Seasonal variation in plasma levels of endothelin-1 and nitric oxide

There is a seasonal variation in the incidence of stroke and coronary heart disease with admissions to hospital being higher in the colder months of the year. The mechanism whereby this winter prevalence of vascular disease occurs is still not fully understood. The aim of our study was to measure plasma levels of vasoactive compounds throughout the year to establish whether or not there were any fluctuations which could play a part in the higher winter incidence.

We measured plasma levels of the vasoconstrictive endothelin-1 (ET) and the vasorelaxant nitric oxide (NO) throughout the year. Blood samples were collected from 176 normal individuals. Samples were collected between 8.00 and 10.00 hours after an overnight fast of at least 12 hours.

Results were divided into two-monthly intervals and analysed using a Kruskal-Wallis one-way analysis of variance and Mann-Whitney U tests (SPSS). We found a significant seasonal variation in both parameters. Mean levels of endothelin were highest in January/February (4.0 pg/ml) and lowest in May/June (2.3 pg/ml), whereas plasma 5 nitric oxide levels were lowest in January/February (5.7 microM) and highest in September/October (9.9 microM); p values were <0.0001 (Jan/Feb vs May/June) and 0.049 (Jan/Feb vs Sept/Oct), respectively.

The high levels of the vasoconstrictor endothelin combined with low levels of vasorelaxant nitric oxide may account in part for the increased incidence of stroke and coronary heart disease seen in these months.