Passive heat exposure leads to delayed increase in plasma levels of atrial natriuretic peptide in humans.

The effects of passive heat exposure on atrial natriuretic peptide (ANP) were studied in six healthy men staying in a Finnish sauna at +92 degrees C for 20 min. Their rectal temperature increased by 0.4 degrees C, and evaporative water loss was 0.92 +/- 0.14 (SD) kg. Heart rate and systolic blood pressure increased significantly during the 20-min exposure. Serum osmolality and plasma arginine vasopressin levels increased during the exposure, then declined, and increased significantly again at 90-120 min. Plasma renin activity and aldosterone increased by two- to fourfold in 20 min. Plasma ANP levels rose from 13 +/- 7 to 39 +/- 15 ng/l at 60 min and to 41 +/- 13 ng/l at 120 min (P less than 0.01 for both). We conclude that transient increases in heart rate and systolic blood pressure or changes in blood volume as inferred from the weight loss do not contribute to the increased plasma ANP levels observed after the heat exposure. Instead, increased secretions of pressor hormones could explain the elevated plasma ANP levels observed after the thermal stress.