We examined the role of dietary electrolytes and humoral factors in causing seasonal changes in blood pressure. Normal subjects had no seasonal difference in blood pressure, although urinary sodium and norepinephrine were significantly higher in winter than in summer. In patients with essential hypertension blood pressure, urinary sodium and norepinephrine excretion and plasma norepinephrine concentration were significantly higher in winter. Plasma renin activity, plasma and urinary aldosterone and urinary kallikrein excretion were not significantly different between the two seasons in both normal subjects and hypertensive patients. In conclusions, the blood pressure of patients with essential hypertension has a seasonal variation with higher pressures in the winter than in the summer. Increased sympathetic nervous activity and an increased load of sodium presented to the kidney for excretion may be contributing factors in the rise in blood pressure in winter in patients with essential hypertension.

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