-based testing of the MEQ, and detail how our MEQ examination has evolved since its inception. Methods: An MEQ committee, comprising specialists in key disciplines from the departments of Medicine and Paediatrics, was formed. We utilized the IVLE, developed for our university in 1998, as the online platform on which we ran the MEQ. We calculated the number of man-hours (academic and support staff) required to run the MEQ examination, using either a computer-based or pen-and-paper format. Results: With the support of our universitys information technology (IT) specialists, we have successfully run the MEQ examination online, twice a year, since 2003. Initially, we conducted the examination with short-answer questions only, but have since expanded the MEQ examination to include multiple-choice and extended matching questions. A total of 1268 man-hours was spent in preparing for, and running, the MEQ examination using CBT, compared to 236.5 man-hours to run it using a pen-and-paper format. Despite being more labour-intensive, our students and staff prefer CBT to the pen-and-paper format. Conclusions: The MEQ can be conducted using a computer-based testing scenario, which offers several advantages over a pen-and-paper format. We hope to increase the number of questions and incorporate audio and video files, featuring clinical vignettes, to the MEQ examination in the near future.

Sodium pump numbers and cation transport of lymphocytes in pregnancy-induced hypertension.

Pregnancy-induced hypertension may be linked with sodium pump inhibition and an increase in vascular myocytic tone and, hence, flow impedance. All of the findings of studies on circulating plasma and blood cells are not, however, consistent with this hypothesis. We therefore assessed sodium pump numbers and cation transport in lymphocytes from 23 women with untreated pregnancy-induced hypertension, 28 normotensive pregnant women and 28 healthy non-pregnant women. We measured the maximum 3H-ouabain binding capacity to determine the sodium pump activity and the apparent dissociation constant (the reciprocal of which estimates binding affinity) by Scatchard analysis, ouabain-sensitive (pump-mediated) 86rubidium influx and ouabain-resistant (pump-independent) influx in lymphocytes in vitro. Pregnant women, whether normotensive or hypertensive, had significantly more sodium pump activity and a higher pump-mediated and pump-independent 86rubidium influx than non-pregnant women. Sodium pump activity and the pump-mediated and pump-independent 86rubidium influx all reached normal, non-pregnant levels in normotensive pregnant women 6 weeks after delivery, but remained high in women with pregnancy-induced hypertension. The normotensive and hypertensive pregnant women and non-pregnant women all had similar ouabain binding affinity. The results of our study do not support the circulating sodium pump inhibitor hypothesis in pregnancy-induced hypertension.

131 BLOOD PRESSURE IN CHINESE SINGAPOREANS IS ASSOCIATED WITH THE POLYMORPHISM rs17249754 UPSTREAM OF THE ATP2B1 GENE

Background: Genes interact with environmental factors to cause essential hypertension, affecting 23.4% of Chinese Singaporeans aged 30-69 years. A genome-wide association study (GWAS) in Koreans found no association of the blood pressure (BP) with gene loci identified by a GWAS in Europeans. However, BP in Koreans was significantly associated with a novel single nucleotide polymorphism (SNP), rs17249754, upstream of the ATP2B1 gene encoding plasma membrane calcium-transporting ATPase-1. We therefore investigated this SNP in the Chinese Singaporean population. Design and methods: We studied 518 Chinese Singaporeans (269 hypertensive cases and 249 normotensive controls without familial hypertension). DNA was extracted and genotyped by digesting amplified products of the rs17249754 region with AluI. Allele frequency differences between cases and controls were tested using binary logistic regression, with adjustment for the confounding effects of smoking, age, and body mass index (BMI). We determined the association of rs17249754 genotypes with BP, using general linear models and taking age, BMI, smoking, alcohol intake, and plasma lipids as covariates. Results: The minor allele (A) frequency of the SNP rs17249754 was 0.345 in the control population. Allele frequency did not differ significantly between cases and controls. The genotypes contribute to significant differences in systolic BP, diastolic BP, and mean arterial pressure in men only. The AA genotype is consistently and significantly associated with lower values of all three quantitative BP traits. Conclusions: The A allele of the SNP rs17249754 is associated with lower BP in Chinese Singaporean subjects. Our results corroborate the SNPs effect on blood pressure in Koreans.