Seasonal variation in haemodynamics and blood pressure-regulating hormones.

Seasonal variation in blood pressure (BP) has been described in some people, although the variation is small for both systolic and diastolic BPs. The aim of this study was to elucidate underlying haemodynamic and hormonal mechanisms that may occur to defend seasonal changes in BP. Participants were 27 men and 7 women with either normal BP or early hypertension. Measurements of haemodynamics (cardiac output by dual-gas rebreathing) and hormones (resting catecholamines, renin activity, and aldosterone by radioenzymatic assay or radioimmunoassay) were performed during the summer, fall, winter, and spring seasons. Student's paired t-test with Bonferroni modification and regression analyses were used to examine the data with a significance level of P<0.05. Systolic and diastolic BP remained relatively constant across seasons. Cardiac output and stroke volume significantly decreased 10 and 15%, respectively, from summer to winter, whereas heart rate and systemic vascular resistance significantly increased 5 and 11%, respectively. Plasma aldosterone (PA) significantly increased 59% from summer to winter, whereas plasma norepinephrine (PNE), plasma epinephrine, and plasma renin activity (PRA) increased 19, 2, and 17%, respectively (pNS for each). Across the four seasons, mean arterial pressure significantly correlated with PRA and PA, whereas systemic vascular resistance significantly correlated with PNE and PRA. There are dramatic counterregulatory haemodynamic and hormonal adaptations to maintain a relatively constant BP. Norepinephrine, PRA, and aldosterone have a function in mediating the changes in haemodynamics.